Final answer:
The genetic term used to describe the relationship between an uncle and his niece, when they have children together, is consanguinity.
Step-by-step explanation:
The genetic term used to describe the relationship between an uncle and his niece, when they have children together, is consanguinity. Consanguinity refers to the biological relationship between individuals who share a common ancestor. In this case, the uncle and niece share a close blood relationship, which is considered consanguineous.