Coefficient of Inbreeding describes the probability that an individual will have homozygous genes ie the gene will have the same two alleles. For each generation the probability reduces by a factor of four:

25%: a child of siblings, or father and daughter, or mother and son.

12.5%: a child of half-siblings, or double first cousins, or uncle and niece, or aunt and nephew

6.25%: a child of first cousins.

1.5625%: a child of second cousins.