Broadly speaking there are two types of models stating the relative position of DNA and proteins in the chromosomes.
(1) Multiple strand models : According to several workers (Steffensen 1952, Ris 1960) a chromosome is thought to be composed of several DNA protein fibrils and atleast two chromatids form the chromosome.
(2) Single strand models : According to Taylor, Duprow etc. The chromosome is made up of a single DNA protein fibril. There are some popular single strand models.
(a) Folded fibre model : Chromosomes are made up of very fine fibrils 2 nm – 4 nm in thickness. As the diameter of DNA molecule is also 2 nm (20Å). So it is considered that a single fibril is a DNA molecule. It is also seen that chromosome is about a hundred times thicker than DNA whereas the length of DNA in chromosome is several hundred times that of the length of chromosome. So it is considered that long DNA molecule is present in folding manner which forms a famous model of chromosome called folded fibre model which given by E.J. Dupraw (1965).
(b) Nucleosome model : The most accepted model of chromosome or chromatin structure is the ‘nucleosome model’ proposed by Kornberg and Thomas (1974). Nucleosomes are also called core particles or Nu-bodies. The name nucleosome was given by P. Outdet et al. The nucleosome is a oblate particle of 55Å height and 110Å diameter. Woodcock (1973) observed the structure of chromatin under electron microscope. He termed each beaded structure on chromosome as nucleosome. Nucleosome is quasicylindrical structure made up of histones and DNA. Histone are mainly of two types :
(i) Nucleosomal histone : These are small proteins responsible for coiling DNA into nucleosome. These are H2A, H2B, H3 and H4. Each histone protein consist of two molecule, thus the four histone proteins form a octamer. These form the inner core of nucleosome.
(ii) Linker histone : H1 proteins is known as linker histone that connect one core particle with another. These are present once per 200 base pairs. These are loosely associated with DNA. H1 histone are responsible for packing of nucleosome into 30 nm fibre.
(iii) DNA in nucleosome : Nucleosome is made of core of eight molecules of histones wrapped by double helical DNA with turns making a repeating unit. Every turn of DNA have 146 base pairs. When H1 protein is added the nucleotide number becomes 200. DNA which joins two nucleosome is called linker DNA or spacer DNA.
(c) Solenoid model : In this model the nucleosomal bead represents the first degree of coiling of DNA. It is further coiled to form a structure called solenoid (having six nucleosome per turn). It represents the second degree of coiling. The diameter of solenoid is 300Å. The solenoid is further coiled to form a supersolenoid of 2000-4000Å diameter. This represent the third degree of coiling. The supersolenoid is perhaps the unit fibre or chromonema identified under light microscopy. The solenoid model was given by Finch and Klug 1976. Klug was awarded by nobel prize in 1982 for his work on chromosome.
(d) Dangler-String or Radial Loop Model : (Laemmli, 1977). Each chromosome has one or two interconnected scaffolds made of nonhistone chromosomal proteins. The scaffold bears a large number of lateral loops all over it. Both exit and entry of a lateral loop lie near each other. Each lateral loop is 30 nm thick fibre similar to chromatin fibre. It develops through solenoid coiling of nucleosome chain with about six nucleosomes per turn. The loops undergo folding during compaction of chromatin to form chromosome.
Heterochromatin and Euchromatin
Flemming (1880) named the readily stainable material in nuclei as chromatin. It is present both during interphase and cell division (as the chromosomal material). It consists of about equal parts by weight of DNA and histones. There are two classes of chromatin structure, heterochromatin and euchromatin.
Heterochromatin or static chromatin is highly condensed and is usually transcriptionally inactive and found in the centromeres of chromosomes. Heterochromatin is of two types, (i) genetically inactive constitutive heterochromatin which is a permanent part of the genome, and (ii) facultative heterochromatin which varies in its state in different cell types and development stages. Euchromatin or dynamic chromatin is relatively extended and open. It at least has the potential of being actively transcribed. It makes up the major part of the genome, and is visible only during mitosis.
Chromosome banding
It was the technique demonstrated by Casperson (1968) using a fluorescent dye quinacrine mustard for the study of finer chromosomal aberrations. The development of banding techniques has made the identification of individual chromosomes easier. Each chromosome can be identified by its characteristic banding pattern. In X chromosomes the bands are large, each containing ~107bp of DNA, and could include several hundreds of genes. The different banding techniques are identified by the letters Q, G, C, R and T.
Table : 4-10 Differentiation of chromosomes by banding
Type of banding
Staining technique
Nature of bands
Q (quinacrine) banding
Chromosomes exposed to quinacrine mustard (acridine dye) which preferentially binds to AT-rich DNA. Other fluorescent dyes used are DAPI or Hoeschst 33258.
UV fluorescence reveals fluorescencing Q bands which correspond to G-bands. DNA of Q/G bands contains more closely spaced SARs, giving tighter loops (Q loops).
G (Giemsa) banding
Chromosomes treated with alkaline solution and subjected to controlled trypsin digestion before staining with Geimsa, a DNA banding chemical dye. Relatively permanent stain.
Dark bands are called G bands and pale bands are G-negative. G bands are presumed to be AT-rich. They are late replicating and contain highly condensed chromatin.
R (reverse) banding
Chromosomes treated with heated saline or restrictase to denature AT-rich DNA and stained with Giemsa. GC-specific chromomycin dyes, e.g, chromomycin A, olivomycin or mithracin give the same pattern.
R-banding pattern is essentially the reverse of the G-banding pattern. R bands are Q negative. They generally replicate in the S-phase and have less condensed chromatin.
T (telomaric) banding
Prolonged heat treatment of chromosomes before staining with Giemsa or combination of dyes and fluorochromes.
T bands are a subset of R bands which are the most inetnsely staining. They are especially concentrated at the telomeres.
C (centromere) banding
Chromosomes pre-treated with sodium hydroxide or barium hydroxide and stained with Giemsa.
Preferred darkening of constitutive centromeric heterochromatin. Rest of the chromosome show Q banding pattern.
Human karyotype and idiogram
Tjio and Levan (1956) of Sweden found that human cells have 23 pairs or 46 chromosomes. 22 pairs or 44 chromosomes are autosome and the last or 23rd pairs is that of sex chromosomes, XX in females and XY in males.
A set of chromosomes of an individual or species is called a karyotype. In human the 23 pairs of chromosomes in somatic cells form the karyotype. It is possible to identify individual chromosomes on the basis of the following characteristics :
(1) The total length of the chromosomes.
(2) Arm ratio.
(3) The position of the secondary constrictions and nucleolar organizers.
(4) Subdivision of the chromosome into euchromatic and heterochromatic regions.
Homologous pairs of identified chromosomes can be arranged in a series of decreasing lengths. Such an arrangement is called an idiogram. Idiogram not possible in symmetrical karyotype.
Karyotyping of human chromosomes : Chromosomes are clearly visible only in rapidly dividing cells. Human chromosomes are studied in blood cells (WBCs), cells in bone marrow, amniotic fluid and cancerous tissues. The WBCs divide when added with phytohaemagglutinin (PHA).
The division stops when colchicine is added at metaphase stage. These dividing WBCs are then treated with hypotonic saline solution. Chromosomes are now stained with stains like orcein, Giemsa dye or recent quinacrine dye.
When viewed with special miscroscope in ultraviolet light the stain produces fluorescent bands on chromosomes. The chromosomes are then arranged on photographic plate for making diagram and their study. The pictorial representation of a person’s chromosomes is called Karyotype.
Classification of chromosomes : The human metaphase chromosomes were first of all classified by a conference of cytogeneticists at Denver, Colorado in 1960 and is known as the 23 pairs (46) chromosomes in human has been numbered from 1 to 23 according to their decreasing size. Patau (1960) divided the human chromosome into the following seven groups designated A to G.
Table : 4-11 Characteristics of the Chromosomes in Human Karyotype
Group
Size
Shape
Number in set
Number in a cell
A
Large
Metacentric
Submetacentric
1-3
6
B
Large
Submetacentric
4-5
4
C
Medium
Submetacentric
6-12
and X
15 male
16 female
D
Medium
Acrocentric
13-15
6
E
Small
Submetacentric
16-18
6
F
Small
Metacentric
19-20
4
G
Smallest
Acrocentric
21-22
and Y
5 male
4 female
46
Type of chromosomes
(a) Depending upon the number of centromeres, the chromosomes may be :
(1) Monocentric with one centromere.
(2) Dicentric with two centromeres, one in each chromatid.
(3) Polycentric with more than two centromeres.
(4) Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes, which do not survive for long.
(5) Diffused or non-located with indistinct throughout the length of chromosome. The microtubules of spindle fibres are attached to chromosome arms at many points. The diffused centromeres are found in insects, some algae and some groups of plants.
(b) Based on the location of centromere the chromosomes are categorised as follows :
(1) Telocentric : These are rod-shaped chromosomes with centromere occupying a terminal position. One arm is very long and the other is absent.
(2) Acrocentric : These are rod-shaped chromosomes having subterminal centromere. One arm is very long and the other is very small.
(3) Submetacentric : These are J or L shaped chromosomes with centromere slightly away from the mid-point so that the two arms are unequal.
(4) Metacentric : These are V-shaped chromosomes in which centromere lies in the middle of chromosomes so that the two arms are almost equal.
Special types of chromosomes
Polytene chromosome : Polytene chromosome was described by Kollar (1882) and first reported by Balbiani (1881) in the salivary gland cells of chironomus larva. They are found in salivary glands of insects (Drosophila) and called as salivary gland chromosomes. These are reported in endosperm cells of embryosac by Malik and Singh (1979). Length of this chromosome may be upto 2000mm.
The chromosome is formed by somatic pairs between homologous chromosomes and repeated replication or endomitosis of chromonemata. These are attached to chromocentre. It has pericentromeric heterochromatin. Polytene chromosomes show a large number of various sized intensity bands when stained.
