A = big apples; R = red apples; a = small apples; r = yellow apples. You have one tree that produces big yellow apples and another tree that produces small red apples. When the two are crossed, you find that half of the new trees produce big red apples and half produce big yellow apples. What are the genotypes of the parents?

When the two trees are crossed, all the new apples are big in size. This is only possible when the parent tree producing big apples is homozygous for it (AA). It will give one dominant allele for the trait and all the offspring will have big apples despite the second parent producing small apples(aa).

Half of the offspring trees have red apples and other half have yellow. This is possible when the tree producing red apples is heterozygous for it (Rr). It will give dominant allele to half of the offspring and recessive allele to the other half. When these alleles will combine with the recessive allele obtained from the yellow apple producing plant (rr), half of the offspring will have red apples and other half will have yellow.

Hence, genotype of the parents are:

Big, yellow apple tree = AArr

Small, red apple tree = aaRr