Final answer:
A testcross is conducted to determine the genotype of an individual expressing a dominant phenotype by crossing it with a homozygous recessive individual, and the offspring's phenotypes reveal the unknown genotype.
Step-by-step explanation:
The type of cross performed to determine the genotype of an individual expressing a dominant trait is a testcross. This cross involves breeding the dominant expressing individual whose genotype is unknown with a homozygous recessive individual. The phenotypes of the offspring from this cross can reveal whether the unknown parent is heterozygous or homozygous for the dominant allele.
For instance, if you have a yellow-seeded pea plant (dominant trait) but are unsure if its genotype is YY (homozygous dominant) or Yy (heterozygous), you would perform a testcross. You would cross the yellow-seeded plant with a green-seeded plant (yy, homozygous recessive). If all the offspring have yellow seeds, the unknown parent is likely homozygous dominant. However, if some offspring have green seeds, the parent is heterozygous.
A Punnett square is a tool that can be used to predict the results of a testcross. The Punnett square organizes the parental alleles and their possible combinations into a grid, showing which alleles could combine during fertilization, thus informing us of the potential genotypes of the offspring.