Question

Morgan carried out several dihybrid crosses in Drosophila and found $F_2$-ratios deviated very significantly from the expected Mendelian ratio. Explain his findings with the help of an example.

Answer 1

Based upon his breeding experiments on fruitfly Drosophila melanogaster, Morgan proposed chromosome theory of linkage which states that linked genes occur on the same chromosome in a linear sequence and maintain parental combination of genes except for occasional cross overs.
Linkage: The phenomenon of inheritance of a number of genes or factors due to their physical occurrence on the same chromosome is called linkage. Linked genes occur on same chromosome while unlinked genes occur in different chromosomes. Linked genes do not show independent assortment. They do not give a 9 : 3 : 3 : 1 dihybrid phenotypic ratio and 1 : 1 : 1 : 1 as double test cross ratio.
Linkage may be: 1. complete 2. Incomplete
Complete linkage: The genes closely located in the chromosome show complete linkage as they have no chance of separating by crossing over and are always transmitted together to the same gamete and the same offspring. Thus, the parental combination of traits is inherited as such by the young one.
Incomplete Linkage: The F${}_1$ females of above cross are crossed with homozygous recessive males. The ratio comes out to be 9 : 1 : 1 : 8. The two genes did not segregate independently. In Drosophila melanogaster- the genes for eye colour and wing size are present in the X chromosome. D. melanogaster has XX- XY sex mechanism. Thus, heterozygous female produces two types of eggs, while the males produce only one type of sperms.