Consanguinity describes the union between two biologically related individuals, often colloquially referred to as inbreeding. This relationship is typically defined as having a common ancestor no more remote than a great-great-grandparent, meaning the couple shares a greater proportion of their genetic material than the general population. The most common form globally is between first cousins, who share approximately 12.5% of their genes inherited from a common grandparent. High rates of consanguinity are observed in populations across North Africa, the Middle East, and South Asia. Genetic risk assessment focuses on the increased probability of offspring inheriting identical copies of certain genes, which can lead to the expression of various health conditions.
The Underlying Genetic Mechanism
The increased probability of negative health outcomes in the offspring of related parents is due to increased homozygosity. An individual’s genes come in pairs, one allele inherited from each parent, and homozygosity means both copies of a specific gene are identical. All humans carry deleterious recessive alleles, which are gene variants that cause disease only when two copies are present. Since these alleles are recessive, one healthy copy is usually enough to mask the effect, meaning the parent is an unaffected carrier.
When parents are biologically related, they are more likely to have inherited the same rare, harmful recessive alleles from their shared ancestor. This shared ancestry increases the probability that their child will inherit two copies of that identical recessive allele, a state known as “identical by descent” or autozygosity. For a child of first cousins, approximately 6% of their genome is expected to be homozygous due to this shared descent. The expression of these previously silent, deleterious recessive traits is the mechanism by which consanguinity leads to a higher incidence of genetic disorders.
Physical and Cognitive Developmental Outcomes
Increased homozygosity manifests in a range of general developmental and structural issues in the offspring of consanguineous unions. One symptom is an increased rate of generalized congenital anomalies, which are structural or functional abnormalities present at birth. These include a higher prevalence of neural tube defects, such as spina bifida, and various forms of congenital heart defects compared to children of non-related parents. An increased risk of congenital hydrocephalus has also been observed.
Physical growth is frequently impacted, often resulting in reduced height and growth failure. Children from these unions often have lower height-for-age scores and a higher likelihood of being severely stunted. Subtle physical signs, such as fluctuating facial asymmetry, are also more common, reflecting minor disruptions in developmental stability. Significant cognitive developmental outcomes are observed, including lower average cognitive test scores and increased rates of intellectual disability.
Elevated Frequency of Inherited Diseases
The unmasking of deleterious recessive alleles leads to an elevated frequency of specific, diagnosable Mendelian recessive disorders. These conditions follow a clear pattern of inheritance and are more prevalent in consanguineous populations. For example, the risk of autosomal recessive disorders in the offspring of first cousins is estimated to be about double the risk in children of unrelated parents.
These conditions include metabolic disorders, which are errors in the body’s chemistry, and blood disorders like thalassemia and sickle cell anemia. Neurodegenerative disorders, such as Tay-Sachs disease, and certain forms of inherited deafness also show a higher prevalence. The increased genetic relatedness heightens the chance of both parents being carriers for the same rare mutation, leading to a greater risk of the child inheriting the disease.
Reduced Viability and Life Expectancy
The biological consequence of increased homozygosity is known as inbreeding depression, which manifests as a reduction in biological fitness and survival rates. This is evident in the higher rates of reproductive loss observed in consanguineous unions. These losses include elevated rates of miscarriage, stillbirth, and increased infant and childhood mortality compared to the general population.
Children born to first cousins face an increased risk of pre-reproductive mortality, with some calculations suggesting a risk that is approximately 3.5% to 4.4% higher than in non-consanguineous offspring. For those who survive to adulthood, there is evidence of a shorter average lifespan, with one study suggesting a reduction of over three years in life expectancy. This reduced viability reflects the cumulative effect of expressing multiple subtle, detrimental recessive traits that affect overall health and longevity.