Question

Yellow body color (y), crossveinless wings(cv) and forked bristles (f) are all recessive and in that order on the X chromosome in Drosophila. Yellow is 14 map units from crossveinless and crossveinless is 22 map units from forked. A yellow, crossveinless female is crossed to a forked male. An F1 female is testcrossed to a male expressing all three traits. If the coefficient of coincidence is 0.9, how many completely wild type flies will be observed out of 1000?

From the same testcross, how many flies will express forked only?

How many flies expressing yellow only will be observed?

Please explain and show work.

Answer 1

Cross 1: y cv f+/y cv f+ x y+ cv+ f/Y

F1 --> y cv f+/y+ cv+ f (Trihybrid female)

Test Cross: F1 Female x Tester Male
y cv f+/y+ cv+ f x y cv f/Y

1) How many completely wild type flies will be observed out of 1000

Completely Wildtype progeny will be observed when there is a single crossover between the cv and f loci, producing y cv f and y+ cv+ f+ progeny. Since the Map distance between the cv and f loci is 14 m.u, 14% of all progeny will have cross-over between cv and f, producing 7% completely wild-type and 7% completely mutant progeny.

Therefore, 7% of F2 progeny (70 flies) will be completely Wildtype.

2) How many flies will express forked only

Only Forked flies will occur when the parental y+ cv+ f chromosome produces progeny. That is no crossover occurs between any of the three loci. As 14% of progeny will have a crossover between y and cv and 22% of progeny will have a crossover between cv and f; the remaining 64% of F2 progny will have no crossover.

Therefore, 32% of F2 progeny or 320 flies will have Forked only.

3)

Progeny with Yellow only will occur when there is a double recombination event that exchanges the cv locus between the parental chromosomes. This would change the y cv f+ chromosome to y cv+ f+ and the y+ cv+ f chromosome to y+ cv f.

Now,

Coefficient of Coincidence
     = Observed Number of Double Crossovers/Expected Number of Double Crossovers = 0.9

Expected Frequency of Double Crossovers
= Recombination Frequency between y-cv * Recombination Frequency between cv-f = 0.14 * 0.22 = 0.0308

Expected Number of Double Crossover Progeny
   = Expected Frequency of Double Crossovers * No. of Progeny = 0.0308 * 1000 = 30.8

Therefore,

Coefficient of Coincidence = Observed No. of Double Crossovers/Expected No. of Double Crossovers = 0.9
=> 0.9 = Observed No. of Double Crossovers/30.8
=> Observed No. of Double Crossovers = 30.8 * 0.9 = 27.72 28

Since the observed number of Double Crossover recombinant progeny is 28, 14 would have only the Yellow trait.