Inbreeding, or consanguinity, refers to reproduction between individuals related by descent from a common ancestor. This practice significantly increases the probability of an offspring inheriting birth defects and genetic disorders. The genetic similarity between closely related parents raises this risk above that of the general population.
The Genetic Mechanism of Increased Risk
Every individual carries recessive genes that, if inherited in two copies, can lead to a genetic disorder. A person is typically a carrier, possessing one mutated copy and one healthy copy, and does not exhibit symptoms. When two unrelated people reproduce, the chance of them both carrying the exact same rare, disease-causing recessive gene is low.
When parents share recent common ancestry, such as being first cousins, the situation changes. Related individuals draw from a restricted family gene pool, making it more likely they inherited the same copies of genes, including rare recessive mutations, from their shared ancestor. If both parents are carriers for the same recessive trait, their child has a 25% chance of inheriting two copies of the mutated gene. This inheritance of two identical copies of an allele is called homozygosity, which “unmasks” the harmful recessive trait that was previously silent.
Specific Health Conditions Associated with Inbreeding
The heightened risk of homozygosity results in a higher occurrence of specific health issues in the offspring of closely related parents. These conditions include autosomal recessive disorders, which manifest only when two copies of the faulty gene are present. Examples include metabolic disorders like Phenylketonuria (PKU) or certain types of immune deficiencies, which are more prevalent in these populations.
Offspring of consanguineous unions also show a greater likelihood of developmental abnormalities. These birth defects can involve physical structures, such as congenital heart defects, limb malformations, or neural tube defects. The overall risk for a child to have a birth defect or genetic disorder rises from a baseline of about 2–3% in the general population to an estimated 4–6% for children of first-cousin couples.
Quantifying Genetic Risk
Scientists use the Coefficient of Inbreeding (CoI), represented by $F$, to quantify the increase in genetic risk due to shared ancestry. The CoI represents the probability that an individual will inherit two copies of a gene that are identical by descent. This measure provides a numerical value for the proportion of the offspring’s genome expected to be homozygous due to the parents’ relationship.
The value of the CoI is directly tied to the degree of biological closeness between the parents. For first cousins, the CoI is 0.0625, or 6.25%. This value jumps higher for closer relationships, such as uncle-niece or aunt-nephew unions, where the CoI is 0.125, or 12.5%. This quantification helps genetic professionals assess the probability of a child inheriting a recessive disorder.
Genetic Screening and Counseling
For related individuals considering having children, genetic counseling provides a valuable resource for risk assessment. A genetic counselor analyzes the family medical history to identify known conditions and estimate the couple’s specific risk for passing on a genetic disorder. This consultation provides accurate information to allow the couple to make informed reproductive decisions.
Expanded carrier screening (ECS) tests prospective parents for carrier status for hundreds of autosomal recessive disorders. Identifying that both partners are carriers for the same condition, which is more probable in consanguineous couples, allows them to understand the 25% risk for an affected child. Based on these results, couples can discuss reproductive options, including preimplantation genetic testing or prenatal diagnostic testing, to manage the identified risks.