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n Drosophila melanogaster, forked bristles are caused by an allele (Xf) that is X linked and recessive to an allele for normal bristles (X+). Brown eyes are caused by an allele (b) that is autosomal and recessive to an allele for red eyes (b+). A female fly that is homozygous for normal bristles and red eyes mates with a male fly that has forked bristles and brown eyes. The F1 are intercrossed to produce the F2. What will the proportions of the F2 flies be (male with red eyes and forked bristles) from the cross?

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Answer:

Proportion of the F2 male with red eyes and forked bristle will be 1/4

Explanation:

forked bristles are caused by an allele (Xf) that is X linked and recessive to an allele for normal bristles (X+).

Brown eyes are caused by an allele (b) that is autosomal and recessive to an allele for red eyes (b+)

A female fly that is homozygous for normal bristles and red eyes mates with a male fly that has forked bristles and brown eyes

    X+X+b+b+  x    Xfbb

the F1 gives   1/2X+Xf 1/2b+b

                      1/2X+Y  1/2b+b

intercross of F1

      Proportion of the F2 male with red eyes and forked bristle will be

      forked bristle= Xfy = 1/2 and for red eyes  2/4= 1/2  = 1/2x1/2 =1/4

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