Aparent organism of unknown genotype is mated in a test cross. half of the offspring have the same phenotype as the parent. what can be concluded from this result?
a. the parent is homozygous dominant for the trait.
b. the trait being inherited is polygenic.
c. the parent is heterozygous for the trait.
d. the parent is homozygous recessive for the trait.

The answer is C. The test cross means that the unknown genotype organism is mated with a homozygous recessive organism("aa"). If the parent organism is homozygous("AA" or "aa"), the offspring will all be the same with the parent(Aa when the parent is "AA". "aa" when the parent is "aa"). So the half of the offspring have the same phenotype means the parent organism is heterozygous. We cannot confirm whether this is polygenic or not by test cross.