Inbreeding Depression

Key Insight

Excessive inbreeding significantly increases the risk of offspring inheriting two copies of deleterious recessive alleles, which can lead to severe health issues. For instance, cystic fibrosis, caused by a mutation on the CFTR gene, results from inheriting two mutated recessive alleles. While carriers with one copy may benefit from disease resistance, individuals with two copies suffer devastating symptoms, often leading to death before reproductive maturity. This increased risk of genetic diseases is a major problem with high levels of inbreeding.

Beyond specific genetic disorders, inbreeding reduces overall genetic variation within a population, making it vulnerable to drastic environmental changes. A genetically homogeneous group may struggle to adapt, as seen in bumblebee populations on Hebridean islands. The most inbred bumblebees, with the lowest genetic diversity, suffered the highest prevalence of the intestinal parasite Crithidia bombi. This lack of diversity, exacerbated by habitat fragmentation, threatens key pollinators essential for human food supply.

In humans, consanguineous unions—marriages between individuals descended from a recent common ancestor—elevate infant mortality, birth defects, and genetic diseases. A study of sibling and parent-child unions showed a 9.3 percent infant mortality rate and 40.8 percent congenital disease/intellectual handicap, compared to 5.3 percent and 4.5 percent in control groups. The Spanish Habsburg dynasty, through generations of close cousin and uncle-niece marriages, accumulated such extreme inbreeding that Charles II was more inbred than a child of full-blooded siblings, suffering severe physical and intellectual disabilities, ultimately ending his dynastic line and triggering the War of Spanish Succession.