Q: My fiancee and I are cousins and we have the same blood group of O positive. Will this cause a problem in future in having a child?

A:The risk for any couple of having a child with a serious or lethal medical condition in any population is around 2% (1 in 50). The excess risk for a couple who are related as first cousins, in the absence of a known genetic disease in the family, is around 3% (1 in 30). The excess risk is as a result of autosomal recessive conditions arising due to homozygosity by descent, that is, the risk of a recessive mutation present in an ancestor being passed down 2 branches of the family, and coming together in the consanguineous marriage. It is thought that we all carry at least one mutated allele, which would cause an autosomal recessive condition if present in two copies (homozygosity). If this mutant allele is passed down to both members of a consanguineous couple from a shared ancestor they will both be carriers for this condition, and will therefore have a 1 in 4 chance of having an affected child. The chance of both parents being carriers for a recessive condition is determined by how closely they are related, which means that the offspring risks can be minimised if marriages occur between unrelated persons or more distant relatives (e.g., second rather than first cousins). Carrier testing is advised for conditions known to be common in the relevant ethnic group. If there is a family history of a possible autosomal recessive condition, this may increase the offspring risks considerably over the background risks of consanguinity. If there is a known mutation in the family, the couple can be tested for their carrier status and prenatal diagnosis offered, if required. If the diagnosis or mutation is not known, investigation of an affected relative may be possible. Otherwise, an estimate of the risks involved and detailed fetal scans (if appropriate) are the only option.