Lethal genes (genes that result in the failure to develop a vital organ or metabolic pathway) are nearly always recessive. Animal breeders who discover a unique trait and selectively breed to increase the occurrence of that trait often encounter a noticeable increase in lethal genes. Why?A. The lethal recessive gene may be incompletely dominant.B. Spreading the gene among offspring of both sexes will increase the likelihood it will be sex-linked and expressed.C, The selective-mating of closely related individuals, or inbreeding, increases chances that two recessive genes will "meet" in offspring.D. "Pleiotropy" - the gene that is being selected for this trait may have the second effect of being lethal.E. "Epistasis" - selection for the desired trait may result in "uncovering" the lethal gene.

When breeders notice a unique trait of interest and selectively breed to increase their population, there is tendency of also increasing the population of lethal genes. This happens via a situation known as inbreeding depression. There is a tendency to mate closely related individuals in a bid to increase the occurrence of a desired phenotype. However, in a situation where these animals have a recessive lethal gene, it may result in a double recessive thus becoming obvious in the population.