The progeny of this cross comes in thirds?

Question

The problem I was provided with asked "A mouse fancier discovers a yellow mouse and crosses it with a black one. Half the progeny are yellow and half are black. A cross of two of the yellow mice results in two-thirds yellow mice and one-third black. How do you account for these results?

I'm not really certain what types of genes you would have to cross to come up with progeny (offspring) in thirds?

Thank you if you can help!

Answer 1

The dominant gene (for yellow) is lethal in the homozygous form.

The original cross was a heterozygous yellow mouse (Yy) with a black mouse (yy), resulting in half the progeny heterozygous and half homozygous recessive.

If you cross two yellow mice (both heterozygotes) you would expect the following ratios:

1/4 YY
1/2 Yy
1/4 yy

But all of the homozygous dominants are removed leaving you with:

2/3 Yy
1/3 yy