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Dialogue around cousin marriage is a positive step, but it must be based on fact not fiction

22 March 2010
By Professor Marcus Pembrey
Chairman of the Progress Educational Trust, Emeritus Professor of Paediatric Genetics at the Institute of Child Health in London and Visiting Professor of Paediatric Genetics at Bristol University
Appeared in BioNews 550
A national awareness campaign, on the scale of smoking, obesity or Aids is needed to warn citizens of the risk of transmitting recessive genetic conditions to children born following cousin marriage, Baroness Ruth Deech will argue during a lecture taking place tomorrow at the London Museum. Her views and objectives were publicised in a report and accompanying analysis by Frances Gibb, published in The Times newspaper on Saturday. The report, however, makes some serious errors and does nothing to either clarify the true health impact of cousin marriage or help couples at risk of recessive genetic conditions.

Gibb reports that '55 per cent of British Pakistanis are married to first cousins and in Bradford the figure is 75 per cent.' Later in the article Baroness Deech is reported as saying that 'the local estimate was that 75 per cent of Bradford disabled children had cousin parents'. With these two statements, surely anyone can see that the latter statement tells us nothing about the connection between disability and cousin marriage, even though it is intended to imply a link.

Whilst Gibb's analysis quotes Baroness Deech saying (correctly) that in cousin marriage there will be 'twice as many sick children (four per cent) as others who are not related', she is contradicted by a box included in Gibb's accompanying analysis stating '13 - estimated number of times more likely that they will have children with genetic disorders than the general population.'

For Gibb to remind us in an article on cousin marriage that Queen Victoria and Albert had a son with haemophilia is disingenuous. As an X-linked disorder, cousin marriage has no effect on the risk of haemophilia in a son.

The suggestion that education should be coupled with the offer of genetic carrier screening (carrier screening for Tay-Sachs disease in the Orthodox Jewish community is given as an example) fails to recognise a more subtle but very important point. Population carrier screening is only practical for the commoner recessive genetic diseases and it is precisely these where cousin marriage has least relative impact on the population frequency of the disease. Cousin marriage increases the relative risk for rare or very rare recessive diseases, where general population carrier screening is not possible, except in the context of a family history of that rare disorder.

The two page spread makes no mention of the importance of reporting a family history of early death or disability, or of the importance of referral to genetic counselling services: the two actions that really can help families. In fact the co-operation of consanguineous families, where some members have a rare recessive disease, has allowed research that is increasingly able to identify the underlying genetic mutation and so develop carrier (and prenatal testing if requested) for that extended family and other families with the same rare disease.

However, further development of genetic services alone will not reach many families in need. I agree that there needs to be a much wider, open discussion of first cousin marriage, but this must be balanced, accurate and sensitive. This should start, at the top, with a concerted effort to dispel wrong beliefs about cousin marriage - beliefs often maintained by misleading statements from people who mean well. To put it straight:-

1) Common recessive diseases are not common because of consanguineous marriage.

2) Population genetic screening for common recessive disease may be a good thing, but does not address the adverse genetic effects of cousin marriage, which relate primarily to multiple rare recessive diseases.

3) Not all the excess of birth defects and early mortality in the children of consanguineous marriages compared to non-consanguineous progeny will be due to first cousin marriage per se.

4) Consanguinity rate is just one of many determinants of the differences in early mortality between ethnic groups.

SOURCES & REFERENCES
Darwin’s theory of evolution included marriage to his first cousin
The Times |  20 March 2010
Rise in marriages between cousins ‘is putting children’s health at risk’
The Times |  20 March 2010
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