Incest

Offspring of biologically related parents are subject to the possible impact of inbreeding. Such offspring have a higher possibility of (see ) because it increases the proportion of zygotes that are for deleterious that produce such disorders). Because most such are rare in populations, it is unlikely that two unrelated marriage partners will both be heterozygous carriers. However, because close relatives , the probability that any such rare deleterious allele present in the common ancestor will be inherited from both related parents is increased dramatically with respect to non-inbred couples. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. This has two contrary effects.[140]

• In the short term, because incestuous reproduction increases , deleterious recessive alleles will express themselves more frequently, leading to increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.


• In the long run, however, because of this increased exposure of deleterious recessive alleles to , their frequency decreases more rapidly in inbred population, leading to a "healthier" population (with fewer deleterious recessive alleles).

The closer two persons are related, the higher the zygosity, and thus the more severe the biological costs of inbreeding. This fact likely explains why inbreeding between close relatives, such as siblings, is less common than inbreeding between cousins.[141]

There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar may be more vulnerable to infectious diseases (see ).[142]

A 1994 study found a mean excess mortality with inbreeding among first cousins of 4.4%. A study of 29 offspring resulting from brother-sister or father-daughter incest found that 20 had congenital abnormalities, including four directly attributable to autosomal recessive alleles.[144]

Laws regarding sexual activity between close relatives vary considerably between jurisdictions, and depend on the type of sexual activity and the nature of the family relationship of the parties involved, as well as the age and sex of the parties. Prohibition of incest laws may extend to restrictions on marriage rights, which also vary between jurisdictions. Most jurisdictions prohibit parent-child and sibling marriages, while others also prohibit first-cousin and uncle-niece and aunt-nephew marriages. In most places, incest is illegal, regardless of the ages of the two partners. In other countries, incestuous relationships between consenting adults (with the age varying by location) are permitted, including in the , , and . is the only country that allows marriage between half-siblings and they must seek government counseling before marriage.[145]

While the legality of consensual incest varies by country, sexual assault committed against a relative is usually seen as a very serious crime. In some legal systems, the fact of a perpetrator being a close relative to the victim constitutes an in the case of sexual crimes such as and – this is the case in .[146]

Jewish[]

According to the , per , "the children of Israel"—Israelite men and women alike—are forbidden from sexual relations between people who are "near of kin" (verse 6), who are defined as: