Matings between first cousins undoubtedly carry a higher rate of autosomal recessive disorders and birth defects. The overall absolute risk from such a pairing is, however, not dramatic - it is only about 3% above the 2% background risk for serious congenital disorders.
This issue reached prominence in the UK in November 2005 with the call from Ann Cryer MP for consideration that first-cousin marriages within the UK Pakistani community be outlawed. While perhaps useful to raise the issue and encourage debate, this view deserves to be challenged with some actual facts.
The vast majority of kids born to consanguineous parents are perfectly normal. More productive public health drives might include reducing use of drugs including alcohol and tobacco during pregnancies in the general population.
Risks within historically inbred populations
Within a historically restricted genepool, the overall risks of an autosomal recessive disease for a first-cousin marriage are theoretically lower in comparison to non-first cousins than they would be in a non-inbred genepool. The reason for this is simple natural selection against autosomal recessive disease genes in general. Basically, the risk for the cousins is slightly decreased, while the risk for the non-cousins is increased. In such a population marriage outside the community offers significant advantage. This is complicated, however, by the fact that in inbred communities, first cousins may share more than 1/8 of their genes, since there will be a higher background rate of shared genes in the community.
Recessive or Dominant?
Another factor here skews the figures dramatically - in a consanguineous family, if a child is born with an abnormality, the initial presumption is that it is recessive. In a non-consanguineous family, recessive inheritance is the explanation of last resort. Hence, in our Pakistani population, there is a much higher ascertainment of recessive disorders than there is in the non-Pakistani population, so we end up with silly headline-grabbing snippets about how much all this costs the public.
Other conditions were mentioned in the debate, which are not relevant to "inbreeding". For example, Crouzon syndrome is an autosomal dominant condition, not recessive, so it is no more common among Pakistanis than anyone else. It (like haemophilia) has nothing to do with inbreeding. There are recessive craniosynostosis syndromes, but they're not Crouzon.
Haemophilia within the European Royal Families is another example which is sometimes cited as evidence of inbreeding, but it is X-linked recessive, and is NOT associated with inbreeding. The Tsars and Queen Victoria are off the hook.
Is the problem getting worse?
Do more people who are at a genetic 'disadvantage' now reach the age to reproduce, and so pass on their genes? is an interesting question, but it does require the reproduction of the affected individual, not just their survival. (More precisely, the propagation of their genes).
One possibility is that it could drive natural selection for genetic modifiers that work to decrease the severity of the disorder in affected individuals, hence potentially increasing the overall genetic variation present within the population. This is beneficial to the species (more variation), as we don't know where the selective pressures might come from in the future. It may help our survival as a species, if not as individuals.
The 3% excess quoted is an overall figure. The vast majority of consanguineous Pakistani families never come near the clinic, because all their kids are normal. You don't see them, hence the bias of ascertainment. It is also fair to say that we would not know the genetics of a lot of recessive disorders, but for the kind co-operation of these families in research (particularly microcephalin, DLL3, etc - there are many examples). The *absolute risk* remains small, although the *relative risk* is of course increased, as it is for any ethnic group, with a first cousin pairing.
But, even accepting that the adverse effects have been hyped out of all proportion, for the individual families involved, they represent a major issue, and they become highly visible within the community and the health service. Banning only serves to make the Pakistani (or whatever) community feel threatened. Education is a far nobler goal, and promises to be far more effective.