Final answer:
Humans and animal breeders generally avoid mating between close relatives due to the biological risks such as the inheritance of deleterious mutations and diminished gene flow, as well as societal incest taboos and cultural norms governing mating practices.
Step-by-step explanation:
People usually do not mate with close relatives due to the biological risks associated with inbreeding. Inbreeding increases the likelihood of offspring inheriting rare, deleterious mutations that can lead to harmful phenotypes, including genetic disorders and increased susceptibility to diseases. Moreover, inbreeding prevents gene flow, hindering the introduction of beneficial new genes into the population which can enhance fitness and diversity.
In human societies, there are also social and cultural norms, such as incest taboos, that prohibit mating between close relatives. These norms are reinforced by exogamy rules that aim to prevent genetic incompatibilities and promote the creation of socioeconomically stable households. Additionally, in the realm of animal breeding, avoiding inbreeding is considered a good practice as it helps maintain healthy, genetically diverse populations, reducing the expression of harmful or fatal mutations in domesticated animals.