Gene lethality occurs when two individuals of the same phenotype mate and a 2:1 phenotypic ratio exists in the offspring (as long as many offspring have been produced). Usually the homozygous dominant individuals will have been aborted before birth.
The number of offspring in a litter can also indicate whether gene lethality occurs. For example, on average, the litter size of a heterozygous x heterozygous cross will be less than a heterozygous x homozygous recessive cross if homozygous dominant is lethal.
