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Heterozygote Advantage: Example, Overview

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  • 0:10 What Is Heterozygote…
  • 0:20 Heterozygote Advantage…
  • 0:58 Sickle Cell Trait
  • 2:09 Cystic Fibrosis
  • 2:55 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
If two copies of a harmful allele are bad, why would a single copy be good for you? It's true - heterozygotes can have an advantage. Discover how some genetic diseases survive by giving their carriers an edge over everybody else.

What Is Heterozygote Advantage?

Even though having two copies of a disease-causing allele is a bad thing, in some cases, it can be good to have a single copy. This advantage, which comes from being a heterozygote (or having mismatched alleles of a gene), is called the heterozygote advantage.

Heterozygote Advantage and Common Diseases

Natural selection would normally keep harmful alleles from becoming popular. If you carry genes for a genetic disease, your children will be more likely to die or get sick and eventually, your harmful allele would be gone from the population.

But if there is a heterozygote advantage, then carriers of the disease (people who are heterozygous, with one normal allele and one for the disease) will be more likely to survive than people without the disease allele. Since the allele helps survival, it will spread throughout the population. This seems to be why some genetic diseases are very common.

Sickle Cell Trait

Your red blood cells are essential to transport oxygen from your lungs to other parts of your body. They are normally round cells that can fit through the tight spaces of your smallest blood vessels.

Normal red blood cells are round. Sickled cells, not shown, are damaged in a way that affects their shape.
Normal red blood cells

But in people with sickle cell disease (who have two copies of the sickle cell allele), the cells can become misshapen. This can block blood flow, causing organ damage, and people with sickle cell disease are at risk of complications and may die in their 40s.

The allele is very common, though: up to 10-20% of people in certain parts of Africa carry at least one sickle cell allele. That's because there is a strong heterozygote advantage: people with one copy of the allele (we say they have the sickle cell trait) are resistant to the disease malaria.

In those same parts of Africa, mosquitoes can carry the malaria germ, a tiny parasite called plasmodium. It causes disease by burrowing into red blood cells. But in people with sickle cell trait, the red blood cells are resistant to malaria infection. That means people with the trait are protected from the disease and have an advantage over people without sickle cell trait.

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