Drosophila Genetics problem

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Drosophila Genetics problem

Post by georgina_009 on Tue Mar 18, 2008 5:36 am

Hi,
I have another question that I need help on - I'm hopeless at making sense of these questions:

Normal wild eye colour in Drosophila is dark red
White eyed mutation (w) is on the X chromosome
purple eye colour (pr) is autosomal
The wild type alleles for both genes (w+ pr+) are dominant
Mutations in the white gene exhibit recessive epistasis with respect to all other Drosophila eye colour mutations.

qn -
If a white eyed true breeding female is crossed with a purple eyed true breeding male what phenotypic ratios are expected in male and female flies in the F2 generation?
Then it gives me a hint saying: the white eyed female does not carry the pr mutation, the female is pr+ pr+

If anyone could give me some pointers it would be much appreciated. Thanks

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Re: Drosophila Genetics problem

Post by Alla on Tue Mar 18, 2008 8:26 am

The answer to this problem depends on the male genotype. To make it easier to understand let’s designate genes as following:

P – dominant wt allele of purple mutation (p)
W – dominant wt allele of white mutation (w)

Then white female is PPww genotype (Ppww, is also white but that would not be homozygous and, therefore, not true breeding).

The purple eye male is a more difficult problem because white eyes exhibit recessive epistasis – it could be ppWY or ppwY – both would have purple eye color and true breeding. In either case all males would have white eyes (PpwY), the difference will be seen in females: red eyes (PpWw) if male is ppWY or white (Ppww) if male is ppwY. Given a hint in your problem, I would say that purple eyed male is more likely to have a wt allele for the white mutation giving you F2 ratio of 1 red: 1 white, but I cannot guarantee that.

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Still don't quite get it

Post by georgina_009 on Wed Mar 19, 2008 5:55 am

Thanks for your help but the only thing I don't understand is you said:

"in either case all males would have white eyes (PpwY), the difference will be seen in females: red eyes (PpWw) if male is ppWY or white (Ppww)"

Why is PpwY white eyes? I thought that the P would mean it was wt?
I probably missed something but could you explain?
Thanks

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Re: Drosophila Genetics problem

Post by Alla on Wed Mar 19, 2008 8:02 am

Your problem says that "mutations in the white gene exhibit recessive epistasis with respect to all other Drosophila eye color mutations."
That means that white eyes are recessive to purple eyes (which are due to mutation). Pp means WT red eye color, so PpwY would exhibit white eye phenotype since white eye would have dominant epistasis with respect to normal phenotype encoded by other genes like P. Just do not confuse dominant/recessive epistasis with respect to other mutations and dominant/recessive alleles of the same gene.

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Re: Drosophila Genetics problem

Post by georgina_009 on Wed Mar 19, 2008 8:43 am

Ok so if white is reccessive to all mutations that means it is dominant to normal? I can't find any information on this but I'll take your word for it.
So does the fact that the w is sex linked and the pr is autosomal have much to do with it?
I think the male would be w+prpr or WppY so then it is 1:1
Where it says what are the expected phenotypic ratios for male and female in f2 does that mean i need two sets of ratios? Or is the 1:1 part males and females?

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Re: Drosophila Genetics problem

Post by georgina_009 on Wed Mar 19, 2008 8:45 am

hang on though - wouldn't the male be red eyed if his genotype was WYpp?

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Re: Drosophila Genetics problem

Post by Alla on Wed Mar 19, 2008 9:12 am

"Ok so if white is recessive to all mutations that means it is dominant to normal? I can't find any information on this but I'll take your word for it."

"exhibit recessive epistasis with respect to all other Drosophila eye color mutations" actually means that any other abnormal/mutant eye color would overcome white eye phenotype. However, white eye has to exhibit dominant epistasis to other WT alleles or you would never get a white eyed fly.

"So does the fact that the w is sex linked and the pr is autosomal have much to do with it?"

No. It is just which color will be apparent in the offspring.

"I think the male would be w+prpr or WppY so then it is 1:1. Where it says what are the expected phenotypic ratios for male and female in f2 does that mean i need two sets of ratios? Or is the 1:1 part males and females?"

No. 1:1 is the ratio of red to white, but if the F1 male genotype is ppwY, then the ratio will be different and the major difference will be seen in females.

"wouldn't the male be red eyed if his genotype was WYpp?"

No, it would be purple. WY = WT red; pp=purple. Mutant phenotypes generally have dominant epistasis over WT encoded by other genes.

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Re: Drosophila Genetics problem

Post by georgina_009 on Thu Mar 20, 2008 7:45 am

Ok thanks for all you're help

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Re: Drosophila Genetics problem

Post by Alla on Thu Mar 20, 2008 11:06 pm

You are wecome!

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