What is the primary focus of a test cross in genetics?

The primary focus of a test cross in genetics is to determine the genotype of an individual that exhibits a dominant phenotype. This is achieved by crossing the dominant individual with a homozygous recessive individual. If the dominant individual is homozygous, all offspring will display the dominant trait. However, if the dominant individual is heterozygous, approximately half of the offspring will show the recessive phenotype. This method is a powerful way to reveal whether the dominant individual carries one or two copies of the dominant allele, thus providing insight into its genotype.

The other options, although related to different aspects of biology, do not pertain to the specific purpose of a test cross. For instance, assessing fertility of a population, analyzing chromosome inversions, and identifying pollination processes involve different experimental techniques and concepts in genetics and ecology that are not related to determining genotypes.