Inbreeding

[Zipporah Schumaker]

The [breeding] of people/ animals/ [plants] that are closely related to each other (first cousins or even closer), thus keeping the genepool weak and leaving them [vulnerable] to various health problems, mentally and emotionally as well as physically.

[Tariq Zaki]

a situation in which plants, animals, or people are produced by breeding between closely related plants, animals, or people: