Endogamy

Explanation:

Endogamy is the practice of a person marrying another person from the same group as them. The type of group could be based on social groups, religion, race, or ethnic group. Prolonged endogamy might decrease the size of the gene pool and increase the chance of genetic disease. 

Additional Resources:

1. https://en.wikipedia.org/wiki/Endogamy

2. https://www.britannica.com/topic/endogamy