A Drosophila geneticist made a cross between females homozygous for three X-link

Question

A Drosophila geneticist made a cross between females homozygous for three X-linked recessive mutations (y, yellow body; ec, echinus eye shape; w, white eye color) and wild-type males. He then mated the F1 females to triply mutant males and obtained the following results:

Females                       Males              Number

+ + ec w             + + +               475

y ec ec w           y ec w           469

y   + ec w          y + +             8

+ ec ec w          + ec w            7

y   +   ec w          y + w                        18

+ ec ec w          + ec +             23

+ + ec w            + + w             0

y ec   ec w          y   ec +            0

Determine the order of the three loci y, ec, and w, and estimate the distances between them on the linkage map of the X chromosome.

Explanation / Answer

The recombinants which are observed at the lowest frquency are double cross over. Here, a cross between and female (++ ec w) and ++w and also y ec-ec w and y ec + showed no number. Hence we can state that the gene order must be y-w-cc. The highly reproduced number is of parental type. The distance between y and w can be estimated by the frequency of recombination between y and w. It will be (8+7)/1000= 0.015 = 1.5cM. The distance between w and ec is (18+23)/1000 = 0.041 = 4.1cM

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