Exemplar Problems

Question:

Define and design a test-cross.

Answer:

A test-cross is a genetic cross between an individual of unknown genotype (usually expressing the dominant phenotype) and a homozygous recessive individual (aa) to determine the genotype of the unknown individual.

Design of a test-cross:

Choose an individual with the dominant phenotype (e.g., A_) of an unknown genotype.

Cross it with a homozygous recessive individual (aa).

Observe the phenotypes of the offspring to determine if the unknown individual is homozygous dominant (if all offspring have the dominant phenotype) or heterozygous (if a 1:1 ratio of dominant to recessive phenotype is observed).