A cross was made between wild type female Drosophila melanogaster and mutant males which are yellow bodied (y) and crossveinless (cv). The two genes are present on the X-chromosome. The F1 progeny was sib-mated and the observation of F2 progeny is tabulated below.
|
Phenotype |
No. of male progeny |
No. of female progeny |
|
wild type |
45 |
100 |
|
yellow body and crossveinless |
40 |
0 |
|
yellow body |
6 |
0 |
|
crossveinless |
9 |
0 |
|
Total no. of progeny analyzed = 200 |
||
1
The genetic distance between the two genes is 7.5 cM.
2
If the mapping was done with a 3rd marker which lies between y and cv, the genetic distance is likely to increase, but never decrease.
3
If a larger progeny size was analyzed, more double crossovers will be identified leading to decrease in the genetic distance.
4
If a reciprocal parental cross was carried out, no recombinants would be observed in the F2 progeny as there is no crossing over in D. melanogaster males.