In a test cross, what is typically crossed with a homozygous recessive?

In a test cross, the primary aim is to determine the genotype of an organism exhibiting a dominant phenotype. This is accomplished by crossing it with a homozygous recessive organism, which has a known genotype.

Choosing a suspected heterozygote to cross allows us to observe the offspring and determine if the dominant phenotype parent is indeed heterozygous (carrying one dominant and one recessive allele) or homozygous dominant (carrying two dominant alleles). If the offspring exhibit both phenotypes (dominant and recessive), it indicates that the suspected parent is heterozygous. If all offspring display the dominant phenotype, the parent must be homozygous dominant.

The other options, while related to genetic crosses, do not serve the specific purpose of a test cross. A dominant homozygote and a homozygous dominant organism would not help identify the genotype of the unknown individual since they carry the same alleles. A hybrid organism might imply the same ambiguity in genotype as the dominant individual being tested. Thus, crossing with a suspected heterozygote is the most informative choice for a test cross.