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.

A true-breeding red-eyed mouse and a true-breeding black-eyed mouse have two copies of their respective eye-color alleles. Since all their offspring have black eyes, the black-eye allele masks the red-eye allele in the offspring. This indicates that the allele for black eyes is dominant.

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