Heterozygote advantage (overdominance) is when individuals heterozygous for a gene have higher fitness than both homozygous types, which keeps two or more alleles in the population instead of selection wiping one out.
Heterozygote advantage, also called overdominance, is a situation where the heterozygote (think Aa) is fitter than either homozygote (AA or aa). The classic example is the sickle-cell allele. People with two normal alleles get malaria more easily, people with two sickle alleles get sickle-cell disease, but the heterozygotes resist malaria and avoid the worst of the disease. So in malaria-heavy regions, being in the middle wins.
Here's the cool part for evolution: natural selection usually pushes one allele up and the other down until the loser disappears. Heterozygote advantage does the opposite. Because the carriers are the most fit, selection actively protects both alleles, even the one that's harmful when doubled up. That keeps genetic variation in the gene pool instead of erasing it.
This concept lives in Unit 7: Natural Selection, and connects to Topic 7.6, Evidence of Evolution. Learning objective AP Bio 7.6.A asks you to describe the types of data that support evolution, and EK 7.6.A.1 specifically lists mathematical data as evidence. Heterozygote advantage is a textbook case of that math: allele frequencies stay stable at a balance point because of differential fitness, something you can model with the Hardy-Weinberg framework. It's also a clean demonstration of how selection doesn't always reduce variation. Sometimes it maintains it.
Keep studying AP Biology Unit 7
Fitness (Unit 7)
Heterozygote advantage is literally a fitness comparison. The heterozygote out-reproduces both homozygotes, and that fitness difference is the engine that keeps both alleles around. No fitness gap, no advantage.
Gene Pool (Unit 7)
Most selection trims alleles out of the gene pool. Heterozygote advantage does the reverse by maintaining a stable mix of alleles, which is why it's a go-to example of how variation gets preserved.
Homozygote (Unit 7)
You can't define heterozygote advantage without the two homozygotes it's beating. The whole point is that both AA and aa are less fit than Aa, so comparing all three genotypes is the move.
DNA Sequences (Unit 7)
EK 7.6.B.2 uses DNA and protein sequence comparisons as evidence for evolution. Sequencing the hemoglobin gene shows the exact single base change behind the sickle-cell allele, linking the molecular data to the fitness story.
Expect this in multiple-choice stems about why a harmful allele persists in a population, usually framed around sickle-cell and malaria. The trap answer is "the allele should disappear because it's harmful," so you need to explain that the heterozygote's higher fitness keeps it in. No released FRQ has used the exact term, but it fits the kind of evolution and Hardy-Weinberg reasoning AP rewards: you might be asked to explain why allele frequencies stay stable, or to interpret genotype frequency data showing the heterozygote favored. Your job is to connect a fitness difference among genotypes to maintained genetic variation.
Being heterozygous isn't the same as heterozygote advantage. Heterozygous just describes a genotype with two different alleles. Heterozygote advantage is the specific evolutionary situation where that genotype has higher fitness than both homozygotes, which is a claim about reproduction and survival, not just genetics.
Heterozygote advantage (overdominance) means the heterozygote has higher fitness than either homozygote.
The sickle-cell allele is the classic example because carriers resist malaria while avoiding full sickle-cell disease.
Unlike most selection, heterozygote advantage maintains genetic variation by protecting both alleles in the gene pool.
It explains why a harmful allele can persist at stable frequencies instead of being eliminated.
It connects to Topic 7.6 as mathematical and molecular evidence for how natural selection shapes allele frequencies.
It's when individuals heterozygous for a gene (Aa) have higher fitness than both homozygotes (AA and aa). This higher fitness keeps both alleles in the population instead of selection eliminating one, which is why it shows up in Unit 7 as a case where selection maintains variation.
Because heterozygotes have an advantage in malaria regions. People with one sickle allele resist malaria and don't get severe sickle-cell disease, so they out-reproduce both homozygotes. Selection actually keeps the allele around because of those fitter carriers.
No. Being heterozygous just means you have two different alleles for a gene. Heterozygote advantage is the specific situation where that heterozygous genotype has higher fitness than both homozygous genotypes, which is a statement about survival and reproduction.
Directional selection pushes a population toward one extreme allele and tends to reduce variation. Heterozygote advantage does the opposite by favoring the middle genotype, which preserves both alleles and keeps frequencies stable.
Yes, it fits Topic 7.6 and Unit 7. It commonly appears in multiple-choice questions asking why a harmful allele persists, and it supports the kind of Hardy-Weinberg and evolution reasoning that free-response questions reward.