Homozygous
An organism is homozygous for a particular gene when it carries two identical alleles at that genetic locus — either two dominant alleles (homozygous dominant, e.g., AA) or two recessive alleles (homozygous recessive, e.g., aa). Homozygous individuals breed true for the trait associated with those alleles, meaning all offspring from two homozygous parents with the same alleles will also be homozygous. This condition is significant in selective breeding and genetic disease risk assessment.
Comparison of Homozygous Dominant, Homozygous Recessive, and Heterozygous Genotypes
| Genotype Type | Example Genotype | Phenotype (Dominant) | Breeds True? |
|---|---|---|---|
| Homozygous dominant | AA | Dominant trait expressed | Yes |
| Homozygous recessive | aa | Recessive trait expressed | Yes |
| Heterozygous | Aa | Dominant trait expressed | No |
| Homozygous dominant (pea) | YY | Yellow seeds | Yes |
| Homozygous recessive (pea) | yy | Green seeds | Yes |
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Related Terms
Heterozygous
An organism is heterozygous for a particular gene when it carries two different alleles at that genetic locus — one dominant and one recessive (e.g., Aa). The dominant allele is expressed in the phenotype, while the recessive allele is carried but not expressed, making the individual a "carrier" of the recessive trait. Heterozygosity is important in population genetics because it maintains genetic diversity and affects the probability of recessive disorders appearing in offspring.
Recessive Allele
A recessive allele is a version of a gene whose trait is expressed in the phenotype only when two copies of it are present (i.e., in the homozygous state), because it is masked by a dominant allele when paired with one. Recessive alleles are conventionally represented by lowercase letters (e.g., "b" for blue eye colour). Many genetic disorders, such as cystic fibrosis and sickle cell anaemia, are caused by recessive alleles.
Punnett Square
A Punnett square is a diagram used to predict the probability of genotypes and phenotypes in the offspring of two parents, based on the alleles each parent can contribute. Developed by British geneticist Reginald Crundall Punnett in the early 20th century, it arranges parental alleles along the axes of a grid, and each cell represents a possible offspring genotype. It is a foundational tool in Mendelian genetics for calculating ratios such as 3:1 (phenotypic) or 1:2:1 (genotypic) for monohybrid crosses.
From Greek "homos" meaning "same" and "zygos" meaning "yoke" or "pair" — referring to a matched or identical pair. Combined with the suffix "-ous" to form an adjective. The term reflects the identical pairing of alleles at a locus.