In Drosophila the genes for eye color wing shape and wing le

In Drosophila, the genes for eye color, wing shape, and wing length are located on chromosome II. Purple eyes (pr), arc bent wings (a), and vestigial wings (vg) are the mutant forms of the wild type traits red eyes, straight wings, and long wings, respectively. You\'ve discovered some data in your genetics laboratory which indicates that the distance between vg and pr is 12.5 m.u., the distance between a and pr is 44.7 m.u., and the distance between a and vg is 32.2 m.u.

Upon careful examination of the offspring obtained from your cross, it was determined that only 25 offspring were double crossover phenotype -- half of the 50 that was expected.

Now, how many offspring that represent a single crossover event between pr and vg are expected, considering that positive interference has occurred?

Solution

It is given that

Observed number of double crossover phenotype = 25

Expected number of double crossover phenotype = 50

Interference (I) is expressed as I = 1-C

where C is coefficient of coincidence

C = Observed number of double crossovers / Expected number of double crossovers

C = 25/ 50 = 0.5

I = 1-C = 1- 0.5 = 0.5 or 50%

It is a positive interference and hence 50% of the offsprings that represent the single crossover. (For example if total offsprings given are 1250, then the single crossover between pr and vg would be 313).