In a lab strain of Drosophila, cinnabar (cn) and brown (bw) are recessive eye co
ID: 92556 • Letter: I
Question
In a lab strain of Drosophila, cinnabar (cn) and brown (bw) are recessive eye color mutations known to be 41 map units apart on chromosome 2. When similar mutant alleles were induced in a strain from nature, the same linkage of cn and bw was observed. However, when a wildtype strain from nature was crossed with a cn bw/cn bw lab strain to create the genotype + +/cn bw, and females of this type were testcrossed to cn bw/cn bw males from the lab strain, the following phenotypic proportions were obtained in the progeny:
+ + 25,200
cn bw 21,009
cn + 11
+ bw 36
a) What is unexpected about these results?
b) What is the most likely explanation? (Summarize your model with a chromosome diagram with labeled genes.)
c) On your diagram show the precise origin of the cn + and the + bw classes.
Explanation / Answer
What is unexpected about these results?
The RF is much less than the expected 41%
Calculate the expected recombination frequencies for 2 loci 41 map units apart.
Total number of progeny = 25,200 + 21,009 + 11 + 36 = 46,256
Without any recombination, you would expect the
Mendellian crosses to give 50% ++
And
50% cn bw.
The recombinant groups of progeny are cn + and +bw.
[(cn +) + (+ bw ]/ 46256 = 41%
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b) What is the most likely explanation?
b) There must be a large inversion in the cn to bw region.
Strain from nature cn ---[inversion spanning most of region]--- bw
Strain from lab strain + ---[noninverted sequence --- +
These two chromosomes are paired in the F , with the inverted region looped.
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c) These genotypes must arise from crossovers in the small homologous noninverted regions shown with dashes in the diagram
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