Some animals may be genetically infertile.
Genetic mutations can occur which can produce infertility.
They can also give rise to abnormal development in embryos so much so that the young fail to develop properly and cause foetal death or atrophy. These are called lethal factors. Poultry seem to be more affected with lethal factors than other animals. A peculiar form of sterility occurs in heifers born twin to a bull. The male then remains normal but the female becomes a freemartin .