Three-quarter siblings share one parent, while the unshared parents are first-degree relatives to each other, for example if a man has children with two women who are sisters, or a woman has children with a man and his son. In the first case, the children are half-siblings as well as first cousins; in the second, the children are half-siblings as well as an avuncular pair. They are genetically closer than half siblings but less genetically close than full siblings, a degree of genetic relationship that is rare in humans and little-studied.