However, most populations contain hundreds of alleles that could potentially cause disease and most people are heterozygotes for one or two recessive alleles that would be lethal in a homozygote.[9] Although the overall frequency of disease-causing alleles does not vary much between populations, the practice of consanguineous marriage (marriage between second cousins or closer relatives) is common in some Jewish communities, which produces a small increase in the number of children with congenital defects.[1]