Biology homework about genetics..?

Question

2 black mice crossed with white mice produced all black mice. cross two heterozygous mice and give the geneotypic and phenotypic ratios

Answer 1

The first part of the question tells you that black is dominant over white.  This is assuming a simple Mendelian inheritance pattern, which is simplistic.  

So, in the second part of the question, each of your heterozygous mice will donate either a Black allele or a white allele to the next generation.  This can be represented by a simple four-square Punnett square.  That means that their are four possible combinations that could be found in the next generation.  

In one of these combinations, both parents donate a black allele.  So any offspring which get that combination will be homozygous for the black, and have black fur.  

In another combination, both parents could donate a white allele.  In this case, the mouse would be homozygous for the white, and have white fur.

The other two combinations are going to look the same, even though they came about differently.  These happen when one parent donates a black allele, and the other a white one, or visa-versa.  Either way results in a heterozygous mouse (just like its parents).  These mice have one of each allele, but because black is dominant over white, they each have black fur.

So the tally for phenotypes is 3 black mice to every 1 white one.  That is Mendel's classic 3:1 ratio.

The tally for genotypes is 1 homozygous black : 2 heterozygotes : 1 homozygous white.