Ocr f215 past paper Q June 2010

Please help I don't understand:
Q:
a mutation in another gene at another locus gives rise to white-eyed flies. Red eye allele(R) is dominant to white eye allele(r). Student crossed a red eye with white eye and expected a F1 generation of red eyed flies, but results were:

Phenotype: Num offspring:
red eyed female. 27
red eyed male. 0
White eyed female. 0
white eyed male. 23

Student suggested reason for red eye and white eye was that red eye parent was heterozygous.
explain why this cannot be the correct explanation for the results.


The answers arefor 3 marks)
If red eye parent was heterozygous:
-there would be no differences between the sexes
-red eye makes and white eye females would occur
-11:1 ratio

Could someone please explain the answers for me please

Sorry I still don't understand, because if you were to cross Rr and rr, you'd end up with two Rr's and two rr's again so how is it it wrong or a 11:1 ratio. Also how can you tell between sex here ������

I understand what you mean by getting two Rr's and two rr's. Assume that the alleles are sex-linked. Therefore, the female (XX) is heterozygous. Try drawing a punnett square with the sex genes. You'll see it is a 1 : 1 : 1 : 1 ratio now. (The male is still homozygous recessive). If you still don't understand, I'll show you

I don't mean to be so demanding, but would it be possible if you could show me please as I really am struggling to understand

No problem, happy to help

This should hopefully explain things (sorry about the handwriting).

Thankyou, you have been very helpful, the only part I now do not understand is: why does this mean there are no differences between the sexes

I think it means that if the female was heterozygous, then both the males and the females could have either white or red eyes, instead of males just having white eyes, and females red eyes, like the table shows. (If that makes sense )