Inbreeding is the mating and production of offspring between individuals who are more closely related genetically than average members of a population. This practice significantly increases the genetic similarity between the parents, which dramatically elevates the risk that their offspring will inherit two copies of the same harmful gene. Every individual carries a few hidden, potentially damaging genetic variants. However, the probability of two closely related people carrying the same rare variant is much higher than for two unrelated people. This increased probability is the main reason why inbreeding is associated with a higher incidence of genetic defects and a decline in the health of the resulting offspring.
Understanding Recessive Alleles
An organism’s traits are determined by genes, which exist in different versions called alleles. Every individual inherits two alleles for each gene, one from each parent. Some alleles are dominant, meaning only one copy is needed for the trait to be expressed, while others are recessive, requiring two copies to show their effect.
Many genetic disorders, such as cystic fibrosis, are associated with defective recessive alleles. An individual who inherits one normal, dominant allele and one defective, recessive allele will not express the disorder because the healthy copy masks the defect. This individual is known as a carrier.
These defective recessive alleles persist at low frequencies in the general gene pool because they remain hidden within carriers. When two unrelated people reproduce, the chance of both individuals carrying the same rare, defective allele is statistically low. This mechanism explains why most recessive genetic conditions are rare in large populations.
The Mechanism of Increased Homozygosity
The negative consequences of inbreeding stem directly from shared ancestry and the resulting increase in homozygosity. Homozygosity is the state where an individual inherits two identical copies of an allele for a specific gene. Closely related parents are more likely to have inherited their genes from a recent common ancestor, meaning they share a higher proportion of their DNA than unrelated people.
Because related parents share more DNA, they are far more likely to both be carriers for the exact same set of rare, defective recessive alleles. For instance, the offspring of first cousins have a significantly elevated probability of receiving an identical allele from both parents, even for a very rare gene. This is a sharp increase compared to the general population.
When an offspring receives the identical defective recessive allele from both parents, they become homozygous for that gene. This pairing means there is no healthy, dominant allele to mask the defect, leading to the full expression of the genetic disorder. Geneticists describe this shared inheritance as “identical by descent.”
The degree of genetic relatedness directly correlates with this risk. The more closely related the parents, the greater the number of identical-by-descent alleles are passed on. The offspring of a parent-child or sibling-sibling union face a higher risk than the offspring of first cousins. This increased chance of pairing two copies of a defective allele is the fundamental reason why inbreeding causes the expression of harmful traits that would otherwise remain hidden.
Genetic Load and Inbreeding Depression
“Genetic load” refers to the accumulation of harmful mutations and defective alleles within a species’ gene pool. Every population carries a genetic load made up of hundreds of detrimental recessive genes. Inbreeding forces these hidden components into the open by making them homozygous.
The observable biological consequence of this increased homozygosity is known as inbreeding depression. This is the decline in fitness, viability, and reproductive success observed in the offspring resulting from inbreeding. Inbreeding depression is a general, cumulative toll caused by the expression of many slightly harmful recessive variants across the genome.
The effects manifest as a range of health issues and physiological deficiencies. These include reduced fertility, higher rates of infant and child mortality, and increased susceptibility to infectious diseases due to a weakened immune system. Specific congenital abnormalities and genetic disorders, such as certain forms of blindness, deafness, and congenital heart disease, are also observed at higher frequencies.
Real-world examples illustrate this impact. Historically, the Spanish Habsburg royal family practiced multiple generations of consanguineous marriages and displayed physical and health problems attributed to inbreeding depression. Similarly, intensive breeding practices used to create purebred dogs have resulted in many breeds having reduced genetic diversity and an increased prevalence of specific recessive illnesses.