Biology Forums - Study Force

Suppose that r, red, and sc, scarlet, are two recessive eye-color mutations that lie 20 map units apart on the Drosophila X chromosome, and that each causes a red eye color in place of the wild type purple color.

A. If a female from pure-breeding rr stock is mated with a red-eyed (sc) male, what phenotypes will the F1 sons and daughters display?

B. If these F1 flies are interbred, what will be that phenotypic ratios among the F2 granddaughters and grandsons?

I have trouble starting on this question.
I realize that the two genes are both on the X chromosome and I think the parental genotypes might be
r = red; s=scarlet
female: X^(r/S) X^(r/S); and male: X^(R/s) Y ?

Then in A. the F1 would be:
female: X^(r/S) X^(R/s); and male X^(r/S) Y
female: purple eyed; and male: red-eyed

I'm not sure whether that's correct and when the map units come into play, nor how to do part B.

Would be great if someone could guide me in the right direction here, thank you!

r - sc = 20 m.u

thus, 20% recombination frequency.

Hence these two loci are quite strongly linked i.e. close to each other on the chromosome. (80% linkage)

1) Female X(r)X(r) is crossed with Male X(sc)Y

Attachment 1

Thus we get the following genotypes:


X(r)X(sc) females and X(r)Y males

Thus the females and males will both show a red eye colour in the F1 generation.

2) For the F2 generation the F1 flies are interbred:

Attachment 2

Thus only one phenotype will be seen i.e the red eye flies

Phenotypic ratio = 1

Genotypic ratio = X(r)X(r): X(r)X(sc): X(r)Y: X(sc)Y = 1:1:1:1

Did this help?

Shouldn't the genotype reflect 2 genes? In your answer it shows only 1. Since the question says that the mutations are 20 map units apart, the two mutation represent 2 different genes on one chromosome right? And each gene has 2 alleles. The way I see your answer is that you put the two mutations as alleles for only one gene.

Not sure if my thinking is correct. That's how I read the problem.

What did you end up finding :-\