Genetic effects of marriage between cousins exaggerated

30 March 2010

Prominent UK academic lawyer Baroness Deech, who chaired the Human Fertilisation and Embryology Authority from 1994 to 2002, has caused controversy in the genetics community following media reports of a recent family law lecture she gave on cousin marriages.

An advance interview in the Times newspaper reported that one element of the lecture would be a call for a campaign to highlight the risks and the preventative measures” about the genetic consequences of consanguineous marriages, citing the example of screening for Tay-Sachs disease in Jewish communities, information from which is sometimes used in arranging marriages (see Tay Sachs Disease Carrier Screening for more information). The paper said that Baroness Deech would also propose in vitro fertilisation (IVF) and prenatal genetic diagnosis (PGD) for married cousins planning to have children – although in fact this was suggested only for couples with a family history of genetic disease.

Comment: There are some major flaws with the substance of this article as reported. Recessive genetic disorders affect children whose parents are both carriers of a causative mutation for that disease. Many healthy people carry such mutations, but it is rare for two parents to each pass on a mutation associated with the same disease, so that a child is affected. For example, the carrier rate in the general UK population for the disease cystic fibrosis (one of the most common genetic diseases) is around 1 in 25, or about 2 million, but the birth rate of children with cystic fibrosis is only around 1 in 2400, or about 240 each year (see Cystic Fibrosis Trust)

Recessive genetic disorders can affect any family, although their frequency may vary in different ethnic groups. Marriages between close relatives do significantly increase the probability that children will be affected by a recessive genetic disorder – but only if both cousins are carriers. Therefore, whilst increased awareness of the possibility of genetic disorders in communities where consanguineous marriages are common (including among health professionals) is a very good idea, the article (as opposed to the original speech) gives a false and potentially alarming impression of the true risks.

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