AP Syllabus focus:
‘Heterozygote advantage occurs when heterozygous genotypes have higher relative fitness than either homozygous genotype.’
Heterozygote advantage is a powerful exception to the idea that natural selection always removes harmful alleles. By favoring heterozygotes, selection can maintain genetic variation and stable allele frequencies within populations across generations.
Core concept: heterozygote advantage and fitness
What heterozygote advantage means
Heterozygote advantage occurs when individuals with two different alleles at a locus (e.g., ) have higher fitness than either homozygote ( or ).
Practice Questions
FAQ
No. It can arise from any mechanism where combining two alleles improves performance, such as broader enzyme function, improved tolerance ranges, or more flexible physiological responses.
Common possibilities include:
Functional complementarity of two protein variants
Increased protein stability in heterozygotes
Optimal gene expression from having two regulatory alleles
By measuring survival and reproductive output for each genotype across multiple seasons or habitats, then comparing estimated $w$ values while controlling for age, sex, and population structure.
Because the advantage depends on carrying both alleles together; losing either allele reduces the production of the high-fitness heterozygote, so selection can maintain both alleles.
Yes. If environmental conditions change (e.g., pathogen prevalence shifts), the heterozygote may no longer have the highest $w$, and allele frequencies may then move towards fixation or loss.
