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.
Key Differences Between Homozygous and Heterozygous Organisms
| Feature | Homozygous | Heterozygous |
|---|---|---|
| Alleles at locus | Identical (AA or aa) | Different (Aa) |
| Phenotype | Matches allele type | Matches dominant allele |
| Breeds true? | Yes | No |
| Carrier status | Not a carrier (aa = affected) | Can be carrier of recessive |
| Offspring variation | Low | Higher variation possible |
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Related Terms
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.
Dominant Allele
A dominant allele is a version of a gene whose trait is expressed in the phenotype whenever at least one copy is present, regardless of whether the other allele is the same or different. Dominant alleles are conventionally represented by uppercase letters (e.g., "B" for brown eye colour). The concept of dominance was first described by Gregor Mendel through his experiments on pea plants in the 1860s.
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 "heteros" meaning "different" or "other" and "zygos" meaning "yoke" or "pair". The term describes a mismatched or unequal pairing of alleles at a genetic locus, contrasting with the identical pairing in homozygous organisms.