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Balancing Selection: Heterozygote Advantage & Frequency-Dependent Selection

Balancing Selection: Heterozygote Advantage & Frequency-Dependent Selection
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  • 0:00 Balancing Selection
  • 1:11 The Heterozygote Advantage
  • 3:21 Frequency-Dependent Selection
  • 4:57 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

You may have heard of natural selection, but what about balancing selection? This lesson will cover two important mechanisms that help keep genes in a population, which is important to populations if they don't want to become extinct!

Balancing Selection

As you learn about natural selection, which adapts a population to its environment through the selection of traits, you'll come to understand that certain traits are selected 'for' while others are selected 'against.' Traits that work well in the current environment are selected 'for', while those that are not beneficial to the population in that environment are selected 'against.'

But many traits remain hidden in a population throughout time instead of being completely removed by natural selection. This is because even though natural selection is a powerful vehicle of evolution, there are other mechanisms that work against natural selection in order to maintain diversity in a population's gene pool.

And this is important because diversity in a population is exactly what enables it to adapt and survive. Think about it - no environment is completely stable. Things are constantly changing, and if a population doesn't have the genetic diversity to change with it, it goes extinct. So, let's look at two mechanisms of balancing selection, which are selection mechanisms that maintain variation in a population: the heterozygote advantage and frequency-dependent selection.

The Heterozygote Advantage

Before you can understand the heterozygote advantage, we need to take a step back and recall what a heterozygote is. You may have learned in your genetics classes that alleles are just different forms of genes and genotypes, or genetic makeups, can be either homo- or heterozygous. Homozygous genotypes are those that have the same two alleles, such as two dominant or two recessive alleles. This makes sense because 'homo' means 'same.' Homozygous individuals exhibits a phenotype, or physical trait, that is the same as the genotype because both alleles are the same.

In contrast, heterozygous genotypes are those that have two different alleles, so one dominant and one recessive. Hetero means 'different,' so this makes sense, too. Heterozygous individuals will have a phenotype that matches the dominant allele, but because they carry that recessive allele they can still pass it on to the next generation of offspring.

Still with me? Good! Let's put it all together. A heterozygous individual has two different alleles, so the heterozygote advantage must be when heterozygous individuals have an advantage over homozygous individuals. Since we're talking about natural selection and evolution, this advantage is a reproductive one, of course, and since there's a reproductive advantage, both alleles are maintained in the population instead of just the dominant or recessive one.

A great example of this is the sickle-cell allele. Malaria is a deadly disease, but amazingly enough, those who are heterozygous for the sickle-cell allele are somewhat protected against it. The sickle-cell allele itself disfigures red blood cells, which are quickly destroyed by the body. If an individual has two sickle-cell alleles, they die because their body can't make functional red blood cells. However, those who have only one sickle-cell are not usually affected by the disease because they make enough healthy red blood cells for the body. And here's where the trick comes in: the parasite that causes malaria also causes sickling in red blood cells! The body destroys these cells quickly, ridding its host of the parasite. Amazingly, these individuals survive better than those who do not carry a single sickle-cell allele.

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