Concept:A test cross reveals the unknown genotype of an individual that shows a dominant trait by crossing it with a homozygous recessive partner.
Explanation:An organism with a dominant phenotype may be either homozygous dominant or heterozygous.
To find out which genotype is present, it is crossed with a homozygous recessive individual.
If any offspring show the recessive trait, the unknown parent must be heterozygous.
If all offspring show the dominant trait, the unknown parent is most likely homozygous dominant.
Thus, the appearance of the offspring helps decide the exact genotype of the individual being tested.
Answer:A. to determine the genotype of an individual whose phenotype is to be known