Homozygous genotypes are the ones where both alleles are the same: AA, aa, BB, bb, and so on. If you’re looking at a list of genotypes on a homework problem or exam, any pair with two identical letters is homozygous. Genotypes with two different letters, like Aa or Bb, are heterozygous.
How to Spot a Homozygous Genotype
Every person carries two copies of each gene, one inherited from each parent. A homozygous genotype means both of those copies are identical. In standard genetic notation, that looks like two of the same letter: AA, aa, BB, bb, RR, rr. It doesn’t matter whether the letters are uppercase or lowercase. What matters is that they match.
A heterozygous genotype, by contrast, has two different alleles at the same spot: Aa, Bb, Rr. The uppercase letter represents the dominant allele and the lowercase represents the recessive one. So if you see a mixed pair, that’s heterozygous. If the pair matches, it’s homozygous.
Here’s a quick reference for common genotype examples:
- AA — homozygous dominant
- aa — homozygous recessive
- Aa — heterozygous
- BB — homozygous dominant
- bb — homozygous recessive
- Bb — heterozygous
Homozygous Dominant vs. Homozygous Recessive
Both AA and aa are homozygous, but they express traits very differently. Homozygous dominant (AA) means you carry two copies of the dominant allele. Homozygous recessive (aa) means you carry two copies of the recessive allele. The distinction matters because recessive traits only show up in the homozygous recessive state. A person with Aa will look the same as someone with AA for most simple dominant traits, because the dominant allele masks the recessive one.
Recessive traits need both copies to match in order to appear. A blue-eyed person, for example, carries the homozygous recessive genotype (bb). Someone with straight hair carries the homozygous recessive genotype (hh), while curly or wavy hair involves at least one dominant allele. Dimples follow the same pattern: the recessive homozygous genotype (dd) means no dimples.
Why Homozygosity Matters in Health
Many genetic diseases only develop when a person is homozygous for a mutated gene. Cystic fibrosis requires two mutated copies of the CFTR gene. Sickle cell anemia requires two mutated copies of the HBB gene. Phenylketonuria requires two mutated copies of the PAH gene. In each case, inheriting just one mutated copy (the heterozygous state) typically makes someone a carrier without full symptoms.
That said, the line isn’t always clean. Researchers have found that some heterozygous carriers of recessive disease genes do experience mild, later-onset symptoms. These tend to fall on a spectrum between fully healthy individuals and those with two mutated copies. Still, the general rule holds: homozygous recessive genotypes produce the most severe expression of recessive conditions.
Some diseases work the opposite way. Huntington’s disease is caused by a dominant allele, so even the heterozygous genotype (one mutated copy, one normal copy) produces the full disease. Truly dominant conditions like this are relatively rare. More often, having two copies of a dominant mutation produces a more severe outcome than having just one, a pattern geneticists call incomplete dominance or semidominance.
Applying This to Genetics Problems
When a question asks “which of the following genotypes are homozygous,” you’re simply looking for matching allele pairs. Scan the list for any genotype where both letters are the same. TT is homozygous. Tt is not. tt is homozygous. It works the same way regardless of which gene or trait the question is about.
If the question also asks you to distinguish between homozygous dominant and homozygous recessive, remember: two uppercase letters (TT, AA, BB) are homozygous dominant, and two lowercase letters (tt, aa, bb) are homozygous recessive. Both are homozygous. The “dominant” or “recessive” label just tells you which version of the allele is present.
One more detail worth noting: organisms that reproduce primarily by self-fertilization, like pea plants (the ones Mendel famously studied), tend to be homozygous at most of their genes. That’s because selfing generation after generation drives allele pairs toward matching. Mendel used this to his advantage, starting his crosses with true-breeding lines that were homozygous, so he could predict exactly what the offspring would look like.