Question

there are genes that govern the eye color of mice, too. suppose that a true - breeding mouse with red eyes is crossed with a true - breeding mouse with black eyes. all of the offspring mice have black eyes. what can you conclude?
the allele for red eyes is dominant.
the allele for black eyes is dominant.
neither. you need more data.

Explanation:

Step1: Define true-breeding genotypes

Let the allele for red eyes be $R$ and black eyes be $r$. True-breeding red-eyed mice have genotype $RR$, true-breeding black-eyed mice have genotype $rr$.

Step2: Cross the two true-breeding mice

When $RR$ is crossed with $rr$, all offspring inherit one $R$ and one $r$, so genotype is $Rr$.

Step3: Relate genotype to phenotype

All offspring have black eyes, meaning the $Rr$ genotype expresses the black eye phenotype. This shows the black eye allele masks the red eye allele, so black eyes are dominant.

Answer:

The allele for black eyes is dominant.