Consanguineous Marriage and Inbreeding: Genetic Effects of Cousin Marriage

A consanguineous marriage is a marriage between two people who share at least one recent common ancestor, most often first cousins or, in South India, an uncle and his niece. Its genetic consequence is inbreeding: the children are more likely to inherit two identical copies of a gene from that ancestor. Inbreeding raises homozygosity and the risk of recessive disorders, measured by Wright's coefficient of inbreeding (F), which is 1/16 for the child of first cousins. In India about one marriage in seven is consanguineous, with the highest rates in the south.

This note covers the meaning and types of consanguineous marriage, the coefficients of relationship and inbreeding with worked formulae, the biological effects of inbreeding, genetic load and inbreeding depression, the reasons for consanguineous marriage, its distribution in India, and classic case studies, for the UPSC Anthropology Optional. It falls under Paper I, topic 9.3 (inbreeding and its effect on gene frequencies) and relates to the marriage rules in topic 2.3 of the Anthropology Optional syllabus.

Key points at a glance

• Consanguinity: from the Latin consanguineus, "of the same blood"; relationship through a common ancestor.
• Inbreeding: mating between relatives; the genetic outcome of consanguineous marriage.
• The two terms: consanguinity is the qualitative, social description of a marriage; inbreeding is its quantitative, genetic measure.
• Measures: the coefficient of relationship (r) between two people and the coefficient of inbreeding (F) of their child, both due to Sewall Wright.
• Values: F = 1/8 for uncle-niece, 1/16 for first cousins, 1/64 for second cousins.
• Effects: more homozygosity; more recessive disorders; higher infant mortality and morbidity; inbreeding depression.
• India: consanguinity is preferred in much of the south and largely avoided in the north; Tamil Nadu has among the highest rates.

Mating and the genetic structure of populations

The pattern of mating plays a large part in shaping the genetic structure of a population. Random mating, assumed by the Hardy-Weinberg law, is rare in human societies, because marriage is governed by rules. Endogamy confines it within a group. Exogamy pushes it outside the lineage or clan. Preferential marriage with cousins or nieces draws it towards relatives. Consanguineous marriage is therefore where the marriage rules studied by social anthropology meet the gene frequencies studied by biological anthropology.

In India marriage practices vary greatly between communities. Consanguineous marriage has been practised since ancient times and is deeply rooted in belief and custom. It is frequent, even preferred, in some communities and taboo in others.

Consanguinity and inbreeding: the two terms

The children of consanguineous parents are, by definition, inbred. In the broadest sense all human beings are consanguineous, since all descend from common ancestors. The concept is useful only when the common ancestor is recent, usually within a few generations.

Types of consanguineous marriage

Consanguineous unions are classified by the closeness of the biological relationship, expressed in degrees.

Uncle-niece marriage

Uncle-niece marriage is a second-degree union. It may take four forms: a man with his brother's daughter or his sister's daughter, or a woman with her sister's son or her brother's son. In South India the usual form is marriage of a man with his elder sister's daughter.

First-cousin marriage

For a man there are four possible first-cousin marriages.

Genetically all four carry the same risk, since each pair of first cousins shares two grandparents. Socially they differ greatly: Hindu communities that permit cousin marriage generally allow only cross-cousins, while Muslim communities may permit all four (Alan Bittles).

Measuring relationship and inbreeding

Genetic relationship is measured by two standard coefficients, both introduced by Sewall Wright in 1922.

Coefficient of relationship (r)

The coefficient of relationship measures the degree of consanguinity between two individuals. It is the proportion of their genes that they share by descent from common ancestors.

r = Σ (1/2)ⁿ

Here n is the number of steps (parent-child links) separating the two individuals through a common ancestor, and the sum is taken over every path through every common ancestor.

Worked example: first cousins A and B. A and B share two grandparents. Through each grandparent the path runs A to parent, parent to grandparent, grandparent to B's parent, B's parent to B: four steps. Each path contributes (1/2)⁴ = 1/16. There are two such paths, one through each grandparent, so r = 2 × 1/16 = 1/8, or 0.125. First cousins share one-eighth of their genes by descent.

Coefficient of inbreeding (F)

The coefficient of inbreeding of an individual is the probability that the two alleles he or she carries at any locus are identical by descent, that is, copies of one allele in a common ancestor. Equivalently, it is the proportion of loci at which an individual is expected to be homozygous by descent.

F = Σ (1/2)ⁿ (1 + F_A)

• Σ: the sum over all paths connecting the two parents through a common ancestor.
• n: the number of individuals in each path, counting both parents and the common ancestor.
• F_A: the inbreeding coefficient of the common ancestor. If the ancestor is not inbred, F_A = 0 and the term in brackets is 1.

