Discuss in detail the contributions of Morgan and Sturtevant in the area of genetics.

Experimental verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan (Father of experimental genetics) and his colleagues. It leads to the discovery of the basis of variation is due to sexual reproduction.

Morgan worked with the tiny fruit files, Drosophila melanogaster suitable for such studies. Morgan performed dihybrid crosses in Drosophila to study sex linked genes. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. For example, Morgan hybridised yellowbodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their $F$ progeny.

He observed that the two genes did not segregate independently of each other and the $F2$ ratio deviated very significantly from the $9: 3: 3: 1$ ratio (expected when the two genes are independent).

Morgan and his group knew that the genes were located on the $\mathrm{X}$ chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.

Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations.

His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome.

Today genetic maps are extensively used as a starting point in the sequencing of whole genomes as was done in the case of the Human Genome Sequencing Project