Saturday, July 09, 2005

Cousinly Love

by Merritt McKinney (this is a bit older news but still interesting)

New York, US

Though marriage between first cousins is illegal or restricted in most US states, the odds that a child whose parents are cousins will be born with a birth defect or genetic disorder are not nearly as high as often thought, according to a panel of experts. Compared with the offspring of unrelated parents, children whose parents are first cousins have an additional 1.7% to 2.8% higher risk of birth defects such as mental retardation or a genetic disorder, a panel organised by the National Society of Genetic Counselors estimates in the April issue of the Journal of Genetic Counseling.

Since the risk of most birth defects is in the low single digits, the increase in risk may double the odds that a child will be born with a health problem. But parents with some genetic diseases have a much higher rate of passing on their disorder to a child.

The panel states that cousins who want to have children do not need any special genetic counselling. As is the case with unrelated couples, they should be offered appropriate genetic testing based on their family history and ethnic background, the report indicates. But due to the somewhat increased risk of health problems, babies born to parents who are first or second cousins should undergo supplementary testing for metabolic disorders soon after birth, the panel advises. They also should be offered hearing tests by age 3 months.

One reason that relationships between cousins are often discouraged is the concern that first cousins - who share 12.5% of their genes - will pass on recessive genes to their offspring. Recessive genes do not cause disease in every generation, but can cause disease when a person inherits a recessive gene from each parent. "The closer the biological relationship between parents, the greater is the probability that their offspring will inherit identical copies of one or more detrimental recessive genes," the report states. Children born to first cousins, the authors note, will have two identical copies of 6.25% of their genes.

But the scientific evidence on the risk of birth defects in children born to cousins has not been conclusive, according to the task force. The panel was led by Robin L Bennett, of the University of Washington in Seattle, who is the president-elect of the National Society of Genetic Counselors, and Dr Arno G Motulsky, also at the University of Washington. The panel, which included genetic counsellors, physicians and epidemiologists, reviewed the results of six previously published studies that examined the risk of birth defects and other health problems in the children of first-cousin relationships.

Although cousins are often discouraged from marrying each other in North America, or even forbidden from doing so, the researchers point out that unions between relatives are common or even preferred in some parts of the world, especially the Middle East, Asia and Africa. "In some parts of the world," according to the report, "20% to 60% of all marriages are between close biological relatives." Due to immigration to the US from these areas, doctors and genetic counsellors may be encountering more patients who married to cousins. "Health providers should provide supportive counselling to these families and respect cultural belief systems," the authors assert.

"If you look at global populations, 20% of people are married to their cousins," Bennett told Reuters Health in an interview. There is a lot of stigma against cousin marriages, but there does not seem to be "good biological or social data to back up that stigma," according to Bennett. From a biological standpoint, "There's probably nothing wrong with it." She said she hopes that the report will "get rid of some of the stigma so people aren't afraid to tell their doctors."

Bennett pointed out that a child born to cousins is much less likely to be born with certain genetic health problems than children of people with certain genetic conditions. For example, Bennett said that people with Huntington's disease have a 50% chance of passing on the disease to their kids.

From the Journal of Genetic Counseling 2002;11:97-119

Source: 4 April 2002 Reuters Health

No comments: