In this article we will discuss about the chromosomal theory of inheritance.
The chromosomes as well as genes occur in pairs. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
According to this theory:
(i) All hereditary characters are carried with sperms and egg cells as they provide bridge from one generation to the other.
(ii) The hereditary factors are carried in the nucleus.
(iii) Chromosomes are also found in pairs like the Mendelian alleles.
(iv) The two alleles of a gene pair are located on homologous sites on the homologous chromosomes.
(v) The sperm and egg have haploid sets of chromosomes, which fuse to re-establish the diploid state.
(vi) The genes are carried on the chromosomes.
(vii) Homologous chromosomes synapse during meiosis and get separated to pass into different cells. This is the basis for segregation and independent assortment.
Experimental Verification of the Chromosomal Theory of Inheritance:
It was done by Thomas Hunt Morgan and his colleagues. He observed that the two genes under consideration in his experiments, did not segregate independently as in the case of characters studied by Mendel. Morgan carried out various dihybrid crosses in Drosophila, with the genes that were sex-linked, i.e., the genes are present on the X-chromosome.
Morgan selected fruit fly, Drosophila melanogaster for his experiments because:
(i) They could be grown on simple artificial medium in the laboratory.
(ii) Their life cycle is about only two weeks.
(iii) A single mating could produce a large number of flies.
(iv) There was a clear differentiation of the sexes male (smaller) and female (bigger).
(v) It has many types of hereditary variation which can be easily seen through low power microscope.