A genotype is homozygous when both copies of a gene are the same allele, and heterozygous when the two copies are different. You can tell which one you’re looking at by reading the allele symbols, observing the organism’s physical traits, analyzing family crosses, or in some cases, using molecular lab techniques. The method depends on what information you have available.
Reading the Allele Symbols
The fastest way to identify zygosity is directly from the written genotype. In standard genetics notation, a capital letter represents a dominant allele and a lowercase letter represents the recessive version of the same gene. If both letters match, the genotype is homozygous. If they differ, it’s heterozygous.
Take a gene for seed color where Y stands for the dominant yellow allele and y stands for recessive green. The three possible genotypes are:
- YY: homozygous dominant (two copies of the dominant allele)
- yy: homozygous recessive (two copies of the recessive allele)
- Yy: heterozygous (one of each)
Genes are typically named after their recessive or mutant trait. A gene involved in albinism, for example, uses “a” for the recessive albino allele and “A” for the allele that produces normal pigment. So an individual written as Aa is heterozygous, carrying one functional copy and one nonfunctional copy.
When the Phenotype Gives It Away
Sometimes you can determine zygosity just by looking at the organism, but it depends on how the alleles interact with each other.
With simple dominance, you can only be sure about one situation from phenotype alone: if the organism shows the recessive trait, it must be homozygous recessive. A green pea, for instance, has to be yy because the dominant Y allele would mask the green color if it were present. But a yellow pea could be either YY or Yy, since one copy of the dominant allele is enough to produce the yellow phenotype. You need additional information (like a test cross) to distinguish between those two.
Incomplete dominance makes things easier. Here, the heterozygous individual displays a blended, intermediate trait that neither homozygous parent shows. The classic example is snapdragon flower color. A plant homozygous for the red allele produces red flowers, one homozygous for the white allele produces white flowers, and the heterozygous plant produces pink flowers. Pink is a dead giveaway for heterozygosity because neither homozygous genotype can produce it.
Codominance is similarly revealing but works differently. Instead of blending, both alleles show up fully and simultaneously. Human blood type is the textbook case: a person with one A allele and one B allele doesn’t get a blended blood type. They express both A and B surface molecules on their red blood cells, resulting in AB blood type. If someone has type AB blood, you know immediately they are heterozygous at that gene. Another example is sickle cell trait: a person heterozygous at the sickle cell gene produces both normal round red blood cells and sickle-shaped cells visible under a microscope.
Using a Test Cross
When an organism shows a dominant phenotype and you can’t tell whether it’s homozygous dominant or heterozygous, a test cross resolves the question. You cross the unknown individual with one that is homozygous recessive (the only genotype you can identify by phenotype alone for dominant traits).
If the unknown parent is homozygous dominant (YY), every offspring from the cross will receive one Y from that parent and one y from the homozygous recessive parent. All offspring will be Yy and show the dominant phenotype. If the unknown parent is heterozygous (Yy), about half the offspring will be Yy (dominant phenotype) and half will be yy (recessive phenotype). Seeing even one offspring with the recessive trait confirms the unknown parent was heterozygous.
In practice, you need a reasonably large number of offspring to be confident. A small litter that happens to all look dominant doesn’t rule out heterozygosity; you may have just gotten lucky. The more offspring you observe without any recessive individuals, the more confident you can be that the parent is homozygous dominant.
Punnett Squares and Predicted Ratios
Punnett squares let you predict what fraction of offspring will be homozygous or heterozygous from any given cross, which you can then compare against actual results.
When two heterozygous parents (Yy × Yy) are crossed, the expected genotypic ratio in the offspring is 1 YY : 2 Yy : 1 yy. That translates to a 25% chance of homozygous dominant, a 50% chance of heterozygous, and a 25% chance of homozygous recessive. The phenotypic ratio is 3:1 because both YY and Yy look the same.
If you cross a heterozygous parent with a homozygous recessive parent (Yy × yy), the expected ratio shifts to 1 Yy : 1 yy, or a 50/50 split between heterozygous and homozygous recessive offspring. Comparing actual offspring ratios to these predicted patterns helps you work backward to figure out whether the parents were heterozygous or homozygous.
Reading Family Pedigrees
In human genetics, you obviously can’t set up controlled crosses. Instead, you trace traits through a family tree (pedigree) and use logic to deduce genotypes.
The most reliable rule: if two parents both show a dominant trait but produce a child with the recessive trait, both parents must be heterozygous. There’s no other way for the child to have inherited two recessive alleles. For example, if two parents with normal pigmentation have a child with albinism, both parents carry one copy of the albinism allele (Aa), even though they show no sign of it themselves.
You can also work in the other direction. If a person shows the recessive phenotype (aa), each of their parents must carry at least one recessive allele. A parent who shows the dominant phenotype but has a child with the recessive trait is confirmed heterozygous. These deductions ripple through a pedigree, letting you assign genotypes to individuals who may have lived generations ago.
Molecular and Lab-Based Methods
At the DNA level, zygosity can be determined directly without relying on visible traits or breeding experiments. When a person’s DNA is sequenced at a particular gene, the readout shows the exact nucleotide sequence on both copies of the chromosome. If both copies have the same sequence, the site is homozygous. If one copy differs from the other, even by a single nucleotide, that position is heterozygous.
Techniques like Sanger sequencing can identify these differences, and the sequencing trace will show two overlapping peaks at a heterozygous position rather than one clean peak. Larger-scale approaches, such as exome sequencing (which reads the protein-coding portions of the genome), can scan thousands of genes at once and flag heterozygous sites throughout. For specific clinical questions, targeted sequencing of a known gene can confirm whether a patient carries one or two copies of a disease-associated variant.
These molecular methods are especially important for identifying carriers of recessive genetic conditions, people who are heterozygous and show no symptoms but can pass the allele to their children. Without DNA analysis, there’s often no phenotypic clue that they carry the allele at all.