Question

Explain the phenomenon shown in the following figure and the reason for difference in the production of recombinants in Cross A and Cross B as explained by Morgan.

Answer 1

This is the linkage phenomenon. Thomas Morgan crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males to obtain the F1 progeny. He then intercrossed the F1 generation to get the F2 generation. This was cross A. He crossed white bodied females with miniature wings with wild-type males. This was cross B. He noticed that the gene did not obey Mendel's law of independent assortment.

In cross A

The yellow and white genes showed a recombination of 1.3% as these genes were tightly linked.

In cross B

The white and miniature genes showed a recombination of 37.2 % as these genes were loosely linked. Morgan discovered that this was due to the linkage of genes. This results in inheritance of genes together in the progeny and such genes are called linked genes.