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?

Answer 1

Because the parents are true-breeding (homozygous) and all F1 offspring show the black phenotype, the black allele must be dominant to the red allele.
For example, if red = rr and black = RR, the cross RR × rr gives all Rr offspring, which are black in phenotype. Thus red is recessive and the F1s are heterozygous showing black eyes.