An equivalent form counts generations instead of individuals: F = Σ (1/2)^{n₁ + n₂ + 1} (1 + F_A), where n₁ and n₂ are the numbers of generations from each parent back to the common ancestor.

Worked example: the child of first cousins. Each path from one parent to the other through a shared grandparent contains five individuals: the father, his parent, the grandparent, the mother's parent and the mother. Each path contributes (1/2)⁵ = 1/32. With two grandparents, F = 2 × 1/32 = 1/16, or 0.0625.

The rule of thumb. Where the common ancestors are not themselves inbred, the inbreeding coefficient of a child is half the coefficient of relationship of its parents: F = r / 2.

Coefficients for different relationships

Adapted from standard human genetics texts, such as Vogel and Motulsky's Human Genetics.

Each step away from the common ancestor halves both coefficients. By the level of third cousins, the genetic effect is negligible.

The mean inbreeding coefficient of a population

To compare populations, the average inbreeding coefficient (α) is calculated:

α = Σ p_i F_i

Here p_i is the proportion of marriages of type i and F_i is the inbreeding coefficient of the children of that type. For example, if 20 per cent of marriages are between first cousins (F = 1/16) and 5 per cent between uncle and niece (F = 1/8), α = (0.20 × 0.0625) + (0.05 × 0.125) = 0.0125 + 0.00625 = 0.01875. Values of this order have been reported for some South Indian populations.

Why inbreeding matters: genotype frequencies

Inbreeding does not change allele frequencies. It changes how alleles are combined into genotypes. For a locus with alleles at frequencies p and q, the expected genotype frequencies among the children of parents with inbreeding coefficient F are:

Heterozygotes fall and both kinds of homozygote rise, each by Fpq.

Worked example. Take a harmful recessive allele with q = 0.01.

• Among children of unrelated parents, affected homozygotes = q² = 0.0001, or 1 in 10,000.
• Among children of first cousins, affected = 0.0001 + (1/16 × 0.99 × 0.01) ≈ 0.00072, or about 7 in 10,000.
• The risk is roughly seven times higher.

The rarer the allele, the greater the relative increase. This is why a large share of children with very rare recessive disorders are born to consanguineous parents, as Gunnar Dahlberg pointed out.

Biological consequences of inbreeding

Reported favourable effects

• Some studies report lower rates of intrauterine death and of mother-foetus incompatibility, such as Rh incompatibility, in consanguineous unions.
• A positive association with fertility has often been found, partly because consanguineous couples tend to marry younger and are less likely to separate.
• Over many generations, consistent inbreeding exposes harmful recessive alleles to selection and can reduce their frequency, a process called purging.

Adverse effects

• Recessive disorders. Increased homozygosity exposes deleterious recessive alleles. Conditions reported more often among the children of consanguineous parents include albinism, alkaptonuria, Tay-Sachs disease, cystic fibrosis, Ellis-van Creveld syndrome and certain congenital malformations.
• Mortality. There is general agreement that the children of consanguineous parents have higher postnatal and infant mortality. For first cousins, the excess is in the order of a few deaths per hundred children, against the background rate.
• Morbidity. Higher rates of congenital defects, intellectual and developmental disabilities, and some chronic conditions.
• Burden on communities. The load of genetic disorders is greatest where consanguineous marriage is culturally preferred over many generations.

Short-term and long-term effects

Genetic load

Genetic load is the reduction in the average fitness of a population caused by harmful genes, compared with a population in which every individual had the best possible genotype. H. J. Muller introduced the idea, and James Crow (1958) gave the standard definition:

L = (w_max − w̄) / w_max

Here w_max is the fitness of the best genotype and w̄ is the mean fitness of the population.

Harmful genes reduce fitness through death, disease and sterility. A dominant harmful gene acts in heterozygotes; a recessive one acts only in homozygotes, which is why inbreeding uncovers the hidden part of the load.

Lethal equivalents

N. E. Morton, J. F. Crow and H. J. Muller (1956) used the excess mortality among children of related parents to estimate the hidden load. A lethal equivalent is a set of harmful genes whose combined effect, if made homozygous, would on average cause one death. Their estimates suggested that each person carries a few lethal equivalents, masked in the heterozygous state.

Inbreeding depression

Inbreeding depression is the reduction in biological fitness, that is, in survival and reproduction, of a population as a result of inbreeding. It has been documented in plants, animals and humans.

