Why does parental consanguinity increase the risk of some recessive metabolic disorders?

Parental consanguinity increases the risk of autosomal recessive metabolic disorders because related individuals are more likely to share a common genetic background.1

Most inborn errors of metabolism are recessive, meaning an individual must inherit two copies of a mutant gene—one from each parent—to manifest the disease. Because these specific mutant alleles are individually rare in the general population, it is statistically uncommon for two unrelated people to both be carriers of a mutation in the same gene.21

However, when parents are related, they have a higher probability of having inherited the same rare mutant gene from a common ancestor. Consequently, they are more likely to both be asymptomatic carriers of the same deleterious trait, significantly increasing the risk that their offspring will be homozygous for the mutation and thus affected by the disorder.21