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Marriage between first cousins, long a major legal, social and religious taboo, is far less likely to produce abnormal children than is commonly believed, a study by leading genetics researchers says.
''Stigma still attaches to these unions,'' says Robin Bennett, a genetics counselor at the University of Washington and the study's lead author.
''But there's no good social or biological reason that should be. . . . There's a lot of misinformation out there that is really holding back some cousins who want to try to have children,'' Bennett says.
Bennett's team, which included researchers from Stanford University and the National Society of Genetic Counselors, spent more than two years studying health statistics on the offspring of first-cousin marriages in North America, Africa, Asia and the Middle East.
The researchers concluded that children of marriages between cousins inherited recessive genetic disorders, such as cystic fibrosis and Tay-Sachs disease, in 7% to 8% of cases. For the general population, the rate was 5%.
The study suggests that doctors and genetics counselors not discourage cousins from procreating. Instead, it says, they should take family disease histories and offer ordinary genetic services such as fetal and newborn disease testing.
The study is in the April edition of the Journal of Genetic Counseling.
Despite the findings, some genetics specialists say they will continue to urge caution.
''A 7 to 8% chance (of genetic disorder) is 50% greater than a 5% chance,'' says Philip Reilly, geneticist and author of Abraham Lincoln's DNA, a popular history of human genetics.
''That's a significant difference. People counseling first cousins who want to marry need to be very careful and clear on this,'' Reilly says.
Scientists say there are at least 5,000 diseases caused by inherited mutations called recessive genes. Possessing a single copy of the mutation is often harmless, but if a copy is inherited from each parent, the result can be death or chronic disease. Because first cousins share a pair of grandparents, the chances are greater that each will pass a copy of a ''bad gene'' to their child, triggering the disorder.
Cousin marriage has been widespread in rural societies, where it serves to keep money and property within families. The practice is still popular in much of the Muslim world, including Saudi Arabia and Pakistan. Genetics researcher Alan Bittles estimates that 20% of marriages worldwide are between relatives who are first cousins.
Genetics counselors say there are no exact figures for the USA, but experience suggests that about one marriage in 1,000 is between first cousins. Jewish and Christian traditions discourage cousin marriage. The Roman Catholic Church requires cousins to get special permission before they marry.
In Brooklyn, N.Y., a rabbi maintains a database that allows Jews to see whether a potential marriage partner carries the recessive gene for Tay-Sachs, a fatal enzyme disorder that is prevalent in Jewish families.
Thirty states do not permit first-cousin marriages.