Homozygous vs heterozygous: Definitions and examples
Table of contents
Most organisms that reproduce sexually carry two copies of each gene, one from each parent. These copies are called alleles.
- Homozygous means both alleles are the same:
AA(homozygous dominant) oraa(homozygous recessive). - Heterozygous means the two alleles differ:
Aa.
At a glance
| Homozygous dominant | Heterozygous | Homozygous recessive | |
|---|---|---|---|
| Genotype | AA | Aa | aa |
| Phenotype (complete dominance) | Dominant | Dominant | Recessive |
| Gamete types | 1 (A) | 2 (A, a) | 1 (a) |
| Also called | Pure-breeding / true-breeding | Hybrid / carrier | Pure-breeding / true-breeding |
Why it matters in crosses
Gamete variety. A homozygous parent can pass on only one kind of allele, while a heterozygous parent can pass on two. For two genes the effect multiplies. An AABB parent makes 1 type of gamete, AaBB makes 2 and AaBb makes 4.
That’s why fully heterozygous parents give the big 16-box dihybrid Punnett square.
True breeding. Two aa parents can only have aa offspring, so the trait “breeds true” every generation. Mendel started from true-breeding lines for this reason.
Carriers. A heterozygous person can carry a recessive allele without showing it. If two carriers (Aa × Aa) have children, each child has a ¼ chance of being aa. This is how recessive conditions such as cystic fibrosis can appear in a child of two unaffected parents.
Being homozygous or heterozygous for several genes
You describe each gene separately. AaBB is heterozygous for gene A and homozygous dominant for gene B. An organism that is heterozygous for two genes, AaBb, is called a dihybrid. That’s where the dihybrid cross gets its name.