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.
Examples of Dominant Alleles in Humans and Pea Plants
| Trait | Dominant Allele | Dominant Phenotype | Symbol |
|---|---|---|---|
| Pea seed colour | Yellow seed | Yellow | Y |
| Pea seed shape | Round seed | Round | R |
| Human earlobe | Free earlobe | Free | E |
| Human tongue roll | Roller | Can roll tongue | T |
| ABO blood type | A or B allele | Type A or B | I^A or I^B |
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Related Terms
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.
Phenotype
The phenotype is the observable set of physical and biochemical characteristics of an organism, resulting from the interaction of its genotype with the environment. It encompasses all visible traits such as eye colour, height, blood type, and behavioural tendencies. Understanding phenotype is fundamental to genetics because it reveals how genetic information is expressed in living organisms.
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 Latin "dominans" meaning "ruling" or "governing", the present participle of "dominare" (to rule). The term reflects the idea that the dominant allele "rules over" or masks the expression of the recessive allele when both are present.