Marriage between members of any extended family group increases the risk that subsequent children will have an inherited disease. The risks decline however the larger the family group, according to a leading genetic researcher.
The Traveller Consanguinity Working Group yesterday published a position paper looking at the inherited disease risks for children born within marriages between cousins. It pointed out that consanguineous marriage [involving first or second cousins] "is not harmful in itself and does not necessarily cause genetic disorders".
Marriage between members of any close family group does however increase the chances of genetic disease, according to Prof Kevin Devine, head of the Genetics Department at Trinity College, Dublin. "It is a probability thing really," he said yesterday.
The Working Group in its position paper made reference to a study done by a Consanguinity Working Group within the US National Society of Genetic Counsellors. This body concluded that the risk of death before 10 years of age to the offspring of first cousins was between four and five per cent higher than the general population.
The increased risks are caused because if marriage partners are closely related there is a greater chance that both may be "carriers" of specific genetic diseases. If both parents carry matching errors or mutations in their genetic makeup then any children will have a one in four chance of actually developing the inherited disease linked to that mutation.
Consanguinity was important because it increased the chances of two carriers of a matching mutation meeting and marrying. "It becomes an issue because if you have 100 people and all of them are related, their chromosomes [genetic makeup] would have a higher frequency of coming up with matching mutations," Prof Devine said.
If the family group included 1,000 members the risk of matching mutations would be lower, and lower again as the size of group increased. "It is really the probability of finding someone with the same mutation," he added. "Within your own family you have a higher probability."
Having carrier parents does not in itself mean however that inheriting the disease is certain, the Traveller Working Group pointed out. Equally, the distribution of matching mutations amongst the Traveller population may actually affect only a few families. It urged the availability of genetic counselling for all Travellers and confidential testing services, in particular for families where children already have inherited diseases.