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.
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.
Cystic fibrosis is the most common genetic disorder among people of European ancestry; about 1 in 25 is a carrier.
People with two copies of the gene have serious problems with fluid and mucous in their organs, and the disease can be fatal in childhood.Since cystic fibrosis is so common, scientists think there must be a heterozygote advantage. One theory is that heterozygotes are better at keeping fluid in their intestinal cells when they are sick with diarrhea. That’s a big deal. A deadly diarrheal disease called cholera used to be very common in Europe. Infected people would die because they lost so much water to diarrhea.
Since cholera is transmitted through water, modern sanitation has made it rare.
When a single copy of a disease allele doesn’t result in a disease but instead is good for the person or organism that carries it, we say that allele has a heterozygote advantage. One example is sickle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes. Another is cystic fibrosis, whose carriers may be protected from dying of dehydration from diseases like cholera.
- heterozygote: having mismatched alleles of a gene
- heterozygote advantage: carriers with one copy of a disease-causing allele are more likely to survive than people without the disease allele or with two copies
- red blood cells: round cells that can fit through the tight spaces of your smallest blood vessels
- sickle cell disease: disease in which red blood cells become misshapen, which can block blood flow and cause organ damage
- sickle cell trait: people with one copy of the sickle cell disease allele (heterozygotes) that protects against malaria (heterozygote advantage)
- cystic fibrosis: most common genetic disorder among people of European ancestry in which people can have serious problems with fluid and mucous in their organs
- cholera: a deadly diarrheal disease in which infected people would die due to dehydration; heterozygotes for cystic fibrosis have survival advantage for cholera
Upon finishing this lesson, you should be able to describe the heterozygote advantage and identify a couple of examples of how it helps with human survival.