What outcome indicates that an unknown parent is heterozygous in a test cross?

In a test cross, an individual with an unknown genotype is crossed with a homozygous recessive individual. When the unknown parent is heterozygous, it carries one dominant and one recessive allele for a particular trait. This results in offspring that can show a combination of phenotypes.

When the heterozygous parent is crossed with a homozygous recessive parent, approximately half of the offspring are expected to display the dominant trait (inheriting the dominant allele from the heterozygous parent) while the other half will display the recessive trait (inheriting the recessive allele from both parents). The presence of some offspring that show the recessive trait indicates that the unknown parent must contain a recessive allele, confirming its heterozygosity.

In contrast, if all offspring displayed the dominant trait, the unknown parent would likely be homozygous dominant. If all offspring displayed the recessive trait, it would mean the unknown parent was indeed homozygous recessive. The outcome of some offspring displaying the recessive trait is therefore the key indicator that the test cross parent is heterozygous.