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Inbreeding

Inbreeding is the mating of closely related individuals, yielding offspring characterized as inbred. The principle biological consequence of inbreeding is decreased genetic variation in inbred individuals as compared to non-inbred individuals. This can manifest a higher incidence of diseases with a recessive genetic basis and other abnormal pregnancy outcomes (stillbirth, etc.). Because individuals in different societies (as well as different individuals within the same society) may have different definitions of “closely related,” some prefer the use of the term consanguinity, which refers to the existence of any common ancestor among a subject's parents (although in practice, such analysis rarely looks at relationships more distant than the second or third cousin or equivalent). It is also worth noting that distinctions between “inbreeding” and “consanguinity” are not uniformly agreed upon and that sources often use the two interchangeably.

Biological Significance

The relative importance of inbreeding in contributing to the incidence of disease depends largely on the prevalence of the recessive allele in the population at large. If the recessive allele in question is relatively common, then the impact of inbreeding or consanguinity will not be significant unless the frequency of inbreeding is such that it is the rule rather than the exception. A good example of this is Tay-Sachs disease and a population of Ashkenazi Jews in the United States. Carrier frequency in this population is thought to be as high as one in 30. If this population mated randomly, the incidence of Tay-Sachs would be sufficiently high that the increased incidence from sporadic inbreeding would be negligible. This incidence can be contrasted with that of the general public, which is one in 300. Because the background rate of Tay-Sachs in such a population is much lower, the influence of any inbreeding is necessarily greater.

It should be cautioned, however, that the risks of inbreeding are not so great as is popularly imagined. While the risk for particular (rare) conditions may be markedly increased when inbreeding is present, it should be remembered that the average, non-inbred individual contains 8 to 10 mutations for autosomal recessive disorders. Thus, while it has been estimated that three to five percent of pregnancies arising from the mating of first cousins produces an abnormality (stillbirth, neonatal death, or congenital malformation), the overall rate of birth abnormalities for the general population stands at two to three percent.

Inbreeding Around the World

The risk of inbreeding is only half the picture when considering the overall impact of inbreeding. Consideration must also be given to the prevalence of inbreeding. Unfortunately, there is a paucity of pop-ulation-level data on inbreeding prior to the end of World War II. While anecdotal evidence and sporadic studies on the phenomenon exist from before this time, there was not much serious outside scholarship looking at the issue, a key when considering a social practice considered unremarkable by those who practice it. What is known is that currently, rates of inbreeding vary significantly between countries and that those countries with high rates of inbreeding are concentrated in a region stretching from Morocco to southern India or China and east Asia depending on the definitions and survey methods used. Furthermore, immigrants from these countries continue the marriage practices of their home countries even in countries where such practices are not the norm.

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