Morgan's Experiments showing linkage and crossing over :

Morgan used Drosophila melanogaster (fruit fly) for his experiments because, Drosophila can easily be cultured in laboratory. It's life span is short, about two weeks. More over, it has high rate of reproduction. Morgan carried out several dihybrid cross experiments in fruit fly to study genes that are sex-linked. The crosses were similar to dihybrid crosses, as carried out by Mendel in Pea. For example, Morgan and his group crossed yellow-bodied, white eyed female to the wild type with brown bodied, red eyed males and intercrossed their F 1 progeny.

He observed that the two genes did not segregate independently of each other and F2 ratio deviated very significantly from 9:3:3:1 ratio.

Morgan and his group knew that the genes were located on X chromosome and stated that when two genes in a dihybrid cross are situated on the same chromosome, then the proportion of parental combination is much higher than non-parental type.

This occurs due to physical association or linkage of the two genes. He also found that, when genes are grouped on the same chromosome, some genes are strongly linked. They show very few recombination (1.3 %). When genes are loosely linked i.e. present far away from each other on chromosome, they show more (higher) recombination (37.2 %).

For example, the genes for yellow body and white eye were strongly linked and showed only 1.3 percent recombination (in cross-I).

White bodied and miniature wings showed 37.2 percent recombination (in cross-II). Cross I shows crossing over between genes y and w. Cross II shows crossing over between genes white (w) and miniature wing (m).

Here dominant wild type alleles are represented with (+) sign.