Increased Disease Risk
Many genetic diseases are recessive, meaning only people who inherit two disease alleles develop the disease. All of us carry several single alleles for genetic diseases. Since close relatives have more genes in common than unrelated individuals, there is an increased chance that parents who are closely related will have the same disease alleles and thus have a child who is homozygous for a recessive disease.
For instance, cousins share approximately one-eighth or 12.5 percent of their alleles. So, at any locus the chance that cousins share an allele inherited from a common parent is one-eighth. The chance that their offspring will inherit this allele from both parents, if each parent has one copy of the allele, is one-fourth. Thus, the risk the offspring will inherit two copies of the same allele is 1/8 × 1/4, or 1/32, about 3 percent. If this allele is deleterious, then the homozygous child will be affected by the disease. Overall, the risk associated with having a child affected with a recessive disease as a result of a first cousin mating is approximately 3 percent, in addition to the background risk of 3 to 4 percent that all couples face.
Inbreeding can be measured by the inbreeding coefficient (often denoted F). This is the probability that two genes at any locus in one individual are identical by descent (have been inherited from a common ancestor). F is larger the more closely related the parents are. For example, the coefficient of inbreeding for an offspring of two siblings is one-fourth (0.25), for an offspring of two half-siblings it is one-eighth (0.125), and for an offspring of two first cousins it is one-sixteenth (0.0625). (This is a different calculation than the calculation of shared alleles between cousins, above.)
In general, inbreeding in human populations is rare. The average inbreeding coefficient is 0.03 for the Dunker population in Pennsylvania and 0.04 for islanders on Tristan da Cunha. Inbreeding occurs in both those populations. Some isolated populations actively avoid inbreeding and have maintained low average inbreeding coefficients even though they are small. For example, polar Eskimos have an average inbreeding coefficient that is less than 0.003.
Beneficial changes can also come from inbreeding, and inbreeding is practiced routinely in animal breeding to enhance specific characteristics, such as milk production or low fat-to-muscle ratios in cows. However, there can often be deleterious effects of such selective breeding when genes controlling unselected traits are influenced too. Generations of inbreeding decrease genetic diversity, and this can be problematic for a species. Some endangered species, which have had their mating groups reduced to very small numbers, are losing important diversity as a result of inbreeding.