In Drosophila , the genes for eye color, wing shape, and wing length are located
ID: 36617 • Letter: I
Question
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?
Express your answer as a whole number
Total number or offspring is 1250
Explanation / Answer
Crossing over don't occur uniformly along a chromosome. For example, fewer crossovers occur in the area around the centromere than in other areas of the chromosome.the formation of one chiasma typically makes it less likely that a second chiasma will form in the immediate vicinitof the first. This seems to be due may be due to the inability of the chromatids to bend backupon themselves within a certain minimum distance. lack of independence is called interference.
Interference varies in different sections of the chromosome and is measured by the
Coefficient of Coincidence(C.C.) =ratio of observed to expected double crossover
= 25/50 =0.5
the interference of coincindece is 1-c.c = 1-0.5 = 0.5.
THIS means 50 percent of the crossover do not occur from the expected value { 50 offspring] . 50/50 = 1
so , single offspring will represent single cross over event between pr and vg genes. one in 1250 offspring will have single crossover event . IT IS A POSITIVE INTERFERENCE.