The lighter area between dark bands are called interbands. They have puffs bearing Balbiani rings. Balbani rings produce a number of m-RNA, which may remain stored temporarily in the puffs, are temporary structures. These are also occur in Malpighian tubules, rectum, gut, foot pads, fat bodies, ovarian nurse cells etc.
Lampbrush chromosomes : They are very much elongated special type of synapsed or diplotene chromosome bivalents already undergone crossing over and first observed by Flemming (1882). The structure of lampbrush chromosome was described by Ruckert (1892). The lampbrush chromosomes occur at the diplotene stage of meiotic prophase in the primary oocytes of all animal species, both vertebrates and invertebrates. Lampbrush chromosomes are also found in spermatocytes of several species, giant nucleus of acetabularia and even in plants. In urodele oocyte the length of lampbrush chromosome is upto 5900mm. These are found in pairs consisting of homologous chromosomes jointed at chiasmata (meiotic prophase-I). The chromosome has double main axis due to two elongated chromatids. Each chromosome has rows of large number of chromatid giving out lateral loops, which are uncoiled parts of chromomere with one-many transcriptional units and are involved in rapid transcription of mRNA meant for synthesis of yolk and other substances required for growth and development of meiocytes. Some mRNA produced by lampbrush chromosome is also stored as informosomes i.e., mRNA coated by protein for producing biochemicals during the early development of embryo. Length of loop may vary between 5-100mm.
Supernumerary, Accessory or B chromosomes or Satellite chromosomes or Giant lines plasmid : In some species, chromosomes have been found that are in addition to the normal autosomes and heterosomes. These chromosomes have been called supernumerary chromosomes, accessory chromosomes or B-chromosomes, and differ from normal or A-chromosomes in the following respects.
(1) They are usually smaller than A-chromosomes.
(2) They are frequently heterochromatic and telocentric.
(3) They are genetically unnecessary, and normally do not strongly influence viability and phenotype.
(4) Their number may vary in different cells, tissues, individuals and populations.
(5) They are not homologous with any of the A-chromosomes and do not synapses with them.
(6) They are found more commonly in plants than in animals.
Limited or L-chromosomes : Limited or L-chromosomes are so called because they are limited to the germ line. They have been found in the family Sciaridae (Diptera: Insecta). The germ line cells in females have 10 chromosomes. Those of males have 9 chromosomes. L-chromosomes differ from B-chromosomes in that they are constant in all individuals of the species having them. B-chromosomes are found only in some individuals of the species.
Minute or m-chromosomes : Minute or m-chromosomes are so called because of their extremely small size (0.5 micron or less). They have been found in a variety of species of bryophytes, higher plants, insects of the family Coreidae (Heteroptera) and birds.
S and E-chromosomes : S and E-chromosomes have been reported in insects in the family Cecidomyiidae (gall insects) and family Chironomidae (Diptera).
Chromosomes which are present in both germ and somatic cells are called S-chromosomes. Those which are eliminated from somatic cells but are present in germ cells are called E-chromosomes. Thus in females of gall insect the germ line cells have 12 S-chromosomes and 36 E-chromosomes.
In male germ line cells there are 6 S-chromosomes and 42 E-chromosomes. The zygote receives half its S-chromosomes from each parent, while all the E-chromosomes are received from the female parent.
Genes
Term ‘gene’ was given by Johannsen (1909) for any particle to which properties of Mendelian factor or determiner can be given. Thomas Hunt Morgan (1910) defined gene as ‘any particle on the chromosome which can be separated from other particles by mutation or recombination is called a gene’. In general, gene is the basic unit of inheritance.
According to the recent information a gene is a segment of DNA which contains the information for one enzyme or one polypeptide chain coded in the language of nitrogenous bases or the nucleotides. The sequence of nucleotides in a DNA molecule representing one gene determines the sequence of amino acids in the polypeptide chain (the genetic code). The sequence of three nucleotides reads for one amino acid (codon).
Gene action
Gene act by producing enzymes. Each gene in an organism produces a specific enzyme, which controls a specific metabolic activity. It means each gene synthesizes a particular protein which acts as enzyme and brings about an appropriate change.
One gene one enzyme theory : This theory was given by Beadle and Tatum (1958), while they were working on red mould or Neurospora (ascomycetes fungus). Which is also called Drosophila of plant kingdom. Wild type Neurospora grows in a minimal medium (containing sucrose, some mineral salts and biotin). The asexual spores i.e. conidia were irradiated with x-rays or UV-rays (mutagenic agent) and these were crossed with wild type. After crossing sexual fruiting body is produced having asci and ascospores. The ascospores produced are of 2 types –
(i) The ascospores, which are able to grow on minimal medium called ‘prototrophs’.
(ii) Which do not grow on minimal medium but grow on supplemented medium called ‘auxotrophs’.
Molecular structure of gene
Gene is chemically DNA but the length of DNA which constitutes a gene, is controversial 3 term i.e. cistron, muton and recon were given by Seymour Benzer to explain the relation between DNA length and gene.
Cistron or functional gene or gene in real sense : Benzer (1955) related gene to arm cistrom or Cistron is that particular length of DNA which is capable of producing a protein molecule or polypeptide chain or enzyme molecule.
Muton or unit of mutation : Muton is that length of DNA which is capable of undergoing mutation. Muton is having one or two pairs of nucleotide.
Recon : Recon is that length of DNA which is capable of undergoing crossing over or capable of recombination. Recon is having one or two pairs of nucleotides.
Complon : It is the unit of complementation. It has been used to replace cistron. Certain enzymes are formed of two or more polypeptide chains. Whose active groups are complimentary to each other.
Operon : Operon is the combination of operator gene and sequence of structure genes which act together as a unit. Therefore it is composed of several genes. The effect of operator gene may be additive or suppresive.
Replicon : It is the unit of replication. Several replicons constitute a chromosome.
Some specific terms
Transposons or Jumping genes : The term ‘transposon’ was first given by Hedges and Jacob (1974) for those DNA segments which can join with other DNA segments completely unrelated and thus causing illegitimate pairing. These DNA segments are transposable and may be present on different place on main DNA. The transposons are thus also called Jumping genes. Hedges and Jacob reported them in bacteria. But actual discovery of these was made by Barbara Mc Clintock (1940) in maize and she named them as controlling elements or mobile genetic elements. For this work, she was awarded nobel prize in (1983).
Retroposons : The term was given by Rogers (1983) for DNA segments which are formed from RNA or which are formed by reverse transcription under the influence of reverse transcriptase enzyme or RNA dependent DNA polymerase enzyme. About 10% of DNA of genome in primates and rodents is of this type.
Split genes or interrupted genes : Certain genes were reported first in mammalian virus and then in eukaryotes by R. Roberts and P. Sharp in (1977) which break up into pieces or which are made of segments called exons and introns. These are called split genes or interrupted genes.
Split gene = Exons + Introns
If mRNA formed from split gene exons are present and not corresponding to introns. So in split genes, exons carry genetic information or informational pieces of split genes are exons.
Pseudogenes or false genes : DNA sequences presents in multicellular organisms, which are useless to the organism and are considered to be defective copies of functional genes (cistrons) are called pseudogenes or false genes. These have been reported in Drosophila, mouse and human beings.
Multiple allelism
More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue are known as multiple alleles.
Characteristics of multiple allelism
(a) There are more than two alleles of the same genes.
(b) All multiple alleles occupy the corresponding loci in the homologous chromosomes.
(c) A chromosome or a gamete has only one allele of the group.
(d) Any one individual contains only two of the different alleles of a gene, one on each chromosome of the homologous pair carrying that gene.
(e) Multiple alleles express different alternative of a single trait.
(f) Different alleles may show codominance, dominance-recessive behaviour or incomplete dominance among themselves.
(g) Multiple alleles confirm to the Mendelian pattern of inheritance.
Examples of multiple allelism : A well known example of a trait determined by multiple alleles is the blood groups in man and skin colour. Other example are eye colour in Drosophila, colour of wheat kernel, corolla length in Nicotiana, Coat colour in Cattle etc.
Blood groups in man
Blood proteins : According to Karl landsteiner (1900) a Nobel prize winner, blood contains two types of proteinous substances due to which agglutinations occurs.
(1) Agglutinogen or antigen : It is a protein found on the cell membrane of RBC’s.
(2) Agglutinin or antibody : This the other proteinous substance, found in the plasma of the blood.
Whenever the blood of a person receives the foreign proteins (antigen) his blood plasma starts forming the antibodies in order to neutralize the foreign antigens.
Agglutinations : Two types of antigens are found on the surface of red blood corpuscles of man, antigen A and B. To react against these antigens two types of antibodies are found in the blood plasma which are accordingly known as antibody – anti-A or a and anti-B or b. Agglutination takes place only when antigen A and antibodya occur together or antigen B and antibody b are present in the blood.
Under such condition antibody a reacts with antigen A and makes it highly sticky. Similarly antigen B in presence of antibody b become highly sticky with the result RBC’s containing these antigens clump to form a bunch causing blockage of the capillaries. Agglutination in blood is therefore antigen-antibody reaction.
Types of blood groups
ABO blood group : Landsteiner divided human population into four groups based on the presence of antigens found in their red blood corpuscles. Each group represented a blood group. Thus there are four types of blood groups viz. A, B, AB and O. He observed that there was a reciprocal relationship between antigen and antibody according to which a person has antibodies for those antigens which he does not possess.
Table : 4-12 Blood groups of man with antigen and antibodies
Type of blood group
Antigen
Antibody
% in society
A
A
Anti-B or ‘b’
23.5
B
B
Anti-A or ‘a’
34.5
AB
A, B
Absent
7.5
O
None
‘a’ and ‘b’
34.5
M, N blood group : K. Landsteiner and A.S. Wiener discovered that antigen M,N or both MN are also found on the surface of red blood corpuscles of human beings. No antibodies are however formed in the blood plasma for these antigens.
