Homozygous vs heterozygous
Quick answer: Homozygous genotypes have two identical alleles; heterozygous genotypes have two different alleles.
These words describe genotype composition at a locus.
Use the table rows first for rapid contrast, then read the similarity and difference sections to lock in exam-ready language. Finish with the “when to use which term” guidance before attempting the self-checks.
Side-by-side comparison
| Feature | Homozygous | Heterozygous |
|---|---|---|
| Alleles | AA or aa | Aa |
| Same or different | Same | Different |
| Dominance reveal | aa shows recessive; AA shows dominant | Shows dominant in simple dominance |
| Carrier role | aa affected for recessive trait; AA not a recessive carrier | Aa can carry recessive allele |
| Punnett use | Gametes all the same allele from that parent if homozygous | Gametes split alleles |
| Diversity | Less allelic diversity at locus | More allelic diversity at locus |
Similarities
Both describe diploid genotypes at a locus.
Both appear in Mendelian analysis.
Differences
Identity vs difference of the two alleles.
Phenotypic consequences depend on dominance.
When each term applies
Use homozygous/heterozygous whenever discussing allele pairs.
Combine with dominant/recessive for phenotypes.
Common confusion
Heterozygous is not “mixed phenotype” unless incomplete dominance applies.
Common questions
Is AA homozygous?
Yes.
Is Aa heterozygous?
Yes.
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