Inbreeding and assortative mating

SIGNIFICANCE: Most models of population genetics assume that individuals mate at random. One common violation of this assumption is inbreeding, in which individuals mate with relatives, which over time results in inbreeding depression, a reduction in fitness. Another violation of random mating is assortative mating, or mating based on phenotype. Many traits of organisms, including pollination systems in plants and dispersal in animals, can be understood as mechanisms that reduce the frequency of inbreeding and the cost of inbreeding depression.

Random Mating and the Hardy-Weinberg Law

In 1908, soon after the rediscovery of Gregor Mendel’s rules of inheritance, British mathematician Godfrey Hardy and German physician Wilhelm Weinberg published a simple mathematical treatment of the effect of sexual reproduction on the distribution of genetic variation. This model showed a simple relationship between allele frequencies and genotypic frequencies in populations. An allele is simply a genetic variant of a particular gene; for example, blood type in humans is controlled by a single gene with three alleles (A, B, and O).

Nearly all mammals, including humans, are diploid. This means that each gene consists of two different alleles, one inherited from the mother and the other from the father. In the simplest case, if a gene has only two possible alleles (for example, A and a), it will have three different possible genotypes (AA, Aa, and aa). The Hardy-Weinberg predictions specify the frequencies of genotypes in a population—that is, how many will be homozygous, meaning they have two copies of the same allele (AA or aa), and how many will be heterozygous, meaning they have two different alleles (Aa).

Underlying the Hardy-Weinberg predictions is the assumption that gametes (sperm and egg cells) unite at random to form individuals, or that individuals pair randomly to produce offspring. An example of the first case is marine organisms such as oysters that release sperm and eggs into the water; zygotes (fertilized eggs) are formed when a single sperm finds a single egg. Exactly which sperm cell and which egg cell combine is expected to be unrelated to the specific allele each is carrying, so the union is said to be random. In cases in which males and females form pairs and produce offspring, it is assumed that individuals find mates without reference to the particular gene under examination. Humans do not choose potential mates at random, but they do mate at random with respect to most genetic variation. For instance, since few people in the Western world know (or care) about the blood type of their potential partners, people tend to mate at random with respect to blood-type alleles.

Inbreeding and are violations of this basic assumption. Inbreeding occurs when an individual mates with a relative rather than with a randomly drawn individual. (In outbreeding, the reverse is true). Assortative mating occurs when individuals make specific mate choices based on the or appearance of others. Each has somewhat different genetic consequences. When either occurs, the Hardy-Weinberg predictions are not met, and the relative proportion of homozygotes to heterozygotes is different from what is expected.

In humans, there can also be a relationship between genetic similarity among relatives and assortative mating. One 2024 study published in Nature Communications found that genetic similarity among relatives could provide evidence of assortative mating within that family many generations ago.

The Genetic Effects of Inbreeding

When relatives mate to produce offspring, the offspring may inherit an identical allele from each parent because related parents share many of the same alleles, inherited from their common ancestors. The closer the genetic relationship, the more alleles two individuals will share. Because of this, inbreeding increases the number of homozygotes for a particular gene in a population. It also increases the number of different homozygous genes in an individual. In either case, the degree of inbreeding can be measured by the level of homozygosity (the percentage or proportion of homozygotes relative to all individuals).

Inbreeding is exploited by researchers who want genetically uniform (completely homozygous) individuals for experiments. Fruit flies or mice can be made completely by repeated brother-sister matings. The increase in the frequency of homozygotes can be calculated for different degrees of inbreeding. Self-fertilization is the most extreme case of inbreeding, followed by sibling mating, and so forth. Sewall Wright pioneered computational methods to estimate the degree of inbreeding in many different circumstances. For self-fertilization, the degree of homozygosity increases by 50 percent each generation. For successive generations of brother-sister matings, the homozygosity increases by about 20 percent each generation.

Inbreeding Depression

Inbreeding commonly produces inbreeding depression in a population, a phenomenon characterized by poor health, lower growth rates, reduced fertility, and increased incidence of genetic diseases. Although there are several theoretical reasons why might occur, the major effects are produced by uncommon and deleterious recessive alleles. These alleles produce negative consequences for the individual when homozygous, but when they occur in a heterozygote, their negative effects are masked by the presence of the other allele. Because inbreeding increases the relative proportion of homozygotes in the population, many of these alleles are expressed, yielding reduced health and vigor. In some cases, the effects can be quite severe. For example, when researchers wish to create homozygous lines of the fruit fly Drosophila melanogaster by repeated brother-sister matings, 90 percent or more of the lines fail because of widespread genetic problems.

Assortative Mating

In assortative mating, the probability of particular pairings is affected by the phenotype of the individuals. In positive assortative matings, individuals are more likely to mate with others of the same phenotype, while in negative assortative mating, individuals are more likely to mate with others that are dissimilar. In both cases, the primary effect is to alter the expected genotypic frequencies in the population from those expected under the Hardy-Weinberg law. Positive assortative mating has much the same effect as inbreeding and increases the relative frequency of homozygotes. Negative assortative mating, as expected, has the opposite effect and increases the relative proportion of heterozygotes. Positive assortative mating has been demonstrated for a variety of traits in humans, including height and hair color.

Impact and Applications

The widespread detrimental consequences of inbreeding are believed to shape many aspects of the natural history of organisms. Many plant species have mechanisms developed through natural selection to increase outbreeding and avoid inbreeding. To prevent self-fertilization, a plant may release its pollen (male gametes) before its ovules (female gametes) are receptive, or it may have a genetically determined self-incompatibility. In most animals, self-fertilization is not possible, and there are often behavioral traits that further reduce the probability of inbreeding. In birds, males often breed near where they were born, while females disperse to new areas. In mammals, the reverse is generally true, and males disperse more widely. Humans appear to be an exception among the mammals, with a majority of cultures showing greater movement by females. These sex-biased dispersal patterns are best understood as mechanisms to prevent inbreeding.

In humans, individuals are unlikely to marry others with whom they were raised. This prevents the potentially detrimental consequences of inbreeding in matings with close relatives. This has also been demonstrated in some birds. Domestic animals and plants may become inbred if careful breeding programs are not followed. Many breeds of dogs exhibit a variety of genetic problems (hip problems, skull and jaw deformities, nervous temperament) that are likely caused by inbreeding. Conservation biologists who manage endangered or threatened populations must often consider inbreeding depression. In very small populations, such as species maintained in captivity (zoos) or in isolated natural populations, inbreeding may be hard to avoid. Inbreeding has been blamed for a variety of health defects in cheetahs and Florida panthers.

Key Terms

• alleleany of a number of possible genetic variants of a particular gene locus
• assortative matingmating that occurs when individuals make specific mate choices based on the phenotype or appearance of others
• heterozygotea diploid genotype that consists of two different alleles
• homozygotea diploid genotype that consists of two identical alleles
• inbreedingmating between genetically related individuals
• inbreeding depressiona reduction in the health and vigor of inbred offspring, a common and widespread phenomenon
• random matinga mating system in which each male gamete (sperm) is equally likely to combine with any female gamete (egg)

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