Research on children in popul ations where first cousin marriages are common showed that although mortality rates are slightly higher, generations of such in-breeding can help to lower the incidence of genetic problems.
'In a perverse sense, this may be a way of cleaning up the genome. You are flushing out a higher percentage of deleterious genes than in a population that is not in-breeding at that high level.'
Dr Alan Bittles, from the human biology group at King's College, London, told the annual meeting of the American Association for the Advancement of Science that laws banning marriages between first cousins cannot be justified by genetics. 'I feel legislation against first cousin marriages is probably unnecessary.'
Speakers at the meeting, in Boston, explained that in-breeding is not harmful in itself, but is more likely to bring forward any harmful genes in the parents' genome. In many cases this results in early death. Congenital problems, such as children born deaf and blind, are also higher among the offspring of so-called consanguin ous marriages. But Dr Bittles said: 'I really don't think the excess mortality and morbidity warrants the intervention of rather heavy- handed legislation.' In the US, 30 states still prohibit first cousin marriages, and in 8 or 9 it is a criminal offence. Dr Bittles believes such laws can only be justified on sociological, not biological, grounds. Dr Bittles has studied 'close kin' marriages across the world.
He focused primarily on surveys in South India and Pakistan, where he found that mortality is higher among inbred progeny, especially in the first year of life. At first-cousin level in Pakistan, he found a 5 to 6 per cent increase in mortality. These excess deaths were due to detrimental recessive genes, inherited from a common ancestor, being expressed.
Professor James Neel, from the department of human genetics at the University of Michigan, gave results from studies he carried out in Japan shortly after the Second World War. He studied couples in Hiroshima, where 6 per cent of marriages were cousin marriages, and in Nagasaki, where the figure was 8 per cent.
He wanted to eliminate in- breeding as a factor in his studies of birth defects after the atomic bomb. 'This was probably the largest sample of cousin marriage offspring ever assembled in an industrialised country.' Professor Neel looked at couples outside the radius where radiation might have had an impact, and found much lower consanguinity effects than earlier literature predicted.
'This was significant. In the controls, by age 10 about 9 per cent of children had died, in the child of a first-cousin marriage the rate was about 10.5 per cent, not nearly as large we were led to believe.' He has examined these results again in the light of advances in our understanding of genetics over the past 25 years, and remains 'puzzled and confused'. He said that about 50 per cent of all pregnancies failed to go to term. 'It could be that this is the vehicle by which Mother Nature is cleaning up the genome.'Reuse content