Inbreeding, Good or Bad?

Inbreeding is breeding between close relatives, whether plant or animal. If practiced repeatedly, it often leads to a reduction in genetic diversity, and the increased gene expression of recessive traits, resulting in inbreeding depression. This may result in inbred individuals exhibiting reduced health and fitness and lower levels of fertility. Livestock breeders often practice inbreeding to "fix" desirable characteristics within a population. However, they must then cull unfit offspring, especially when trying to establish the new and desirable trait in their stock. In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the heterosis effect. Inbreeding in plants also occurs naturally in the form of self-pollination. Inbreeding may result in a far higher expression of deleterious recessive genes within a population than would normally be expected.
As a result, first-generation inbred individuals are more likely to show physical and health defects, including: reduced fertility both in litter size and sperm viability increased genetic disorders fluctuating facial asymmetry lower birth rate higher infant mortality slower growth rate smaller adult size loss of immune system function.