Non-Random Mating: Videos & Practice Problems

Non-random mating occurs when certain genotypes are more likely to mate, disrupting Hardy-Weinberg equilibrium by altering genotype frequencies without changing allele frequencies. This often leads to inbreeding, increasing homozygosity and potentially causing inbreeding depression, which reduces fitness due to the expression of deleterious recessive alleles. While non-random mating itself does not drive evolution, it sets the stage for natural selection to influence allele frequencies, ultimately affecting population dynamics and adaptation.

Non-Random Mating

Non-random mating occurs when certain genotypes are more likely to mate with each other, deviating from the assumptions of the Hardy-Weinberg equilibrium, which posits that all alleles in a population are mixed randomly. In non-random mating scenarios, the genotype frequencies are altered, pushing the population out of Hardy-Weinberg equilibrium, but importantly, the allele frequencies remain unchanged. This distinction is crucial because changes in allele frequencies are indicative of evolution.

It is essential to clarify that non-random mating is not synonymous with sexual selection. While non-random mating involves the pairing of alleles in a non-random manner, it assumes that all alleles still have the opportunity to be passed on. Sexual selection, on the other hand, focuses on the ability of organisms to obtain mates and successfully pass on their alleles.

One common form of non-random mating is inbreeding, which occurs when organisms mate with relatives, often due to geographical proximity. For instance, consider the distribution of white oak trees in the Eastern United States. Although these trees are wind-pollinated, pollen typically travels only about 200 meters. Therefore, an oak tree in Maine is unlikely to mate with one in Texas, as the wind cannot carry pollen that far. This localized mating leads to inbreeding, which increases homozygosity in the population, meaning there are more homozygotes than expected.

Increased homozygosity can result in inbreeding depression, characterized by a decrease in fitness due to the higher likelihood of pairing deleterious recessive alleles. Normally, these rare recessive alleles are masked by dominant alleles, but in inbred populations, the chance of these harmful alleles being expressed increases, leading to reduced fitness among individuals. While this decrease in fitness may suggest a potential for evolution, it is the subsequent natural selection that will drive changes in allele frequencies, ultimately leading to evolutionary changes in the population.

Non-Random Mating Example 1

In the study of population genetics, understanding the differences between random mating and inbreeding is crucial. In a scenario involving two populations, both with 1,000 individuals and identical allele frequencies (p = 0.5 and q = 0.5), we can analyze the genotype frequencies to determine the mating patterns. Under random mating, we expect the population to be in Hardy-Weinberg equilibrium, which means the genotype frequencies can be predicted using the equations:

\[p^2 + 2pq + q^2 = 1\]

Here, \(p^2\) represents the frequency of homozygous dominant individuals, \(2pq\) represents the frequency of heterozygous individuals, and \(q^2\) represents the frequency of homozygous recessive individuals. For our populations, calculating these values yields:

\[p^2 = 0.25, \quad 2pq = 0.5, \quad q^2 = 0.25\]

Multiplying these frequencies by the total number of individuals (1,000) confirms that the genotype frequencies in one population align perfectly with Hardy-Weinberg expectations, indicating random mating. In contrast, the other population exhibits an excess of homozygotes, suggesting inbreeding, which typically results in increased homozygosity and a deviation from Hardy-Weinberg equilibrium.

To summarize the relationship between allele and genotype frequencies, we can fill in the following statement: "For an actual population, knowing the genotype frequency will tell you the allele frequency. But knowing the allele frequency does not tell you the genotype frequency." This highlights that while genotype frequencies can be used to derive allele frequencies, the reverse is not necessarily true in real populations due to factors like inbreeding and non-random mating.

In a 2019 paper in Mammalian Genome, Chu et al. showed that golden retrievers with higher rates of inbreeding had fewer offspring, suggesting inbreeding depression. Which of the following conclusions could you draw about the population?

• A. Lack of gene flow between dog breeds is responsible for the fewer offspring.
• B. The small population size of golden retrievers has led to high rates of genetic drift in the breed.
• C. Those golden retrievers with fewer offspring likely have decreased fitness due to excess homozygosity.
• D. Natural selection is not occurring in golden retrievers.

Answer: (C)

Here’s what students ask on this topic:

What is non-random mating and how does it affect Hardy-Weinberg equilibrium?

Non-random mating occurs when certain genotypes are more likely to mate with each other, rather than mating randomly. This disrupts the Hardy-Weinberg equilibrium by altering genotype frequencies without changing allele frequencies. In a population under Hardy-Weinberg equilibrium, alleles are randomly paired, but non-random mating means that alleles are not paired randomly, leading to an increase in homozygosity. This can push the population out of Hardy-Weinberg equilibrium, but it does not cause evolution on its own because allele frequencies remain unchanged. Evolution requires changes in allele frequencies, which can occur through natural selection acting on the altered genotype frequencies.

How does inbreeding lead to inbreeding depression?

Inbreeding, which is mating between relatives, increases homozygosity in a population. This means there are more homozygotes than expected under random mating. Inbreeding depression occurs because this increase in homozygosity raises the likelihood of deleterious recessive alleles being paired together. Normally, these rare recessive alleles are masked by dominant alleles, but in an inbred population, they are more likely to be expressed. When these deleterious alleles are expressed, they can reduce the fitness of individuals, leading to a decrease in the overall fitness of the population. This fitness decrease due to the expression of harmful recessive traits is known as inbreeding depression.

What is the difference between non-random mating and sexual selection?

Non-random mating and sexual selection are different concepts. Non-random mating refers to the scenario where certain genotypes are more likely to mate with each other, altering genotype frequencies without changing allele frequencies. It assumes that all individuals have an equal chance to mate, but the pairing is not random. Sexual selection, on the other hand, involves the ability of individuals to obtain mates and pass on their alleles. It is a form of natural selection where certain traits increase an individual's chances of mating and reproducing. Sexual selection can lead to changes in allele frequencies and drive evolution, unlike non-random mating, which does not change allele frequencies on its own.

Why does non-random mating not cause evolution by itself?

Non-random mating does not cause evolution by itself because it does not change allele frequencies within a population. Evolution is defined as a change in allele frequencies over time. Non-random mating alters genotype frequencies by increasing homozygosity, but the overall allele frequencies remain the same. For evolution to occur, there must be a mechanism that changes allele frequencies, such as natural selection, genetic drift, or gene flow. Non-random mating can set the stage for natural selection to act on the altered genotype frequencies, potentially leading to changes in allele frequencies and driving evolution.

How does non-random mating increase homozygosity in a population?

Non-random mating increases homozygosity in a population by making certain genotypes more likely to mate with each other. When individuals mate with relatives or those in close proximity, the likelihood of pairing similar alleles increases. This results in a higher proportion of homozygous individuals (those with two identical alleles for a trait) compared to what would be expected under random mating. Over time, this increased homozygosity can lead to a higher expression of recessive alleles, including deleterious ones, which can affect the fitness of the population.