• Small populations. In a small population, even random mating leads to relatedness between spouses, and drift reduces variation. A population bottleneck caused by flood, drought, epidemic or war can therefore produce inbreeding depression. The greater the genetic variation in a breeding population, the less likely it is to suffer.
• Purging. Continued inbreeding can reduce depression by removing harmful recessives, though often at a heavy cost in the short run.
• In humans. Studies have reported lower cognitive scores and higher rates of intellectual disability with rising inbreeding coefficients, and reduced height, weight and body mass index in inbred children. Non-genetic factors such as poverty and maternal age must be allowed for in such comparisons.
• A theoretical point. Inbreeding can raise an individual's inclusive fitness, since its offspring share more of its genes. Where the genetic cost is low, theory predicts some advantage to mating with kin. The cost of inbreeding is therefore of theoretical as well as practical interest.

Darwin's own case

Charles Darwin, who demonstrated inbreeding depression experimentally in plants, was himself married to his first cousin, Emma Wedgwood. Three of their ten children died before the age of ten, and three of the surviving children had long marriages without children. Darwin worried that the cousin marriage was responsible, and his son George Darwin later made one of the first statistical studies of cousin marriage in England. Later analyses of the Darwin-Wedgwood family have linked the high child mortality to the family's repeated intermarriage.

Why people marry relatives

Consanguineous marriage is common in many large populations of Asia and Africa, where between a fifth and a half of all unions may be consanguineous (Bittles).

Royal inbreeding

• Brother-sister marriage was practised in the royal families of ancient Egypt, the Inca and Hawaii. The Inca ruler was expected to marry his full sister.
• Reasons given include preserving the sacred royal line, keeping land and wealth within the family, allowing succession through both male and female lines, and myth.
• Pierre van den Berghe argued that in highly stratified societies the highest-ranking families become the most inbred, because suitable spouses of equal rank are few.
• The Spanish Habsburgs are the classic European case. Generations of uncle-niece and cousin marriage gave the last Habsburg king of Spain, Charles II, an inbreeding coefficient estimated to be close to that of the child of a brother and sister. He suffered serious ill-health and died without heirs.

Consanguineous marriage in India

Overall level

Consanguineous marriage is practised to some extent by most religious and ethnic groups in India (L. D. Sanghvi). The rate varies with region, religion, caste, tribe, language, socio-economic status, education, isolation and population size. National Family Health Survey data for 2015–16 put the share of consanguineous marriages at roughly one in seven.

Regional pattern

The contrast follows the kinship zones described by Irawati Karve: southern kinship favours marriage with kin, and northern kinship forbids it. Dravidian-speaking populations show the highest frequencies. See our note on Indian anthropologists.

Religious and social pattern

• Muslims have the highest rates nationally, and may marry any of the four first cousins. High rates have been reported among the Dawoodi Bohra and other communities.
• Buddhists and Christians also show substantial rates in national data, largely reflecting their concentration in the south and west.
• Hindus show high rates in the south, mainly matrilateral cross-cousin and uncle-niece marriage, and very low rates in the north.
• Sikhs have the lowest rates among the major religions.
• Caste categories. Rates are broadly similar across Scheduled Castes, Scheduled Tribes, Other Backward Classes and others nationally, with Other Backward Classes somewhat higher.
• Tribes. Many tribes of central and southern India prefer cross-cousin marriage. The Irula of Tamil Nadu have reported very high rates of first-cousin marriage. Tribes of the North-East generally show low inbreeding.

Survey figures differ depending on whether they count all consanguineous marriages or first-cousin marriages only, and which survey round is used. The pattern is more reliable than any single number.

Why South India favours consanguinity

• The long-standing preference for marriage with the mother's brother's daughter and with the elder sister's daughter.
• Keeping property within the family.
• Reducing dowry.
• Strengthening social and economic ties among kin.

Indian studies of the effects

Caution. Fertility and mortality also depend on non-genetic factors, such as the mother's age, birth spacing, poverty and access to health care. Consanguineous marriages in some communities involve younger brides, and early motherhood itself carries risks. State-level data on genetic disorders, especially from rural areas, are scarce, so published rates may not represent the true burden. Evidence on adult-onset conditions, such as some cancers and early heart disease, is still preliminary.

Classic studies of inbreeding

Haemophilia in the royal families of Europe

Queen Victoria of Britain carried a mutation for haemophilia, an X-linked recessive disorder. No ancestor of hers is known to have had the disease, so the mutation probably arose anew in her or in one of her parents. Through the marriages of her daughters and granddaughters into other royal houses, it reached the royal families of Spain, Prussia and Russia, where several sons, including the heir to the Russian throne, were affected. Analysis of DNA from the remains of the Russian imperial family identified the condition as haemophilia B. Strictly, this is a case of a new mutation spread by intermarriage among a small circle of royal families, not of a disease caused by inbreeding.