In this way when blood with M group is injected in rabbit it will produce antibodies in the blood serum which will bring about agglutination with blood group M and MN but not with blood of N group. In the same way on injecting blood of N group into the rabbit it will bring about agglutination with blood group N and MN and not with blood having blood group M.
Blood transfusion
Blood transfusion is best done in the persons of same blood group. At the same time it is possible to know in which different blood groups the blood transfusion can be made possible.
Persons with blood group AB are called universal recipients because both antigens A and B are found in their blood and the two antibodies ‘a’ and ‘b’ are absent. Therefore, such persons can receive blood of all the blood groups.
In the same way persons who have blood group O– are universal donors as they lack both the antigens and Rh– person can donate to Rh+ person as well as Rh– person but Rh+ person cannot donate blood to Rh– person. But at the same time such persons can not be given the blood of any other blood group except blood group O because their blood possesses both the antibodies ‘a’ and ‘b’. Persons belonging to blood group A and B contain only one antigen and one antibody against it, in their blood. Such persons can therefore receive blood either of the blood group of their own or the blood group O.
Blood bank
A place where blood of different blood groups is safely stored in bottles for emergency use, is called blood bank. Blood after proper testing is stored in a sealed bottle at a definite temperature (4°-6°c) to be preserved for a definite time period.
Artificial anticoagulants are used to prevent blood clotting in the blood banks. These anticoagulants are added to the blood preserved in bottle. Such anticoagulants include sodium citrate, double oxalates (sodium and ammonium), dicumarol and EDTA (ethylene diamine tetra acetic acid). The whole blood in this way can be stored for a maximum period of 21 days.
Inheritance of blood groups
Blood groups in human are inheritable trait and are inherited from parents to offsprings on the basis of Mendel’s Laws. Blood group inheritance depends on genes received from parents. Genes controlling blood group in man are three instead of two and are called multiple alleles. All these three genes or alleles are located on the same locus on homologous chromosomes. A person can have only two of these three genes at a time which may be either similar or dissimilar in nature. These genes control the production of blood group/antigens in the offspring. The gene which produces antigen A is denoted by Ia, gene for antigen B by Ib and the gene for the absence of both antigens by Io. it is customary to use the letter I (Isohaemagglutinogen) as a basic symbol for the gene at a locus. Based on this, six genotypes are possible for four blood groups in human population.
Table : 4-13 Genotype of blood groups in man
Type of blood group
Genotype
Nature of gene
A
Ia Ia
Homozygous (Dominant)
A
Ia Io
Heterozygous
B
Ib Ib
Homozygous (Dominant)
B
Ib Io
Heterozygous
AB
Ia Ib
Codominant
O
Io Io
Homozygous (Recessive)
The alleles Ia and Ib of human blood group are said to be codominant because both are expressed in the phenotype AB. Each produces its antigen and neither checks the expression of the other. There is codominance as well as dominant recessive inheritance in the case of the alleles for the blood groups in human beings. The alleles Ia and Ib are codominant and are dominant over the allele Io (Ia = Ib > Io). The human blood groups illustrate both multiple allelism and codominance. This blood group are inherited in the simple Mendelian fashion. Thus offsprings with all four kinds of blood groups are possible. If the parents are heterozygous for blood groups A and B which is shown below.
Table : 4-14 Cross between parents heterozygous for blood group A and B
Male (Heterozygous for blood group A)
Gametes
Ia
Io
Female
(Heterozygous for blood group B)
Ib Io
Ia
Io
Ib
Ia Ib
Ib Io
Group AB
Group B
Io
Ia Io
Io Io
Group A
Group O
If we know the blood groups of a couple the blood groups of their children can easily be predicted as shown below.
Table : 4-15 Possible blood groups of children for known blood groups of parents
Blood groups of parents (known)
Genotype of parents (known)
Blood groups of children
Possible
Not possible
O and O
Io Io ´ Io Io
O
A, B, AB
O and A
Io Io ´ Ia Io
O, A
B, AB
A and A
Ia Io ´ Ia Io
O, A
B, AB
O and B
Io Io ´ Ib Io
O, B
A, AB
B and B
Ib Io ´ Ib Io
O, B
A, AB
A and B
Ia Ia ´ Ib Ib
Ia Ia ´ Ib Io
Ia Io ´ Ib Io
O, A, B, AB
None
O and AB
Io Io ´ Ia Ib
A, B
O, AB
A and AB
Ia Io ´ Ia Ib
A, B, AB
O
B and AB
Ib Io ´ Ia Ib
A, B, AB
O
AB and AB
Ia Ib ´ Ia Ib
A, B, AB
O
Significance of blood groups : The study of blood groups is important in settling the medico-legal cases of disputed parentage because with the help of blood group of a child it can be decided as to who can be his or her genuine father, if the blood group of mother is known. It means that blood groups of the mother and a child being known, the possibilities of blood group in the father can be worked out or if blood group of child and that of father is known then that of mother can be known with the help of the table given below. Blood groups can also save an innocent from being hanged in the case of murder and can help in hanging the real culprit.
Table : 4-16 Possibilities of blood groups of other parent on the basis of blood group of child and one parent being known
Blood group of child (known)
Genotype of child (known)
Blood group of father or mother (known)
Blood group of other parent
Possible
Not possible
O
Io Io
O
A
B
A, B
O, B
O, A
AB
A
Ia Io, Ia Ia
O, B
A, AB
O, B
B
Ib Io , Ib Ib
O, A
A
B, AB
B, AB
O, A
O, A
AB
Ia Ib
B
AB
A, AB
A, B, AB
O, B
O
Rhesus or Rh factor
Landsteiner and Weiner (1940) discovered a different type of protein in the blood of Rhesus monkey. They called it Rh antigen or Rh factor after Rhesus monkey. When injected the blood of these monkeys into the blood of guinea pigs they noticed the formation of antibodies against the Rh antigen in the blood of guinea pigs.
Formation of Rh antigen is controlled by dominant gene (R) and its absence by recipient gene (r). People having this antigen with genotype (RR or Rr) are called Rh positive (Rh+) and those whose blood is devoid of it with genotype (rr) are Rh negative (Rh–). About 85% human beings in Europe and 97% in India are Rh+.
Importance of Rh factor : Generally human blood is devoid of Rh antibodies. But it has been noticed that on transfusion of blood of a Rh+ person to Rh– person, the recepient develops Rh antibodies in its blood plasma. If Rh+ blood is transfused for the second times it causes agglutination and leads to the death of Rh– person.
Erythroblastosis foetalis : This disease is related to the birth of a child related with Rh factor. It causes the death of the foetus within the womb or just after birth. It was studies by Levine together with Landsteiner and Wiener.
The father of Rh affected foetus is Rh+ and the mother is Rh–. The child inherits the Rh+ trait from the father. A few Rh+ red blood corpuscles of foetus in the womb enter in the blood of the mother where they develop Rh antibodies. As mother’s blood is Rh–i.e. devoid of Rh antigen, it causes no harm to her. These Rh antibodies alongwith the mother’s blood on reaching the foetal circulation cause clamping of foetal RBCs or agglutination reaction. The first child is some how born normal because by that time the number of antibodies in mother’s blood remain lesser but they increase with successive pregnancies.
Thus the foetus following the first child dies either within the womb or just after its birth. This condition is known as erythroblastosis foetalis. So a marriage between Rh+ boy and Rh– girl is considered biologically incompatible.
Table : 4-17 Type of biological marriage on the basis of Rh factor
Boy
Girl
Type of biological marriage
Rh+
Rh+
Compatible marriage
Rh–
Rh–
Compatible marriage
Rh–
Rh+
Compatible marriage
Rh+
Rh–
Incompatible marriage
However, there is no danger if both parents are Rh– or mother is Rh+ and father is Rh–. Rh factor serum has been developed which when given to the Rh– mother after each child birth saves the next child. This serum contains Rh antibodies which destroy the Rh antigens of foetus before they can initiate formation of Rh antibodies in the mother.
Rhogam method : It is a method of preventing erythroblastosis foetalis. In this method the Rh– mother is given a special blood test after delivery of her Rh+ child. If foetal Rh+ cells are present in mother’s blood. She is given injections of rhogam. Rhogam is a preparation of anti-Rh antibodies. It is obtained from immunized donors. The rhogam forms a coat around foetal RBCs in mother’s blood. As a result no Rh+ antigens are available to stimulate mother’s circulation and no antibodies are formed.
Inheritance of Rh factor : Rh factor or Rh antigen is determined by a series of four pair of multiple alleles. They are denoted as R1, R2, R0, Rz, r’, r”, ry and r. The alleles denoted by capital letter give rise to Rh+ condition while those denoted by small letter to Rh– condition. Rh+ condition is dominant over Rh– condition. Thus Rh+ person may be homozygous (RR) or heterozygous (Rr) while Rh– persons are always homozygous(rr). Hereditary trait for Rh– factor is inherited according to Mendelian principle.
Genetic Mutation
The idea of mutation first originated from the observations of a Dutch botanist Hugo de Vries (1880) on variations in plants of Oenothera lamarckiana. The mutation can be defined as sudden, stable discontinuous and inheritable variations which appear in organism due to permanent change in their genotype. Mutation is mainly of two types :
(1) Spontaneous mutations : Mutation have been occurring in nature without a known cause is called spontaneous mutation.
(2) Induced mutation : When numerous physical and chemical agents are used to increase the frequency of mutations, they are called induced mutations.
Gene mutations
Gene or point mutations are stable changes in genes i.e. DNA chain. Many times a change in a gene or nucleotide pair does not produce detectable mutation. Thus the point or gene mutation mean the process by which new alleles of a gene are produced. The gene mutation are of following types :
Tautomerism : The changed pairing qualities of the bases (pairing of purine with purine and pyrimidine with pyrimidine) are due to phenomenon called tautomerism.
Tautomeres are the alternate forms of bases and are produced by rearrangements of electrons and protons in the molecules.
