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Bar eye (B) is a dominant and sex-linked trait of Drosophila. Curved wings (cu) and black...

Bar eye (B) is a dominant and sex-linked trait of Drosophila. Curved wings (cu) and black body (bl) are recessive traits, both found on chromosome 2, separated by 24 map units. A homozygous Bar-eyed, black-bodied female is crossed to a true-breeding male expressing curved wings. The heterozygous F1 female is then crossed to a male expressing all three traits. At what frequency among the F2 would you expect to find a completely wild type male?

How often would you find a Bar eyed, curved wing female?

Solutions

Expert Solution

Cross 1: (Black bodied, Bar eyed female crossed to Male with Curved wings)
bl c+/bl c+; XB/XBx bl+ c/bl+ c; X/Y

F1 --> bl c+/bl+ c ; XB/X (F1 Heterozygous Female)

Cross 2 (F1 Heterozygous female crossed to male expressing all three traits):

bl c+/bl+ c ; XB/X x bl c/bl c ; XB/Y

The female parent of Cross 2 produces the following gametes:

Gametes and the proportion of gametes
Gamete Gamete Type Proportion Rationale

bl c+; XB

Parental 19% Since the two loci are 24 map units apart, recombination would occur in 24% of all meioses. Therefore, 76% of all gametes would be parental, with each class of parental gametes represented equally

bl c+; X

Parental 19%

bl+ c ; XB

Parental 19%

bl+ c ; X

Parental 19%

bl+ c+; XB

Recombinant 6% 24% of all meioses would be recombinant, and all recombinant gametes would be represented equally.

bl+ c+; X

Recombinant 6%

bl c ; XB

Recombinant 6%
bl c ; X Recombinant 6%

Now, the tester male (Male used in Cross 2) would produce half gametes with the X chromosome and half with the Y chromosome.

Therefore, the progeny proportions are:

A completely normal male is 3% of the F2 progeny.

A Bar-eyed, Curved Wing Female would occur in 19% of female progeny.


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