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?

Because the parent mice are true-breeding, each passes the same eye-color allele to its offspring. All offspring have black eyes, so the black-eye allele masks the red-eye allele in the first generation. This means the allele for black eyes is dominant.

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