Substitutions (Replacements) : These are gene mutations where one or more nitrogenous base pair are changed with others. It may be further of three sub types :
(1) Transition : In transition, a purine (adenine or guanine) or a pyrimidine (cytosine or thymine or uracil) in triplet code of DNA or mRNA is replaced by its type i.e. a purine replaces purine and pyrimidine replaces pyrimidine.
GC ® AT or AT ® GC
(2) Transversion : Transversion are substitution gene mutation in which a purine (adenine or guanine) is replaced by pyrimidine (thymine or cytosine) or vice versa.
GC ® CG or TA , AT ®TA or CG
(3) Frame shift mutations : In this type of mutations addition or deletion of single nitrogenous base takes place. None of the codon remains in the same original position and the reading of genetic code is shifted laterally either in the forward or backward direction.
Chromosomal mutation or aberrations
A gene mutation normally alters the information conveyed by a gene, it alters the message. On the other hand, chromosomal mutation only alters the number or position of existing genes. They may involve a modification in the morphology of chromosome or a change in number of chromosomes.
(1) Morphological aberrations of chromosomes
Deletion or deficiency : Sometimes a segment of chromosome break off and get lost. If a terminal segment of a chromosome is lost, it is called deficiency. Deficiency generally proves lethal or semilethal. If intercalary segment is lost it is termed deletion.
Deletion occurs during pairing in meiosis. For example in human babies deletion of a segment of chromosome number 5 causes a disease called cri-du-chat syndrome (the baby cries like a cat and is mentally retarted with small head).
Wolf-Hirschhorn’s syndrome is another well characterized deletion syndrome in human beings caused by a deletion of short arm of chromosome 4 (4p-). The phenotypic effect includes wide-spaced eyes and cleft lip.
Duplication : In this mutation deleted chromosomal segment is attached to its normal homologous chromosome. Here a gene or many genes are repeated twice or more times in the same chromosome.
Inversion : A piece of chromosome is removed and rejoined in reverse order. For example a chromosome with the gene order A, B, C, D, E, F, G, H is broken between B,C,D and the centre portion turned through 180°, the resulting gene order is A, D, C, B, E, F, G, H.
Translocation : Mutual exchange (reciprocal) of the chromosome segments between non homologous chromosome. An exchange of parts between two non homologous chromosomes is called reciprocal translocation. In simple translocation a segment of one chromosome breaks and is transferred to another non-homologous chromosome.
(2) Numerical aberrations of chromosomes
Euploidy : The somatic chromosome number in euploids is the exact multiple of basic haploid number. In euploidy an organism acquires an additional set of chromosomes over and above the diploid complement. It can be divided into following types :
(i) Monoploidy or haploidy : Monoploids possess only one set or single basic set of chromosomes. Haploids on the other hand have half the somatic chromosome number. In diploid organisms monoploids and haploids are identical while in a tetra-or hexaploid with 4n or 6n chromosomes the haploids will possess 2n or 3n chromosome whereas its monoploid will possess only one set (n) of chromosome.
(ii) Polyploidy : Organism with more than two sets of chromosomes are known as polyploids. It may be triploid with three sets of chromosomes (3n) or tetraploid with four sets of chromosome (4n) and so on. Polyploidy is of three types :
(a) Autopolyploidy : It is a type of polypoidy in which there is a numerical increase of the same genome, e.g., Autotriploid (AAA), autotetraploid (AAAA). e.g., Maize, Rice, Gram. Autopolyploidy induces gigas effect.
(b) Allopolyploidy : It has developed through hybridisation between two species followed by doubling of chromosomes (e.g., AABB). Allopolyploids function as new species. e.g., Wheat, American cotton, Nicotiana tobacum. Two recently produced allopolyploids are Raphanobrassica and Triticale.
(c) Autoallopolyploidy : It is a type of allopolyploidy in which one genome is in more than diploid state. commonly autoallopolyploids are hexaploids (AAAABB), e.g., Helianthus tuberoseus.
Aneuploidy : Aneuploidy is the term applied for the chromosomal mutations involving only a part of a set, i.e., loss (hypoploidy) or addition (hyperploidy) of one or more chromosomes. Aneuploidy may result from non disjunction of chromosome during cell division.
(i) Monosomy : Diploid organism that are missing one chromosome of a single pair with genomic formula 2n – 1. Monosomics can form two kind of gametes, (n) and (n –1). e.g., Turner’s syndrome (44+x).
(ii) Nullisomy : An organism that has lost a chromsome pair is nullisomic. The result is usually lethal to diploids (2n – 2).
(iii) Trisomy : Diploids which have extra chromosome represented by the chromosomal formula 2n + 1. One of the pairs of chromosomes has an extra member, so that a trivalent may be formed during meiotic prophase. e.g., Down’s syndrome (45 +xx or 45 + xy), klinefelter’s syndrom (44 + xxy). All the possible trisomic have been studied in Datura.
(iv) Tetrasomy : In tetrasomic individual particular chromosome of the haploid set is represented four times in a diploid chromosomal complement. The general chromosomal formula for tetrasomics is 2n + 2 rather than 2n + 1+ 1. The formula 2n + 1 + 1 represents a double trisomic. e.g.,Super female (44 + xxxx).
Mutagens
Any substance or agent inducing mutation is called a mutagen. The mutagens may be broadly grouped into two classes :
(1) Physical mutagens : It comprise mainly radiations. Radiation has been used to induce mutations for the first time by H.J. Muller (1927) on animals and L.J. Stadler (1928) on plants. Radiation that can produce mutation is known as effective radiations which are as follows.
(i) Ionizing (Particulate) : a-particles, b-rays, protons and neutrons.
(ii) Ionizing (non particulate) : X-rays, r-rays and cosmic rays.
(iii) Nonionizing : Ultraviolet rays
(2) Chemical mutagen : A large number of chemicals react with the four nucleotides and modify their base-pairing capabilities. These are as follows :
(i) Base analogues : 5-bromodeoxyuridine (Brdu), 2-amino purine.
(iii) Intercalating agents : Proflavin and acridine orange
Genetic diseases in man
There are many diseases in man due to gene mutations. It is either dominant or recessive. The mutated person may become incapable to produce specified enzyme, so result in inborn errors of metabolism.
Chondrodystrophic dwarfism : Chondrodystrophic dwarfism is a dominant autosomal mutation, most people are homozygous for recessive allele (c/c). The presence of one dominant C results in the premature closure of the growth areas of long bones of arms and legs, resulting in shortened and bowed arms and legs.
Huntington disease : Huntington disease is caused by a dominant gene on chromosome 4. The mutated gene causes abnormality by producing a substance that interferes with normal metabolism in the brain that leads to progressive degeneration of brain cells. The death comes ten to fifteen years after the onset of symptoms.
Neurofibromatosis : Also called “von Recklinghausen disease” caused by a dominant gene on chromosome 17. The affected individual may have ten spots on the skin which later may increase in size and number. Small benign tumours called neurofibromas may occur under the skin or in various organs.
Tay-Sachs disease : Tay-Sachs disease results from the lack of the dominant gene on chromosome 15 for the production of hexosaminidase and subsequent storage of its substrate, a fatty substance known as glycosphingolipid, in lysosomes. The patient suffers from defective vision, muscular weakness and gradual loss of all mental and physical control, death occurs by the age of three or four years.
Cystic fibrosis : The most common lethal genetic disease due to a recessive mutation on the chromosome 7. The body produces abnormal glycoprotein which interferes with salt metabolism. he mucus secreted by body becomes abnormally viscid and blocks passages in the lungs, liver and pancreas.
Alzheimer’s disease : Alzheimer’s disease, named after the German neurologist Alzheimer, is a degenerative brain disease characterized by memory loss, confusion, restlessness, speech disturbances, erosion of personality, judgement, and inability to perform the functions of daily living. Alzheimer’s disease, a form of dementia, occurs in karyotypically normal individuals. The brain of Alzheimer’s patients show a marked loss of neurons. These patients also show an accumulation of senile plaques, which are thickened nerve cell processes (axons and dendrites) surrounding a deposit of particular type of polypeptide called amyloid b protein. The occurrence of Alzheimer’s disease in people with Down’s syndrome suggests that a gene or genes on chromosome 21 is involved. According to Bush (2003) Alzheimer’s disease is caused by a copper and zinc build up in the brain.
Marfan’s syndrome : Marfan’s syndrome is due to dominant mutation resulting in the production of abnormal form of connective tissues and characteristic extreme looseness of joints. The long bones of body grow longer, fingers are very long called ‘spider fingers’ or arachnodactyly. The lenses in eyes become displaced.
Albinism : Albinism is an autosomal recessive mutation. An albino cannot synthesize melanin which provides black colouration to skin and hair. Albinism is due to tyrosinase deficiency. The enzyme tyrosinase normally converts the amino acid tyrosine to melanin through an intermediate product DOPA (dihydro phenyl alanine).
Sickle-cell disease : Sickle-cell disease is a genetic disease reported from negroes due to a molecular mutation of gene HbA on chromosome 11 which produces the b chain of adult haemoglobin. The mutated gene HbS produces sickle-cell haemoglobin. The sixth amino acid in b chain of normal haemoglobin is glutamic acid. In sickle-cell haemoglobin this amino acid is replaced by valine. The children homozygous (HbSHbS) produce rigid chains. When oxygen level of the blood drops below certain level, RBCs undergo sickling. Such cells do not transport oxygen efficiently; they are removed by spleen causing severe anaemia. Individuals with the HbAHbA genotype are normal, those with the HbSHbS genotype have sickle-cell disease, and those with the HbAHbS genotypes have the sickle-cell trait. Two individuals with sickle-cell trait can produce children with all three phenotypes. Individuals of sickle-cell trait are immune to malaria.
Thalassemia : Thalassemia is a human anaemia due to an autosomal mutant gene and when this gene is present in double dose, the disease is severe thalassemia major with death occurring in childhood. Heterozygous persons show a milder disease, thalassemia minor or also called Cooley’s anaemia. The persons suffering from thalassemia major are unable to produce b chain. Their haemoglobin contains d chains like that of foetus which is unable to carry out normal oxygen transporting function.
