Heterozygote advantage facts for kids

Heterozygote advantage is a cool idea in genetics. It means that if you have two different versions of a gene (one from each parent), you might be stronger or healthier than someone who has two identical versions of that gene. This happens when having both versions gives you a special benefit. It's like having the best of both worlds! This idea is part of something called balancing selection, which helps keep different gene versions in a population.

Why having two different genes can be better

Imagine a specific spot on a chromosome where a gene sits. This spot is called a gene locus. The different versions of a gene are called alleles.

• A heterozygote is someone who has two different alleles for a gene.
• A homozygote is someone who has two identical alleles for a gene.

When a heterozygote is stronger or healthier than both types of homozygotes, it's called heterozygote advantage. This means they have a better chance of surviving and having children. Scientists call this "greater fitness".

Sickle cell anaemia: A clear example

Let's look at sickle cell disease to understand this better. It's a great example of heterozygote advantage.

Understanding the genes

There are two main versions (alleles) of a gene that affects our haemoglobin (a protein in red blood cells):

• Hb⁺: This is the normal gene for healthy haemoglobin.
• Hb^s: This is a mutated gene that can cause sickle cell anaemia.

Based on these two alleles, people can have three different genotypes (gene combinations):

• Hb⁺Hb⁺: These people have two normal genes. Their red blood cells are healthy.
• Hb⁺Hb^s: These people have one normal gene and one sickle cell gene. They are called heterozygotes. They usually don't have sickle cell anaemia, but their red blood cells can sometimes "sickle" under stress.
• Hb^sHb^s: These people have two sickle cell genes. They suffer from sickle cell disease, which is a serious illness.

How environment affects health

The health and survival (fitness) of these three gene combinations can change depending on where people live.

Living in cooler climates

In cooler places, like parts of Europe or North America, people with the normal Hb⁺Hb⁺ genes and the heterozygotes (Hb⁺Hb^s) are generally the healthiest. People with two sickle cell genes (Hb^sHb^s) often have many health problems and might not live as long.

Living in tropical climates

In hot, tropical areas, especially near the equator in Africa, a disease called malaria is a huge problem. Malaria is spread by mosquitoes and can be deadly.

Here's where the heterozygote advantage comes in:

• People with Hb^sHb^s still suffer from sickle cell disease.
• People with Hb⁺Hb⁺ are healthy, but they can get very sick from malaria.
• People with Hb⁺Hb^s (the heterozygotes) have a special advantage! They don't get malaria as badly as those with two normal genes. This is because the sickle cell trait somehow protects them from the malaria parasite. They also don't suffer from the severe health problems of full-blown sickle cell anaemia.

Because of this protection against malaria, the Hb⁺Hb^s heterozygotes are the healthiest in these tropical regions. They have the best chance of surviving and having children. This is why the Hb^s gene is much more common in people from parts of Africa where malaria is widespread. In contrast, it's very rare in people from Europe.

Polymorphism: Keeping variety in genes

Heterozygote advantage is probably the main reason why we see so much polymorphism in living things. Polymorphism just means that there are different genetic types within a population. For example, different blood groups in humans are a type of polymorphism.

When heterozygotes are fitter, it helps to keep a wide range of different gene versions (alleles) in a population. This variety is like having many different tools in a toolbox. It gives the population more genetic "resources." This wide range of genes can be very helpful for a species, giving it a better chance to survive and adapt if the environment changes.

Related pages

• Hybrid vigour

See also

In Spanish: Ventaja heterocigótica para niños