In the process of being tamed, certain genes of a species may change or be lost. In particular, those breeders concerned with quantity rather than quality often breed close relatives. Inbred, the animals will slowly weaken.
在人們馴化過程，動物的某些基因會改變、遺失，尤其許多只求數量、不問品質的人工繁殖，常常都是近親交配，基因在人為介入之下，混合度不足，動物族群會逐漸衰弱。