Alkaptonuria : Alkaptonuria was the first of the recessive human trait discovered in 1902 by Archibald Garrod, ‘father of physiological genetics’ or ‘father of biochemical genetics’. Patients of alkaptonuria excrete large amounts of homogentistic acid in urine. Such urine turns black upon exposure to light. In normal person, homogentistic acid (alkapton) is oxidized by a liver enzyme homogentistic acid oxidase to maleyl acetoacetic acid.
Phenylketonuria (PKU) : Phenylketonuria was discovered by the Norwegian physician A. Folling in 1934; an autosomal recessive mutation of gene on chromosome 12. PKU results when there is a deficiency of liver enzyme phenylalanine hydroxylase that converts phenylalanine into tyrosine. There is a high level phenylalanine in their blood and tissue fluids. Increased phenylalanine in the blood interferes with brain development; muscles and cartilages of the legs may be defective and the patients cannot walk properly.
Gaucher’s disease : Gaucher’s disease is a genetic disease associated with abnormal fat metabolism, caused by the absence of the enzyme glucocerebrosidase required for proper processing of lipids. Non processing of lipids results in accumulation of fatty material in spleen, liver, bone marrow and brain. The swelling of these organs occurs and patients usually die by the age of 15 years.
Galactosemia : Galactosemia is inherited as an autosomal recessive, and the affected person is unable to convert galactose to glucose. Galactosemia is due to the deficiency of the enzyme Galactose Phosphate uridyl Transferase (GPT). Milk is toxic to galactosemic infants; child usually dies at three years of age.
Taste blindness of PTC : Taste blindness of PTC is a genetic trait, not a disease, discovered by Fox in 1932. PTC (phenyl thiocarbamide) is a compound of nitrogen, carbon and sulphur with sour taste. About 30% people lack the ability to taste PTC which is transmitted by a dominant gene T. The genotypes TT and Tt are tasters of PTC, while tt are non-tasters or taste blind persons.
Chronic Myelogenous Leukaemia (CML) : Chronic myelogenous leukaemia in human beings is a fatal cancer involving uncontrolled replication of myeloblasts (stem cells of white blood cells). Ninety percent of CML is associated with an aberration of chromosome 22. This abnormal chromosome was originally discovered in the city of Philadelphia in 1959 and thus is called the ‘Philadelphia chromosome’. In the Philadelphia translocation, the tip of the long arm of chromosome 9 has been joined to the body of chromosome 22 and the distal portion of the long arm of chromosome 22 has been joined to the body of chromosome 9. CML is characterized by an excess of granular leucocytes in the blood. With the increase in the number of leucocytes, there is a reduction in the number of RBCs resulting in severe anaemia.
Burkitt’s Lymphoma : Burkitt’s lymphoma, a particularly common disease in Africa, is another example of a white blood cell cancer associated with reciprocal translocations.These translocations invariably involve chromosome 8 and one of the three chromosomes (2, 14 and 22) that carry genes encoding the polypeptides that form immunoglobulins or antibodies.Translocations involving chromosomes 8 and 14 are the most common.
Sex chromosome abnormalities
Turner’s syndrome : Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n – 1 = 45. They are phenotypic females but are sterile because they have under developed reproductive organs. They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain under developed. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner’s syndrome.
Klinefelter’s syndrome : Since 1942, this abnormality of sex is known to geneticists and physicians. It occurs due to Trisomy of sex chromosomes which results in (XXY) sex chromosomes. Total chromosomes in such persons are 2n + 1 = 47 in place of 46. Klinefelter (1942) found that testes in such male remain under developed in adulthood. They develop secondary sex characters of female like large breasts and loss of facial hair. Characters of male develop due to Y chromosome and those like female due to XX chromosomes. About one male child out of every 5,000 born, develops Klinefelter’s syndrome.
Such children are born as a result of fertilization of abnormal eggs (XX) by normal sperms with (X) or (Y) chromosomes or by fertilization of normal eggs with (X) chromosomes by abnormal sperms with (XY) chromosome. They are sterile males mentally retarded and are eunuchs.
Super females or metasuper females : Presence of extra (X) chromosomes in females shows such condition leading to (XXX, XXXX, XXXXX), having total 47, 48 or 49 chromosomes in each cell. Females with this type of aneuploidy show abnormal sexual development and mental retardation. Severeness of abnormality increases with the increase in number of (X) chromosomes.
Criminal’s or Jacob’s syndrome (super males) : Presence of an extra (Y) chromosome in males causes such a condition (XYY) resulting in individuals with 2n + 1 = 47 chromosomes. They have unusual height, mentally retarded and criminal bent of mind since birth. Their genital organs are under developed. Their frequency is one in every 300 males.
Autosomal abnormalities
Down’s syndrome : This autosomal abnormality is also known as Mongolian idiocy or mongolism. In Langdon Down of England (1866) studied the Mongolian idiocy and described the trisomic condition of their chromosomes. Down’s syndrome, a very common congenital abnormality arises due to the failure of separation of 21st pair of autosomes during meiosis. Thus an egg is produced with 24 chromosomes instead of 23. A Down’s syndrome has 3 autosomes in 21st pair instead of 2. Total number of chromosomes in this case is 2n + 1 (21st) = 47.
The affected children have a very broad fore head, short neck, flat palms without crease, stubby fingers, permanently open mouth, projecting lower jaw and a long thick extending tongue. They have low intelligence and are short heighted. They have defective heart and other organs. They are born to mothers aged 40 year and above during first pregnancy. They may survive upto 20 years under medical care.
They are called mongolian idiots because of their round, dull face and upper eyelids stretched downwards similar to mongolian race.
Edward’s syndrome : This autosomal abnormality occurs due to trisomy of eighteenth pair of autosomes in which the number of chromosomes are 2n + 1 = 47. The child with this defect survives only about 6 months. Such children have defective nervous system, malformed ears and a receding chin.
Patau’s syndrome : This is trisomy of thirteenth pair of autosomal chromosome. This trisomic condition involves numerous malformations such as harelip, clefted palate and cerebral, ocular and cardiovascular defects. Such children usually survive for about 3 months only.
Sex determination
Fixing the sex of an individual as it begins life is called sex determination. The various genetically controlled sex-determination mechanisms have been classified into following categories :
Chromosomal theory of sex determination
The X-chromosome was first observed by German biologist, Henking in 1891 during the spermatogenesis in male bug and was described as X-body. The chromosome theory of sex determination was worked out by E.B. Wilson and Stevens (1902-1905). They named the X and Y chromosomes as sex-chromosomes or allosomes and other chromosomes of the cell as autosomes.
Sex chromosomes carry genes for sex. X-chromosomes carries female determining genes and Y-chromosomes has male determining genes. The number of X and Y chromosomes determines the female or male sex of the individual, Autosomes carry genes for the somatic characters. These do not have any relation with the sex.
XX-XY type or Lygaeus type : This type of sex-determining mechanism was first studied in the milk weed bug, Lygaeus turcicus by Wilson and Stevens. Therefore, it is called Lygaeus type. it is most common in plants and animals. e.g., In all mammals including man and among plants in Melandrium album, M.rubrum, Elodea, Rumex angiocarpus, Populus, Salix, Smilax, Morus, Canabis etc. These are two different patterns of sex determination in Lygaeus type.
(1) Female homogametic XX and male heterogametic XY e.g., Drosophila.
(2) Female heterogametic and male homogametic e.g., Fowl, Birds and some fishes.
XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the loss of Y-chromosome. e.g., Grasshopper and plant kingdom in Dioscorea sinuta and Vallisneria spiralis.
Fig : 4-31 Protenor type of sex determination in Grasshopper
Haploid-diploid mechanism of sex determination
Hymenopterous insects, such as bees, wasps, saw flies, and ants, show a unique phenomenon in which an unfertilized egg develops into a male and a fertilized egg develops into a female. Therefore, the female is diploid (2N), and the male is haploid (N). eggs are formed by meiosis and sperms by mitosis. Fertilization restores the diploid number of chromosomes in the zygote which gives rise to the female. If the egg is not fertilized, it will still develop but into a male. Thus, the sex is determined by the number of chromosomes.
In honeybee, the quality of food determines whether a diploid larva will become a fertile queen or a sterile worker female. A larva fed on royal jelly, a secretion from the mouth of nursing workers, grows into a queen, whereas a larva fed on pollen and nectar grows into a worker bee. Thus, the environment determines fertility or sterility of the bee but it does not alter the genetically determined sex.
Table : 4-18 Different types of chromosomal mechanisms of sex-determination in animals
Organisms
Heterogametic sex
Gamete
Zygotes
Sperms
Eggs
F
M
Drosophila, man etc.
Male
X and Y
All X
XX
XY
Protenor(Bug Grasshopper)
Male
X and O
XX
XX
XO
Birds, moths
Female
All X
X and Y
XY
XX
Fumea (a moth)
Female
All X
X and O
X
XX
Quantitative or ratio theory of sex determination
C.B Bridges worked out ratio theory of sex determination in Drosophila. According to this theory the ratio of chromosomes to autosomes is the determining factor for the sex. Single dose of X-chromosome in a diploid organism produces male, whereas 2X-chromosomes produce a female. If a complete haploid set of autosomes is designated by A then 2A : X will give rise to male and 2A : 2X to female.
Intersexes in Drosophila and ratio theory of sex determination : Due to abnormal meiosis during oogenesis both the X-chromosomes fail to separate and move to one pole of meiotic spindle. Thus few eggs are formed with single autosomal genome but with 2X chromosomes, i.e. (AXX) and other with single autosomal genome but no sex chromosome (A). When such abnormal eggs are fertilized with normal sperm, the following result are obtained.
