Question

N the cross shown here, involving two linked genes, ebony (e) and claret (ca) in drosophila, where crossing over does not occur in males, offspring were produced in this phenotypic ratio: 2 + : 1 ca : 1 e

Answer 1

I found the full exercise on the internet and it ends like this:
"These genes are 30 cM apart on the third chromosome. Explain how this ratio arose." Attached is an image of the cross mentioned above.
Crossing over in the female contributed to the possible existing allele combination |e-ca| that did not exist in the parents genotypes. With this new allele combination, the new possibilities are |♀e-ca/♂e-ca+|, |♀e-ca/♂e+-ca|, |♀e+-ca+/♂e-ca+|, and |♀e+-ca+/♂e+-ca|, leading to the respectively phenotypes ebony, claret, wild, wild. These results correspond to the obtained phenotype ratio.