NEET Biology Notes Genetics Chromosomal Theory of Inheritance
Chromosomal Theory of Inheritance
It was proposed by Walter Sutton and Theodor Boveri in 1902.
Sutton and Boveri made a correlation between Mendel’s conclusions about genes (inherited traits) and the behaviour of chromosomes during mitosis and meiosis.
Chromosome theory of inheritance described that
- Chromosomes are in pairs and genes or their alleles are located on chromosomes.
- Homologous chromosomes separate during meiosis so that alleles are segregated.
- Meiotic products have one of each homologous chromosome but not both.
- Fertilisation restores the pairs of chromosomes.
Interaction of Genes
Mendel considered that each character is controlled by single gene, but we know today that a character may be controlled by two or more than two genes. In such a situation, two or more than two genes may interact to give rise to a particular phenotype.
Types of Gene Interaction
The gene interaction is of two types:
- intragenic
- intergenic
- In intragenic interaction, two alleles of a gene, which are present on same gene locus on the two homologous chromosomes react to produce modified phenotype, e.g. incomplete dominance, codominance, multiple allele and lethal genes, etc.
- Intergenic interaction is one where two or more independent genes belonging to same or different chromosomes interact to form different expression, e.g. epistasis, duplicate genes, complementary genes, supplimentary genes, inhibitory genes, etc.
If A and B are two genes, both dominant over their respective recessive alleles a and b, then the interaction will depend upon
- the presence of both dominant alleles A and B
- the absence of A
- the absence of B.
- the absence of both A and B
- Interaction of genes resulted into abbreviated phenotypic ratio, i.e. the phenotypic ratio other than expected 9 : 3 : 3 : 1.
Epistasis
When a gene at one locus ‘masks’ or inhibits the expression of a gene at a distinct locus, such an interaction is called epistasis. Epistatic interaction means that one gene masks or changes the effect of another gene.
In epistasis, out of two independent genes one expressing itself is called epistatic, while other is masked called hypostatic. Such interactions give modified F2 ratio as 12 : 3 : 1.
Complementary Genes
W Bateson and RC Punnett observed complementary gene interaction in sweet pea [Lathyrus odoratus). In such type of interaction, two separate pairs of genes interact to produce the phenotype in such ^ way that neither of the dominant genes is expressive unless the other one is present.
Complementary gene interaction is found in sweet pea (Lathyrus odoratus) and shows 9 : 7 ratio in F2 -generation.
Supplementary Genes
In supplementary gene interaction, two non-allelic gene pairs control the phenotypic expression of a trait, but the dominant gene of one pair expresses itself only when the dominant gene of the other pair is present, whereas the dominant gene of the other pair is expressed even in the absence of the dominant gene of the first pair.
Interaction of supplementary gene is found in flower colour of snapdragon (Antirrhinum majus), which shows 9:3:4 ratio in F2 -generation.
Duplicate Genes
When two pairs of genes are involved and both of them are able to produce the same effect singly as well as in combination, these are called duplicate genes.
GS Shull reported interaction of duplicate genes in Capsella bursa pastoris (Shepherd’s purse), which shows triangular and top-shaped capsule in 15 :1 ratio in F2 -generation. The possible genotype can give large number of phenotypes such event is known as continuous variation.
Linkage and Recombination
Linkage is the tendency of alleles of different genes to be passed together from one generation to next. If the chromosomes in a gamete producing cell are not rearranged (or lack recombination) during meiosis, it would be expected that all the genes on a particular chromosome would be transferred together into whichever gamete gets that particular chromosome. That is, the genes are linked together on their chromosomes. .
Linkage was discovered by W Bateson, ER Saunders and RC Punnett (1905), when they were working with sweet pea (Lathyrus odoratus). They found test cross ratio 7 : 1 : 1 : 7 instead of 1 : 1 : 1 : 1 and described it as coupling and repulsion.
Linkage is an exception to the law of independent assortment. It is inversely proportional to the distance between the genes on chromosomes. .
TH Morgan (1910) reported linkage in Drosophila and gave the term ‘linkage’ to this phenomenon. He also reported that coupling and repulsion are two aspects of the linkage.
A linkage group is a linearly arranged group of linked genes, i.e., all the linked genes of a chromosome form a linkage group. The number of linkage group corresponds to the haploid number of chromosomes.
Types of Linkage
- Complete linkage When only parental combinations are produced, linkage is called complete linkage. Crossing over remains absent in this case, e.g. male Drosophila, female silkworm (Bombyx mori).
- Incomplete linkage When some recombinations are also produced along with parental combinations, linkage is called incomplete linkage.
- Linkage Maps
The graphical representation of relative distances between linked genes of a chromosome is called linkage map or genetic map. In linkage map, one map unit is equal to 1% recombination. It is also called Centi Morgan after the name of Thomas H Morgan.
Total progeny
The first chromosome maps were prepared by Sturtevent in 1911.
Recombination involves the reshuffling of parental genes at their linkages so as to produce new genotypes. It develop due to crossing over, random fusion of gametes and independent assortment of chromosomes during meiosis.
Crossing Over
The term’ crossing over’ was introduced by Morgan and Cattell for separation of linked genes. The location of a gene on a chromosome is called a locus (PI. loci). The loci of the genes on a chromosome are arranged in a linear sequence.
It is a process occurring in the prophase-I of meiosis, in which exchange of non-sister chromatids called genetic recombination takes place. jfla r-
The frequency of Crossing Over Value is
calculated using formula
Significance of Crossing Over
(i) It provides proof for linear arrangement of genes on chromosomes
(ii) It is the basis for linkage maps or genetic maps construction.
(iii) Recombinations or new gene combinations are produced due to the crossing over, which is a source of variations.
(iii) In insects XX-XO system, In which females have 2X-chromosomes. Males have only one X and no additional sex chromosomes.
(iv) ZO-ZZ type This type of sex determination occurs in some butterflies and moths. It is exactly opposite to the XX-XO type of sex determination.
Here, the females have odd sex chromosome (AA + Z) while males have two homomorphic sex chromosomes (AA + ZZ) Compound chromosome system can be very complex with multiple numbers of X and Y-chromosomes, e.g. in Ascaris incurva, a nematode, there are 26 autosomes, 8 X-chromosomes and one Y-chromosome.Males have 26A + 8X + Y for 35 chromosomes. Females have 26A + 16X for 42 chromosomes. This type of system is also common in spiders