Results of fertilization of abnormal female gametes
AAXXY – Female
AAXXX – Super female
AAX – Sterile male
AAY – Nonviable
Triploid intersexes and balance theory : The triploid flies with (3A + 3X) are much like the normal diploid females both in appearance as well as in fertility. On mating to diploid males their progeny consisted of following types :
(1) AAAXXX – Triploid females
(2) AAXX – Dilpoid females
(3) AAXXY – Diploid females
(4) AAAXX – Intersexes
(5) AAAXXY – Intersexes
(6) AAXY – Normal males
(7) AAXXX – Super females
(8) AAAXY – Super males
The intersexes are sterile and intermediate between females and male, because the sex balance ratio in the intersexes comes to 2 : 3.
Gyandromorphs in Dorsophila and ratio theory of sex determination : In Drosophila occasionally flies are obtained in which a part of the body exhibits female characters and the other part exhibits male characters. Such flies are known as gynandromorphs. These are formed due to misdivision of chromosomes and start as female with 2A+2X-chromosomes. The occurrence of gynandromorphs clearly indicates that the number of X-chromosomes determines the sex of the individual. The term Gynandromorphism was indroduce by Goldschmidt in 1915.
Genic balance theory
According to the genic balance theory of Bridges in Drosophila melanogaster, sex is determined by the ratio of the X-chromosomes and the set of autosomes. The Y-chromosomes play no part in sex determination it only governs male fertility. The XO flies are male, but sterile. Sex is governed by the ratio of the number of X chromosomes to sets of autosomes. The table given below indicates how the ratio of X/A help to determine the sex.
Table : 4-19 Ratio of X-chromosome to autosomes and the corresponding phenotype in Drosophila
Sex
Number of X-chromosomes
Number of autosomal set
Sex index X/A ratio
Super female
XXX (3)
AA (2)
3/2 = 1.5
Normal female
Tetraploid
Triploid
Diploid
Haploid
XXXX (4)
XXX (3)
XX (2)
X (1)
AAAA (4)
AAA (3)
AA (2)
A (1)
4/4 = 1.0
3/3 = 1.0
2/2 = 1.0
1/1 = 1.0
Intersex
XX (2)
AAA (3)
2/3 = 0.66
Normal male
X (1)
AA (2)
1/2 = 0.50
Super male
X (1)
AAA (3)
1/3 = 0.33
Human sex determination : The genic balance theory of sex determination is not universally accepted. Unlike Drosophila X : A does not influence sex determination. The key to sex determination in humans is the SRY (for sex region on the Y) gene located on the short arm of the Y-chromosome. In the male, the testis-determining factor (TDF) is produced by SRY on the Y-chromosome. TDF induces the medulla of the embryonic gonads to develop into testes. In the absence of SRY on Y, no TDF is produced. The lack of TDF allows the cortex of the embryonic gonads to develop into ovaries.
Hormonal theory of sex determination
The sex determination theories of chromosomes and genic balance successfully apply to the lower animals but in higher vertebrates and under certain conditions in invertebrates, the embryo develops some characters of the opposite sex together with the characters of its own sex-chromosome. It means, the sex changes under specific circumstances. This is due to the hormones secreted by the gonads of that animal.
Free martinism : The influence of hormones on sex determination comes from free-martins often found in cattles. LILLIE and others found that where twins of opposite sex (one male and other female) are born, the male is normal but female is sterile with many male characteristics. Such sterile females are known as free martins.
The scientific explanation for the formation of free martins is the effect of hormones of the male sex on the female.
Environmental theory of sex determination
In some animals, there is environmental determination of sex.
In Bonellia, a marine worm, the swimming larva has no sex. If it settles down alone, it develops into a large (2.5 cm) female. If it lands on or near an existing female proboscis, a chemical secreted from her proboscis causes the larva to develop into a tiny (1.3 mm) male. Male lives as a parasite in the uterus of the female.
In turtles, a temperature below 28°C produces more males, above 33°C produces more females, and between 28°C to 33°C produces males and females in equal proportion, while in crocodile male sex is predominant at high temperature.
Barr body in sex determination
Murray Barr (1949), a geneticist noticed a small body in the nucleus of the nerve cells of female cats which stained heavily with nuclear stains. Further investigations showed that not only nerve cells, but many other cells from female cats only, had these bodies, now known as sex chromatin or Barr bodies. It was soon learnt that such bodies can be found in females of many mammals including human. In women the Barr body lies against the nuclear membrane like a round disc in the neutrophil blood cells, skin cells, nerve cells, cells of mucous membrane, cells of lining in vagina and urethra. They are absent in man. These bodies are thus named after the discover Barr.
Barr bodies are used to determine the sex of unborn human embryos. In this technique called amniocentesis sample of the amniotic fluid is examined for Barr bodies. The sex is determined by the presence or absence of Barr bodies in epithelial cells of embryo present in the amniotic fluid sample.
Mary Lyon hypothesis : According to the British geneticist Mary Lyon (1961), one of the two X-chromosomes of a normal female becomes heterochromatic and appears as Barr body. This inactivation of one of the two X-chromosomes of a normal female is the dosage compensation or Lyon’s hypothesis.
Table : 4-20
Individual
No. of X chromosome
No. of Barr body (X – 1)
Normal woman
XX
2–1 = 1 (one barr body)
Women with Turner’s syndrome
XO
1–1 = 0 (no barr body)
Super female
XXX
3–1 = 2 (two barr bodies)
Man
XY
1–1 = 0 (no barr body)
Man with Klinefelter’s syndrome
XXY
2–1 = 1 (one barr body)
Sex linked inheritance
Sex chromosomes of some animals and man besides having genes for sex character also possess gene for non sexual (somatic) characters. These genes for non sexual characters being linked with sex chromosomes are carried with them from one generation to the other. Such non-sexual (somatic) characters linked with sex chromosomes are called sex linked characters or traits, genes for such characters are called sex linked genes and the inheritance of such characters is called sex linked inheritance. The concept of sex-linked inheritance was introduced by T. H. Morgan in 1910, while working on Drosophila melanogaster.
Genes for sex linked characters occur in both segments of X and Y chromosomes. Many sex linked characters (About 120) are found in man. Such characters are mostly recessive.
Types of sex linked inheritance
(1) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. Alleles of these genes do not occur on Y chromosome. Genes of such characters are transferred from father to his daughter and from his daughter to her sons in F2 generation. This is known as Cris-cross inheritance. As the genes for most sex linked characters are located in X chromosome, they are called X-linked characters e.g., colour blindness and haemophilia in man and eye colour in Drosophila.
Sex linked inheritance in Drosophila : Drosophila melanogaster has XX and XY sex chromosomes in the female and male respectively. Its eye colour is sex linked.
Allele of the eye colour gene is located in the X chromosome, and there is no corresponding allele in the Y chromosome. The male expresses a sex-linked recessive trait even if it has a single gene for it, whereas the female expresses such a trait only if it has two genes for it. The normal eye colour is red and is dominant over the mutant white eye colour. The following crosses illustrate the inheritance of X-linked eye colour in Drosophila.
Sex linked inheritance in man : Colour blindness and Haemophila are the two main sex linked or X-linked disease are found in man.
Colour blindness : Person unable to distinguish certain colours are called colour blind. Several types of colour blindness are known but the most common one is ‘red-green colour blindness’. It has been described by Horner (1876).
The red blindness is called protanopia and the green blindness deutoranopia. X-chromosome possesses a normal gene which control the formation of colour sensitive cells in the retina. Its recessive allele fails to do its job properly and results in colour blindness. These alleles are present in X chromosome.
Table : 4-21 Inheritance of colourblindness
PARENTS
OFFSPRINGS
Female
Male
Daughters
Sons
Genotype
Phenotype
Genotype
Phenotype
Genotype
Phenotype
Genotype
Phenotype
XX
Normal
XcY
Colourblind
XXc
Carrier
XY
Normal
XXc
Carrier
XY
Normal
(i) XX
(ii) XXc
Normal
Carrier
XY
XcY
Normal Colourblind
XXc
Carrier
XcY
Colourblind
(i) XXc
(ii) XcXc
Carrier
Colourblind
XY
XcY
Normal
Colourblind
XcXc
Colourblind
XY
Normal
XcX
Carrier
XcY
Colourblind
Haemophilia : In haemophilia the blood fails to clot when exposed to air and even a small skin injury results in continuous bleeding and can lead to death from loss of blood.
It is also called bleeder’s disease, first studied by John Cotto in 1803. The most famous pedigree of haemophilia was discovered by Haldane in the royal families of Europe. The pedigree started from Queen Victoria in the last century. In a patient of haemophilia blood is deficient due to lack necessary substrate, thromboplastin. It is of two types :
Haemophilia-A : Characterized by lack of antihaemophilic globulin (Factor VIII). About four-fifths of the cases of haemophilia are of this type.
Haemophilia-B : ‘Christmas disease’ (after the family in which it was first described in detail) results from a defect in Plasma Thromboplastic Component (PTC or Factor IX).
Like colour blindness, haemophilia is a well known disorder which is sex-linked recessive condition. The recessive X-linked gene for haemophilia shows characteristic Criss-cross inheritance like the gene for colour blindness. Its single gene in man results in disease haemophilia, whereas a woman needs two such genes for the same.
(2) Holandric or Y-linked traits : Genes for these characters are located on non-homologous segment of Y chromosome. Alleles of these genes do not occur on X chromosome. Such characters are inherited straight from father to son or male to male e.g. hypertrichosis of ears in man.
(3) XY-linked inheritance : The genes which occur in homologous sections of X and Y-chromosomes are called XY-linked genes and they have inheritance like the autosomal genes. e.g., Xeroderesia pigmentosa, Nephritis.
(4) Sex-influenced traits : The autosomal traits in which the dominant expression depends on the sex hormones of the individual are called sex-influenced traits. These traits differ from the sex limited traits which are expressed in only one sex. e.g., Baldness in man, Length of index finger.
