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So this is a practice question I got from school but I kinda lost here. So the question is:

In Drosophila , four new recessive mutations have been found all of which produce purple eye colour.

Mutations 1, 2 and 3 are all found to be X-linked whereas mutation 4 segregates with one of the autosomes.

Crosses between females heterozygous for either mutation 1 and mutation 2, OR mutation 1 and mutation 3, and wild-type males have been performed.

1st cross: no wild-type males were observed in 7500 male progeny.

2nd cross: two wild-type males were recovered among 1510 male progeny.

What is the minimum and maximum number of genes identified by these four mutations given the above data? Explain your reasoning.

How would you go about determining the actual number of genes?

My best guess is either mutation 1 & 2 OR 1 & 3 must be allelic because we didn't observe any WT on the first cross. If it's not allelic, there should be complementation there and we will observe a WT phenotype.

I'm not sure what "OR" means.

I'm not sure why only 2 WT males were recovered among 1510 male progeny. I think it has something to do with recombination, but can't really find out what the gene configuration is to obtain that result.

And the way to determine the actual number of genes is by performing a complementation test, right?

Really appreciate if someone can help me / give me some hints here. I'm really lost!

Thanks in advance :)

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  • $\begingroup$ I suppose that 2nd cross imply crossing offspring of both sexes with parents (of both sexes). Even then, I can't think of a scenario that would explain this pattern without some level of epistasis or environmental variance. Did you copie-pasted the exact question that has been asked? $\endgroup$ – Remi.b May 14 '16 at 18:30

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