These results provide empirical risk figures in the counseling of first cousins. Although the relative risk for abnormal offspring is higher for related than for unrelated parents, it is still quite low: approximately double in the offspring of first cousins, compared with baseline risk figures for any abnormality of 15 to 20 per 1000 for any child, regardless of consanguinity. This increased risk is not exclusively for single-gene autosomal recessive diseases but includes the entire spectrum of single-gene and complex trait disorders. However, any couple, consanguineous or not, who has a child with a birth defect is at greater risk for having another child with a birth defect in a subsequent pregnancy.
These risk estimates for consanguinity may be slightly inflated given they are derived from communities in which first-cousin marriages are widespread and encouraged. These are societies in which the degree of relationship (coefficient of inbreeding) between two first cousins may actually be greater than the theoretical due to multiple other lines of relatedness (see Chapter 9). Furthermore, these same societies may also limit marriages to individuals from the same clan, leading to substantial population stratification, which also increases the rate of autosomal recessive disease beyond what might be expected based on mutant allele frequency alone (see Chapter 9).