(5) Sex limited traits : Traits or characters which develop only in one sex are called sex-limited characters. They are produced and controlled by the genes which may be located on autosomes in only one sex. Such genes are responsible for secondary sexual characters as well as primary sexual characters. They are inherited according to Mendel’s laws. e.g., Moustaches and beards in human males, breast in human females, milk secretion in human females.
Pedigree analysis
A pedigree is a systematic listing (either as words or symbols) of the ancestors of a given individual or it may be the “family tree” for a number of individuals.
Pedigree analysis is carried out in order to word off possible diaster due to picking up of harmful genetic defects like dominant polydactyly (extra digits), syndactyly (joined digits) and brachydactyly (short digits), recessive haemophilia, deaf mutism, birth blindness, colour blindness, thalassemia, alkaptonuria, phenylketonuria, sickle cell anaemia attached ear lobes, tongue rolling etc.
Pedigree chart and symbols : It is customary to represent men by squares and women by circles in a chart for study of pedigree analysis. Marriage is indicated by a connecting horizontal line and the children by attachment to a vertical line extending downward from the horizontal line. Individuals having particular characters to be studied are denoted by solid squares or circles while those not having them are indicated by outlines only. Twins are denoted by bifurcating vertical lines.
In such a pedigree analysis a person who is the beginner of the family history is called proband. It is called propositus, if male and poposita, if female. The children of such parents are known as sibs or siblings. So a family is constituted by such parents and their siblings. Sometimes, a very large family is formed as a result of interconnected marriages. Such a circle of large persons interconnected is called Kindred.
Twins and I.Q.
Twins : Two birth occurring at the same time in human are called twins, they are of peculiar genetic interest. The hereditary basis of a number of human traits has been established by the study of twins. There are 3 kinds of twins.
(1) Identical or monozygotic twins : Identical twins are formed when one sperm fertilizes one egg to form a single zygote. They have the same genotype and phenotype and are of same sex. Differences if any, may be due to different environmental conditions.
(2) Siamese twins or conjoint twins : Like monozygotic twins, siamese twins also originate from one zygote but the daughter cells formed as a result of first cleavage fail to separate completely and they remain joined at some point. They were first studied in the country Siam, hence called Siamese twins. Siamese twins usually do not survive after birth although a few cases of their survival are well known. They are always of the same sex, same genotype and phenotype.
(3) Fraternal twins : They are dizygotic twins formed from the two eggs fertilized by two sperms separately but at the same time. They may be both males, both females or one male and one female. They may have different genotypic constitution and different phenotype.
Intelligence quotient (IQ) : The ratio between actual (chronological) age and mental age multiplied with 100 is known as I.Q. Intelligence quotient is the mental competence in relation to chronological age in man. It can be denoted by following formula.
I.Q.
By applying this formula we can easily calculate the IQ, such as if a 10 year child has mental age 14, his IQ will be
I.Q.
Table : 4-22
I.Q.
Person
I.Q.
Person
0 – 24
Idiot
90 – 109
Average
25 – 49
Imbecile
110 – 119
Superior
50 – 69
Moron
120 – 139
Most superior
70 – 79
Dull
140 or more
Genius
80 – 89
Ordinary
Eugenics, Euthenics and Euphenics
Eugenics
The term eugenics (Gr. Eugenes, well born) was coined by British scientist Sir Francis Galton in 1883. Galton is called ‘Father of eugenics’ as this branch has been started by him.
Eugenics is the branch of science which deals with improvement of human race genetically. Eugenics can be divided into two types :
(1) Positive eugenics : In this approach of eugenics the future generations are improved by encouraging the inheritance of better traits.
(2) Negative eugenics : This is a negative aspect of improving mankind by restricting the transmission of poor and defective germplasm.
Euthenics
Euthenics is the improvement of human race by improving the environmental conditions, i.e., by subjecting them to better nutrition, better unpolluted ecological conditions, better education and sufficient amount of medical facilities.
Euphenics
The study of born defectives and their treatment is called euphenics. The term euphenics was given by A.C. Pai (1974) for symptomatic treatment of human genetic disease especially in born errors of metabolism.
Genetic engineering
Recombinant DNA technology
Genetic engineering, a kind of biotechnology, is the latest branch in applied genetics dealing the alteration of the genetic make up of cells by deliberate and artificial means. Genetic engineering involves transfer or replacement of genes, so also known as recombination DNA technology or gene splicing.
Tools of genetic engineering : Two enzymes used in genetic engineering are restriction endonuclease and ligases. R.E. is used to cut the plasmid as well as the foreign DNA molecules of specific points while ligase is used to seal gaps or to join bits of DNA.
Steps of recombinant DNA technology
(1) Isolating a useful DNA segment from the donor organism.
(2) Splicing it into a suitable vector under conditions to ensure that each vector receives no more than one DNA fragment.
(3) Producing of multiple copies of his recombinant DNA.
(4) Inserting this altered DNA into a recipient organism.
(5) Screening of the transformed cells.
Vectors : Vector in genetic engineering is usually a DNA segment used as a carrier for transferring selected DNA into living cells. Which are as follows :
(1) Plasmid : Plasmid are extrachromosomal, closed circular double stranded molecules of DNA present in most eukaryotes. All plasmid carry replicons pieces of DNA that have the genetic information required to replicate. Plasmid pBR 322 was one of the first widely used cloning vectors, it contain both ampicillin and tetracycline resistance genes.
(2) Phage : It is constructed from the phage l chromosomes and acts as bacteriophage cloning vectors.
(3) Cosmid : The hybrids between plasmid and the phage l chromosome give rise to cosmid vectors.
Beside all these there are artificial chromosomes like
BACs (Bacterial Artificial chromosomes)
YACs (Yeast Artificial chromosomes)
MACs (Mammalian Artificial chromosomes) are very efficient vectors for eukaryotic gene transfers.
Natural genetic engineer : When as gene transfer occurs without human effort, the bacterium is known as “natural genetic engineer” of plants. e.g., A soil inhabiting, plant pathogenic bacterium, Agrobacterium tumefaciens.
Application of recombinant DNA technology : The technique of recombinant DNA can be employed in the following ways :
(1) It can be used to elucidate molecular events in the biological process such as cellular differentiation and ageing. The same can be used for making gene maps with precision.
(2) In biochemical and pharmaceutical industry, by engineering genes, useful chemical compounds can be produced cheaply and efficiently which is shown in table.
(3) Production of transgenic plants.
(4) Production of genetically modified microorganisms.
Table : 4-23 Applications of recombinant DNA products
Medically useful recombinant products
Applications
Human insulin
Treatment of insulin-dependent diabetes
Human growth hormone
Replacement of missing hormone in short stature people
Calcitonin
Treatment of rickets
Chronic gonadotropin
Treatment of infertility
Blood clotting factor VIII/IX
Replacement of clotting factor missing in patients with Haemophilia A/B
Tissue plasminogen activator
Dissolving blood clots after heart attacks and strokes
Erythropoitin
Stimulation of the formation of erythrocytes (RBCs) for patients suffering from anaemia during kidney dialysis or side effects of AIDS patients treated by drugs
Platelet derived growth factor
Stimulation of wound healing
Interferon
Treatment of pathogenic viral infections, cancer
Interleukins
Enhancement of action of immune system
Vaccines
Prevention of infectious diseases such as hepatitis B, herpes, influenza, pertussis, meningitis, etc.
Genetic engineering in plants
The main steps in plant genetic engineering are given below :
(1) Agronomically important gene is identified and isolated.
(2) Plasmid is isolated from the bacterium, Agrobacterium tumefaciens.
(3) Plant DNA containing the gene of interest in integrated into the T DNA of the plasmid by using restriction endonuclease and ligase enzymes.
(4) Recombinant plasmid is introduced into the cultured plant cells.
(5) T DNA integrates into the plant cells chromosomes DNA.
(6) As the plant cells divide, each daughter cell receives a copy of T DNA and the gene of interest it carries.
(7) The cells give rise to a plantlet, which, when transferred into soil, grows into a new plant that may express the new gene.
Cloning
Cloning is the process of producing many identical organisms or clones. In this process nucleus of ovum (n) is removed and replaced by nucleus of diploid cell of same organism. Now the egg with 2n nucleus is transferred to the uterus of mother to have normal pregnancy and delivers clone of itself.
Examples of organism cloning
(1) Cloning of sheep was done by Dr. Ian Wilmut (1995) of Roslin Institute, Edinberg U.K. and normal healthy lamb (DOLLY) was born in Feb, 1996. This lamb was exactly similar to her mother.
(2) The first cloned calves George and Charlie were born in January 1998.
(3) ANDI was the world’s first genetically altered primate produced by inserting a jelly fish gene into the embryo of a rhesus monkey.
(4) Scientist at Scotland cloned POLLY and MOLLY. Unlike Dolly, polly and molly were transgenic (they carried human protein gene) polly and molly were born in july 1997.
(5) Brigitte Boissliar, a 46-year old French chemist announced the creation of the world’s first cloned human baby nicknamed “Eve” (December 2002).
Few examples of applications of plant cloning in genetic engineering are given below where desired DNA has been introduced in plant genome for various purposes :
Table : 4-24
Applications
Examples
Herbicide resistant plants
Petunia, tobacco, tomato and corn
Insect resistant plants
Cotton, tobacco and mustard
Virus resistant plants
Tomato, potato, alfaalfa, cucumber, rice and papaya
Plants which improved storage proteins
French been and potato
Plants with improved oil and fats
Rapeseed (rich in oleic acids and sterates) and soyabean (rich in cocoa oil)
Stress tolerant plants
Tobacco
Gene libraries
A gene library is a collection of gene clones that contains all the DNA present in some source. If the original source of the DNA was original DNA from a living organism, then the library seek to include clones of all that DNA, it is called a genomic gene library. Gene libraries can also be created by using RNA.
cDNA
If a gene library is created by enzymatic copying of RNA by reverse transcriptase (RNA-dependent DNA polymerase), it would be called c-DNA library. c-DNA stands for complimentary DNA or copy DNA. c-DNA is made to use PCR to amplify an RNA. PCR does not work on RNA, so one can copy it to DNA using reverse transcriptase and then PCR amplify the c-DNA; this is called RT-PCR (reverse transcriptase PCR).
