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.