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Inbreeding is breeding between close relatives. If practised repeatedly, it leads to a reduction in genetic diversity. Inbreeding often leads to reduced health and fitness (called consanguinity depression); however, livestock breeders often practice inbreeding, then cull unfit offspring, especially when they are trying to establish a new and desirable trait in their stock.

Within humans, the genetic problems caused by inbreeding is believed to be a factor in the prohibition against incest.

Inbreeding occurs in animals. For example, the cheetah is a highly inbred species, probably because of a population bottleneck in the species' recent past. Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model. Human genetic diversity is also limited, indicating a population bottleneck some 100,000 years ago.