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.
Examples of Recessive Alleles and Associated Phenotypes
| Trait | Recessive Allele | Recessive Phenotype | Condition for Expression |
|---|---|---|---|
| Pea seed colour | Green seed | Green | Must be homozygous (yy) |
| Pea seed shape | Wrinkled seed | Wrinkled | Must be homozygous (rr) |
| Human hair type | Straight hair | Straight | Must be homozygous (ss) |
| Cystic fibrosis | Mutant CFTR allele | Disease phenotype | Must be homozygous (ff) |
| Albinism | Non-functional OCA2 | No pigmentation | Must be homozygous (aa) |
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Related Terms
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.
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.
Genetic Disorder
A genetic disorder is a disease or condition caused by an abnormality in an individual's genome, which may involve a mutation in a single gene (monogenic), multiple genes (polygenic/multifactorial), or chromosomal abnormalities. Genetic disorders can be inherited from parents (hereditary) or arise as new mutations (de novo). They range widely in severity — from manageable conditions like colour blindness to life-threatening diseases like Huntington's disease — and understanding their genetic basis is crucial for diagnosis, treatment, and genetic counselling.
From Latin "recessus" meaning "a going back" or "withdrawal", derived from "recedere" (to recede or go back). The term describes how the allele's effect appears to "withdraw" or be hidden when paired with a dominant allele.