Genetics and Mice?

Question

Hey everyone! I've spent about 3 hrs on this problem and can't seem to find an answer, so here goes:
When yellow mice are mated together, 2/3 of the progeny are yellow, 1/3 are agouti. Another locus is known to govern pigment production. When parents heterozygous at this location are mated, 3/4 of the offspring are colored, 1/4 are albino. These albind mice can not express whatever alleles they have at the agouti locus. A yellow mouse is crossed with an albino and produces an F1 containing 1/2 albino, 1/3 yellow, and 1/6 agouti.

I've figured out that the only way to get the 2:1 phenotypic ratio between yellow and agouti is to consider the dominant yellow allele, when homozygous, to be lethal. I understand that it is not lethal when the recessive albino allele is homozygous. Past that I'm stuck. I feel like I've tried everything!

Answer 1

This is an example of epistasis, where a second gene can affect the phenotype by overriding the first.

The genotype of the yellow parent must be YyAa (because homozygous YY is lethal, and if the parent was AA at the albino locus no albino offspring are possible)

The genotype of the albino parent must be Yyaa (because if the parent was homozygous yy you would expect equal numbers of yellow and agouti offspring, and because albinoism is homozygous recessive)

So now your cross is YyAa x Yyaa

Look at just the albinoism gene. You would expect 1/2 of your offspring to be Aa and the other half to be aa. Right off the bat, half of your offspring are albino, doesn't matter what their genotype at the Y locus is.

Now lets look at the half that can have offspring. Just like before you expect 1:2:1 ratio of YY to Yy to yy. Except that all of the homozygous dominants die, leaving you with 2:1 Yy to yy. Thus, of the colored offsring you get:

2/3 Yellow (Yy)
1/3 agouti (yy)

But remember, these are only half of your offspring (the other half are all albino), so you need to divide these totals by 2, leaving you with:

1/3 Yellow (YyAa)
1/6 agouti (yyAa)

1/2 albino (Yyaa and yyaa)

Answer 2

ok this has nothingggg to do with the topic
but your like the only person ive ever met who spells
their name like i do lol