Gene bank
A gene bank is repository of clones of known DNA fragments, genes, gene maps, seeds, spores, frozen sperms or eggs or embryos. These are stored for possible use in genetic engineering and breeding experiment where species have become extinct.
DNA finger printing
Alec Jeffreys et al (1985) developed the procedure of genetic analysis and forensic medicine, called DNA finger printing. It is individual specific DNA identification which is made possible by the finding that no two people are likely to have the same number of copies of repetitive DNA sequences of the regions. It is also known as DNA profiling. The chromosomes of every human cell contain scattered through their DNA short, highly repeated 15 nucleotide segments called “mini-satellites” or variable-number Tandem Repeat (VNTR).
Applications of DNA fingerprinting
This technique is now used to :
(1) Identify criminals in forensic laboratories.
(2) Settle paternity disputes.
(3) Verify whether a hopeful immigrant is, as he or she claims, really a close relative of already an established resident.
(4) Identify racial groups to rewrite biological evolution.
Gene therapy
The use of bioengineered cells or other biotechnology techniques to treat human genetic disorders is known as gene therapy. Gene therapy is the transfer of normal genes into body cells to correct a genetic defect. It can be used to treat genetic diseases like sickle-cell anaemia and Severe Combined Immuno Deficiency (SCID). It (SCID) is caused by a defect in the gene for the enzyme adenosine deaminase (ADA). SCID patients have no functioning T lymphocytes and one treated with the injections of their white blood cells that have been engineered to carry the normal ADA alleles.
Transgenics
A gene that has been introduced into a cell or organism is called a transgene (for transferred gene) to distinguish it from endogenous genes. The animal carrying the introduced foreign gene is said to be transgenic animal and the possessor called Genetically Modified Organisms (GMOs). Most of the transgenic animals studied to date were produced by microinjection of DNA into fertilized eggs. Prior to microinjection, the eggs are surgically removed from female parent and fertilized in vitro then DNA is microinjected into the male pronucleus of the fertilized egg through a very fine-tipped glass needle. The integration of injected DNA molecules appears to occur at random sites in the genome.
The first transgenic animal produced was the ‘supermouse’ by the incorporation of the gene for human growth hormone by Richard Palmiter and Ralph Brinster in 1981.
Table : 4-25 Some important example of transgenic animals
Transgenic animals
Useful application
Cow, Sheep, goat
Therapeutic human proteins in their milk
Pig
Organ transplantation without risk of rejection
Fish (Common Carp, Catfish, Salmon, gold fish)
They contain human growth hormone (hGH). They attain a size twice of that shown by nontransgenic fish.
Mouse
Contains a human gene that cause breast cancer. This enables the researchers to study the very early development of cancer.
Table : 4-26 Some important example of transgenic plants
Transgenic plants
Useful application
Bt Cotton
Pest resistance, herbicide tolerance and high yield. It is resistant to boll worm infestation.
Flavr Savr Tomato
Increased shelf-life (delayed ripening) and better nutrient quality.
Golden rice
Vitamin A-rich
Potato
Higher protein content
Corn, Brinjal
Insect resistance
Soyabean, Maize
Herbicide resistance
Genomics and human genome project
The term genome has been introduced by Winkler in 1920 and the genomics is relatively new, coined by Thomas Rodericks in 1986. Genomics is the subdiscipline of genetics devoted to the mapping, sequencing and functional analysis of genomes.
Two important scientist associated with human genome are Francis Collins, director of the Human Genome Project and J. Craig Venter, founding president of Celera genomics. The complete sequencing of the first human chromosome, small chromosome 22, was published in December 1999.
Table : 4-27 Genome of Model organisms
Organism
No. of base pair
No. of genes
Bacteriophage
10 thousand
–
E. coli
4.7 million
4000
Saccharomyces cerevisiae
12 million
6000
Caenorhabditis elegans
97 million
18,000
Drosophila melanogaster
180 million
13,000
Human
3 billion
30,000
Lily
106 billion
–
Prospects and implications of human genome
(1) The genome project is being compared to the discovery of antibiotics.
(2) Efforts are in progress to determine genes that will revert cancerous cells to normal.
(3) The human genome sequencing not only holds promise for a healthier living. It also holds the prospects of vast database of knowledge about designer drugs, genetically modified diets and finally our genetic identity.
? In thalassemia, the b chain of haemoglobin is changed due to frame shift mutation as a result, bone marrow is not formed.
? Bateson coined the term Genetics, allele, F1, F2, homozygous heterozygous and epistasis. He is also known as father of animal genetics.
? Johannsen coined the term genotype, phenotype, pure line.
? Mendel also observed that flower colour and colour of the seed coat may not assort independently.
? The genes for seed form in pea was present on chromosomes no. 7.
? Independent assortment is shown by the alleles present on different loci.
? Nilsson-Ehle (1908) was the first scientist to prove quantitative inheritance.
? Gene flow is spread of genes from one breeding population to another by migration.
? The genes, which enhance the effect of other gene, is also known as extender.
? Single copy genes : Represented only once in the whole genome.
? Multigenes : A group of nearly similar genes.
? Sutton and Winiweter (1900) expressed that number of chromosome is reduced to half in meiosis and doubled in fertilization.
? Sometimes two satellites are present in a chromosome these chromosome are called tandem SAT-Chromosomes.
? SAT Chromosomes are used as marker chromosomes.
? Genes modify the effect of other gene called modifiers.
? Separation of a chromosome segment and its union to non-homologous chromosomes is called illegitimate crossing over.
? Study of phenotype to DNA sequence in gene come under forward genetics.
? tt × tt ® Tt, This type of inheritance is an example of de-novo mutation.
? M.H.F. Wilkins and his associates supported DNA double helical structure using x-ray crystallography technique.
? Fisher discovered purine and pyamidine bases in DNA.
? Repetitive DNA or Satelite DNA It is found in eukaryotes only.
? Palindromic DNA are inverted repetitions of bases in double stranded DNA
;
? Nucleotide ATP is always found free in cell.
? RNA is single stranded but it is double stranded in reovirus and wound tumour plant and Rice dwarf Virus.
? In vitro synthesis of DNA, RNA and Gene were done by Korenberg, Ochoa and Khorana respectively.
? Ribozyme : RNA acts as an enzyme having catalytic activity, discovered by Altman and Cock.
? Circular flow of information®DNA®RNA®Protein®RNA ® DNA (commoner).
? Eukaryotic mRNA can be modified by the addition (at their 5¢ end) of methylated argenine.
? Actinomycin D prevents transcription.
? The transcription of genes increased by Glucocorticoid.
? When a particular gene codes for a m-RNA strand, it is said to be monocistronic or monogenic. When several genes (Cistrons) transcribe one m-RNA molecule it is called as polycistronic polygenic.
? Informososmes : In eukaryotes mRNA is associated with protein forming ribonucleoprotein complex. The name is given by Spirin and ratio of protein and mRNA is 4 : 1.
? UUU was first triplet codon discovered.
? Puromycin antibiotic inhibits translation.
? One gene one enzyme theory was given by G. W. Beadle and E. L. Tatum they worked on Neurospora crassa (pink bread mould). Which is replaced by one gene one-polypeptide theory was given by Yanofsky et al. (1965) utilizing bacterium E. coli.
? Two types of genes;
(1) Constitutive genes : It constantly express themselves e.g. enzymes of glycolysis, which are also known as house keeping gene, which lacks TATA boxes.
(2) Non constitutive genes : They express themselves only when needed, known as luxury genes Example– Inducible and Repressible genes.
? Morgan is called father of experimental genetics.
? Bateson is called father of modern genetics.
? Heteropyknosis : Darkly staining property of chromatin.
? H1, H2A and H2B proteins are lysine rich (H1 is very lysine rich) while H3 and H4 are arginine rich polypeptide chains.
? Satellite is also called trabant.
? The frequency of an allele in an isolated population is due to genetic drift.
? Pallindromic DNA is a segment of DNA in which the base pair sequence reads the same in both directions from a point of symmetry.
? Western blotting is the technique used to detect specific proteins.
? Northern blotting is the technique used to blot transfer of RNAs.
? Recombinant DNA is also called chimeric DNA.
? Polymerase Chain Reaction (PCR) was developed by Kary Mullis in 1983 and got Nobel prize for chemistry.
? Southern blotting technique is used for separating DNA fragments and identification of cloned genes.
? Gel electrophoresis and autoradiography are employed in nucleic and blotting.
? Delayed ripening is possible by reducing the amount of cell wall degrading enzyme ‘Polygalacturonase’ responsible for fruit softening.
? Duchenne Muscular Dystrophy (DMD) is the disease which is characterized by a progressive weakness and loss of muscle.
? Inheritance of beard in a man is sex-limited.
? Inheritance of A, B, AB and O blood types in man was discovered by Bernstein in 1925.
? Immunological incompatibility between mother and foetus sometimes results in a condition called haemolytic disease of the new born (HDN).
? HDN was earlier known as erythroblastosis foetalis.
(a) Aseptate and multinucleate (b) Septate and multinucleate (c) Aseptate and uninucleate (d) Septate and uninucleate Answer. (b) The mycelium of Aspergillus consist of branch septate hyphae. Cells of hyphae are multinucleate. (185)
(a) Parenchyma (b) Chlorenchyma (c) Plectenchyma (d) Collenchyma Answer. (c) Plectenchyma (Pseudoparenchyma) is nothing but the interwoven mass of fungal mycelium which in cross-section look like parenchyma. (108)
(a) Colletotrichum falcatum (b) Phytophthora infestans (c) Potato mosiac virus (d) Alternaria solani Answer. (b) Late blight of potato is a seed born disease which is caused by Phytophthora infestans. The disease is characterised by brownish to blackish dead areas …
