Testcross to Detect Linkage In Drosophila the mutant black (b) has a black body,
ID: 18683 • Letter: T
Question
Testcross to Detect Linkage
In Drosophila the mutant black (b) has a black body, and the
wild type has a gray body; the mutant vestigial (vg) has
wings that are much shorter and crumpled compared to the
long wings of the wild type. True breeding black, normal flies
were crossed with true breeding gray, vestigial flies. F1 gray,
normal female flies were testcrossed to black, vestigial male
flie
P
F1
The testcross progeny:
gray, normal 283
gray, vestigial 1,294
black, normal 1,418
black, vestigial 241
Total 3,236
Are the,black and vestigial genes linked?
What is the map distance between the black and vestigial genes?
Explanation / Answer
The cross is b vg+/b vg+ x b+ vg/b+ vg, which gives an F1 that is b vg+/b+ vg.
An F1 female is crossed with a homozygous recessive b vg/b vg. The female is used as the doubly heterozygous parent, as no crossing-over occurs in male Drosophila. The classes can be grouped in reciprocal pairs as follows:
Nonrecombinants (parentals): grey, vestigial 1,294
black, normal 1,418
Recombinants (nonparentals): grey, normal 283
black, vestigial 241
Total progeny 3,236
Recombination frequency = [(283 + 241)/3,236] x 100% = 16.2%
There are 16.2 map units between the genes.
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