Figure 1.

Pedigree of the Offspring of First Cousins

An example chromosome is illustrated. The female common ancestor is red. The chromosome inherited from one of her parents is colored red, and the chromosome inherited from her other parent is colored pink. The male common ancestor is blue. The chromosome inherited from one of his parents is colored dark blue, and the chromosome inherited from his other parent is colored light blue. The second generation are sisters. They share around 50% of their chromosomes IBD. The segments colored red and pink are segments inherited from their mother, and the segments colored dark and light blue are segments inherited from their father. The third generation are first cousins. In each case, the second (white) chromosome derives from their fathers (not shown), the red and pink segments are inherited from their maternal grandmother, and the dark and light blue segments are inherited from their maternal grandfather. The offspring of these first cousins has segments inherited from both founders on both copies of the chromosome. Where the same segments have been passed down both sides of the pedigree, the offspring of first cousins has extended identical-by-descent tracts or runs of homozygosity.