You are studying three autosomal recessive mutations in a Mongonian fly that is

Question

You are studying three autosomal recessive mutations in a Mongonian fly that is reminiscent of Drosophila here on Earth. Flies that are homozygous for the rd mutation are red-winged, those that are homozygous for the bl mutation are blind, and those that are homozygous for st mutation are straight-legged. Wildtype flies are blue-winged, not-blind, and bowed-legged.

You mate flies from two true-breeding strains, and the resulting F1 flies are all blue-winged, not-blind, and bowed-legged. F1 females are then mated to males that are red-winged, blind, and straight-legged. In the F2 generation, among the 1000 progeny resulting from this cross, you observe the following phenotypes.

red-winged, blind, and straight-legged 26 flies
red-winged, blind, and bowed-legged 455 flies
red-winged, not-blind, and bowed-legged 24 flies
blue-winged, blind, and straight-legged 27 flies
blue-winged, blind, and bowed-legged 4 flies
blue-winged, not-blind, and straight-legged 442 flies
blue-winged, not-blind, and bowed-legged 22 flies

(a) The male flies that were bred to the F1 generation in order to produce the F2 generation were red-winged, blind, and straight-legged. On each of their chromosomes, they have the alleles rd-, bl-, and st-. Using this notation, state the genotype of each of the two true-breeding parental strains.
(b) Draw a genetic map of the three loci. Show all your calculations and indicate if any chromosomal interference is observed.

Explanation / Answer

Given that cross between Blue-winged, not-blind, and bowed-legged * red-winged, blind, and straight-legged is done rd+ bl+ st+ * rd- bl- st- progeny obtained are rd-, bl-, st- = 26 flies rd-, bl-, st+ = 455 flies rd-, bl+, st+ = 24 flies rd+, bl-, st- = 27 flies rd+, bl-, st+ = 4 flies rd+, bl+, st- = 442 flies rd+, bl+, st+ = 22 flies total progeny = 1000 Recombination between rd- and bl- = 24+ 27+ 4 = 55 / 1000 = 0.055 Recombination between bl- and st- = 455 + 4 + 442 = 901/ 1000 = 0.901 Recombination between rd- and st- = 455 + 24 + 442 = 948 / 1000 = 0.948 So, the genetic map of the three loci are Rd- --------------bl- ----------------------st- Interference = 1 - coefficient of coincidence Here coefficient of coincidence = actual double recombinant frequency / expected double recombinant frequency

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