Japanese children after the Second World War

W. J. Schull and J. V. Neel studied the children of consanguineous and non-consanguineous marriages in Hiroshima and Nagasaki, using data gathered by the Atomic Bomb Casualty Commission, in an area where first-cousin marriage was common. They examined fertility, mortality, morbidity, reproductive performance and the physical and mental characteristics of the children. Inbreeding did not reduce the fertility of the marriages, but it was associated with a significant increase in mortality in the first year of life, with higher morbidity, and with more children affected by disabilities. The results were published in 1965 as The Effects of Inbreeding on Japanese Children.

The Amish, Mennonites and Hutterites

These Anabaptist groups settled in North America in the 18th and 19th centuries in search of religious freedom. Small, isolated farming communities and marriage within the faith limited the choice of partners and raised the level of consanguinity. The Hutterites live in communal colonies in the Dakotas and Canada, keep detailed genealogies, and are among the most fertile populations known. They have been studied extensively for the effects of inbreeding on fertility and complex traits (Carole Ober).

Consanguinity, counselling and policy

• A balanced view. For most couples the added risk from a first-cousin marriage is real but modest. Most children of such marriages are healthy. Alan Bittles has argued that the social benefits must be weighed alongside the genetic risks.
• Genetic counselling and screening. Carrier screening, especially in communities with known founder mutations or with thalassaemia and sickle cell, is more effective than discouraging cousin marriage in general.
• Law. The Hindu Marriage Act, 1955 prohibits marriage within certain degrees of relationship but allows it where the custom of both parties permits, which preserves South Indian cousin and uncle-niece marriage.
• Trend. Rising education, urban living and later marriage are reducing consanguinity in many communities.

For health programmes among tribal communities, see tribal India today.

Using this topic in a UPSC answer

• Define consanguinity and inbreeding, and distinguish them as social and genetic measures.
• Give the formulae for r and F, and work the first-cousin example.
• Reproduce the table of r and F for degrees of relationship.
• Show the genotype frequencies with inbreeding (q² + Fpq) and the worked example of a rare recessive.
• Discuss effects, inbreeding depression and genetic load.
• Present the Indian pattern: high in the south, low in the north, with reasons.
• Cite one classic study, such as Schull and Neel in Japan, and one Indian study.
• End with a balanced view and the role of genetic counselling.

For presentation, see our guide to anthropology answer writing with diagrams, thinkers and case studies.

Frequently asked questions

What is a consanguineous marriage?

A consanguineous marriage is a marriage between two people who share at least one recent common ancestor, such as first cousins, second cousins or an uncle and niece.

What is the difference between consanguinity and inbreeding?

Consanguinity describes a relationship or a marriage between relatives and is used mainly in the social sciences. Inbreeding is the genetic outcome of such a marriage, measured quantitatively by the coefficient of inbreeding.

What is the coefficient of inbreeding?

The coefficient of inbreeding (F), introduced by Sewall Wright, is the probability that the two alleles an individual carries at a locus are identical by descent from a common ancestor of the parents.

What is the coefficient of inbreeding for the child of first cousins?

It is 1/16, or 0.0625. First cousins share two grandparents, and each of the two paths through them contributes (1/2)⁵ = 1/32.

What is the coefficient of relationship?

The coefficient of relationship (r) is the proportion of genes two individuals share by descent from common ancestors. It is calculated as the sum of (1/2)ⁿ over all paths, where n is the number of steps between them. For first cousins it is 1/8.

How does inbreeding increase genetic disorders?

Inbreeding raises the chance that a child receives two copies of the same harmful recessive allele from a common ancestor. Recessive homozygotes increase from q² to q² + Fpq, so rare recessive disorders become several times more frequent.

What is inbreeding depression?

Inbreeding depression is the reduction in survival and reproduction caused by inbreeding, mainly because harmful recessive alleles become homozygous and are expressed.

What is genetic load?

Genetic load is the reduction in the average fitness of a population caused by harmful genes, compared with the fitness of the best possible genotype. James Crow defined it as (w_max − w̄) / w_max.

Where is consanguineous marriage most common in India?

It is most common in South India, particularly Tamil Nadu, Andhra Pradesh, Telangana and Karnataka, and in Lakshadweep, and is least common in the North-East and much of the north.

Why is consanguineous marriage preferred in South India?

Because of a long tradition of marriage with the mother's brother's daughter and the elder sister's daughter, and because it keeps property in the family, reduces dowry and strengthens kinship ties.

Does inbreeding change allele frequencies?

No. Inbreeding changes genotype frequencies by increasing homozygotes and reducing heterozygotes. Allele frequencies change only if selection then acts on the exposed homozygotes.

What did Schull and Neel find in Japan?

They found that inbreeding did not reduce the fertility of marriages but was associated with higher mortality in the first year of life, higher morbidity and more disabilities among the children of related parents.