Inbreeding & Hardy-Weinberg Equilibrium

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

The Hardy-Weinberg equilibrium allows scientists to predict the frequency of alleles in a population. We will be discussing inbreeding and how that impacts a population's Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium

Mating happens in every species. A species in a given area is known as a population. Why do those two facts matter to each other? Well, members of a species can only mate with other members that are in the same area.

In other words, a brown bear in the United States would not be able to mate with a brown bear in England because they are too far away from each other. What this also means is that any of the version of genes, known as alleles, in the United States' bear population is likely to be different from the alleles in the brown bears in England.

Scientists realized that if the distance between populations has an impact on the number of alleles, then there must be other factors that also impact the number of alleles. They determined that rather than looking at the literal number of alleles in a population, it might be easier to look at allelic frequency.

Putting all those thoughts together led to the Hardy-Weinberg principle, which states that the frequency of alleles will remain stable in a population as long as a certain set of factors are met. When a population meets the factors, it is said to be in Hardy-Weinberg equilibrium. We should expect to see a certain amount of homozygous and heterozygous genes. Homozygous means that both alleles for a particular gene are the same. Heterozygous means that there are two different alleles for the same gene.

Hardy-Weinberg Equilibrium Factors

Let's get a brief rundown of the factors that have to be met in order for the Hardy-Weinberg principle to hold true. The first one deals with the distance aspect previously mentioned. If separate populations were to get close enough to mate, then the frequency of alleles would change and not be predictable. This would cause alleles to enter and exit the population. In order for a population to be in Hardy-Weinberg equilibrium, then alleles should not completely enter or exit the population.

Members of the species should not have any reproductive advantages over other members of the species. Having a reproductive advantage would mean that alleles in members with the advantage will be passed on more, and the frequency of their alleles will increase. The inverse is also true. The frequency of the alleles in those with a disadvantage will decrease.

A larger population is going to pass its alleles more regularly than smaller populations and the likelihood of each allele occurring remains the same, mainly due to more mating occurring. In a smaller population, though, less mating occurs. So, the frequency of the alleles is going to change over generations because there are some alleles that won't get passed on.

Another factor that has to be met is that members of the population must mate randomly. This gives every single member an equal chance to mate and therefore pass along its alleles. If the mating is not random, then certain alleles will get passed on more than others because mates that are more desirable will be the only ones passing on their alleles.

You can think of this similarly to what happens when people are trying to breed dogs or horses. People choose the dogs or horses with the more desirable characteristics, so they can pass their alleles on to future generations. The ones without the alleles for the desired characteristics are not going to get to pass their alleles. This will cause a change in the frequency of both alleles.

Inbreeding

In regards to random mating, one thing that can interrupt this randomness is when inbreeding occurs, which is the mating of closely related individuals.

The mating of P1 and P2 is an example of inbreeding
Diagram of inbreeding

The more closely related two individuals are to each other, the more alleles they are going to have in common. The more inbreeding that occurs within a population, the more it will affect the frequency of alleles.

Think about it. If you look at siblings, they are going to have some characteristics that are exactly alike because they came from the same two parents. There would be some alleles that are probably not present in any of the siblings. If the siblings were mated with each other, they can only pass on the alleles that they have. As a result, the frequency of those alleles will increase, while the frequency of the alleles that they do not have would decrease within the population.

This fluctuation in allelic frequency could cause a population to experience inbreeding depression. This is when alleles that make individuals in the population less fit are continuously passed on to the point that the population becomes less fit for survival.

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