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Why does consanguinity increase the chance that a child will inherit identical alleles from a shared ancestor?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

Consanguinity increases the probability that a child will inherit identical alleles from a shared ancestor because it significantly raises the likelihood that both parents carry the same genetic material inherited from that common forebear.12

In genetics, an individual is considered homozygous for a specific allele if they possess identical copies at the same position on both paired chromosomes. When parents are consanguineous, they share a common ancestor, meaning there is a higher statistical probability that they both carry the same rare or recessive allele derived from that ancestor.312

The coefficient of inbreeding (F) quantifies this risk, representing the probability that two alleles in an offspring are identical by descent from a shared ancestor. For example, the offspring of a brother-sister mating has a 1 in 4 chance of inheriting a pair of identical alleles from a grandparent, while the offspring of first cousins has a 1 in 16 chance. This increased homozygosity is particularly significant in medical genetics, as it elevates the risk of expressing recessive genetic diseases that might otherwise remain masked in the population.2345