Heterozygous Calculators

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Heterozygous describes a diploid organism that carries two different alleles for a specific gene locus — one on each homologous chromosome. In contrast, a homozygous individual carries identical alleles at a locus (either homozygous dominant AA or homozygous recessive aa). Heterozygosity (the proportion of heterozygous loci across the genome) is a key measure of genetic diversity in populations — reduced by inbreeding and genetic drift, increased by gene flow and disruptive selection. Heterozygote advantage (overdominance) describes cases where heterozygotes have higher fitness than either homozygote, as classically demonstrated by sickle cell trait.

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Heterozygous vs. Homozygous

  • Heterozygous (Aa): Two different alleles at a locus; one from each parent; phenotype depends on dominance relationship
  • Homozygous dominant (AA): Two dominant alleles; shows dominant phenotype
  • Homozygous recessive (aa): Two recessive alleles; shows recessive phenotype

In incomplete dominance: heterozygote (Aa) has intermediate phenotype (pink if AA = red, aa = white). In codominance: heterozygote (IᴬIᴮ) expresses both alleles fully (blood type AB).

Carrier Status

In autosomal recessive diseases, heterozygous individuals are 'carriers' — they carry one copy of the disease allele but show no symptoms (the normal allele provides sufficient function). Two carriers × two carriers → 25% chance of affected (aa) child. Examples: cystic fibrosis (CFTR gene); sickle cell anemia (HBB gene); PKU (PAH gene); Tay-Sachs (HEXA gene).

Heterozygosity (H)

Expected heterozygosity: H_e = 1 − Σpᵢ² (where pᵢ = frequency of allele i). Measures genetic diversity at a locus. Genome-wide H_e is used in conservation genetics to assess inbreeding depression risk.

Heterozygote Advantage

Sickle cell trait (HbA/HbS heterozygotes): resistant to severe malaria (Plasmodium falciparum); homozygous normal (HbA/HbA) susceptible to malaria; homozygous sickle (HbS/HbS) has sickle cell disease. Selection maintains both alleles in malaria-endemic regions (balanced polymorphism).

Glossary

Heterozygous
Carrying two different alleles at a locus (Aa); phenotype depends on dominance; carriers of recessive disease alleles are heterozygous; contrasts with homozygous (AA or aa).
Carrier
A heterozygous individual carrying one copy of a recessive disease allele; usually unaffected; two carriers have a 25% probability of producing an affected offspring.
Heterozygote Advantage
Higher fitness of heterozygotes relative to both homozygotes; maintains both alleles in the population (balanced polymorphism); classic example: sickle cell trait provides malaria resistance.

Frequently Asked Questions

A heterozygous individual carries two different alleles at a specific gene locus — one allele inherited from each parent. Written as Aa (where A = dominant allele, a = recessive allele). The phenotype depends on the dominance relationship: complete dominance: Aa shows the dominant A phenotype (same as AA). Incomplete dominance: Aa shows an intermediate phenotype. Codominance: Aa expresses both alleles fully. X-linked loci: males (XY) are hemizygous — only one allele per X-linked gene; not truly heterozygous or homozygous in the traditional sense.

A carrier is a heterozygous (Aa) individual who carries one copy of a recessive disease allele but is typically unaffected because the functional dominant allele (A) provides sufficient protein. Carriers are clinically important because: two carriers have a 25% chance per pregnancy of having an affected child (aa). Carrier frequency can be estimated from disease frequency using Hardy-Weinberg: if disease frequency = q² = 1/10,000 → q = 0.01 → carrier frequency = 2pq ≈ 2 × 0.99 × 0.01 = 0.02 = 1 in 50. Carrier screening is offered for common recessive diseases (cystic fibrosis, sickle cell, Fragile X) before or during pregnancy.

Heterozygote advantage (overdominance) occurs when the heterozygote Aa has higher fitness than both homozygotes AA and aa, maintaining both alleles in the population (balanced polymorphism). Classic example: sickle cell trait (HbA/HbS) in malaria-endemic regions: HbA/HbA: normal hemoglobin; susceptible to Plasmodium falciparum malaria. HbA/HbS (heterozygote): sickle cell trait; significantly reduced malaria severity; near-normal health. HbS/HbS: sickle cell disease; severe hemolytic anemia; high mortality without treatment. The heterozygote advantage in malaria-endemic regions (Africa, Mediterranean, Middle East) explains why the HbS allele has not been eliminated by selection despite causing disease in homozygotes.

Observed heterozygosity (H_o): proportion of individuals in a sample that are heterozygous at a locus: H_o = (number of heterozygotes) / N. Expected heterozygosity (H_e): the heterozygosity expected under Hardy-Weinberg equilibrium: H_e = 1 − Σpᵢ², where pᵢ = frequency of allele i. For a two-allele locus with p=0.6, q=0.4: H_e = 1 − (0.36 + 0.16) = 0.48. The inbreeding coefficient F = 1 − H_o/H_e — measures the deficit of observed vs. expected heterozygosity. F = 0: random mating; F = 1: complete inbreeding. Genome-wide H_e from SNP or microsatellite data is used in conservation genetics to assess population genetic health.