Recessive allele
In genetics, the term "recessive gene" refers to an allele (form of a gene) that causes a phenotype (visible characteristic) that is only seen in a homozygous genotype (an organism that has two of the same allele). However, the term "recessive gene" is sloppy language use. Properly, it is the phenotype (or trait) which is recessive, not the gene.
The term "recessive gene" is part of the the laws of Mendelian_inheritance created by Gregor Mendel. Examples of recessive genes in Mendel's famous pea plant experiments include color and shape of seed pods, and plant height.
Recessive genes are usually represented by a lowercase letter in a Punnett square, as opposed to the uppercase letters of dominant genes. Using the letter "T" as an example, only in "tt" (the homozygous genotype) would the recessive physical trait appear. The form "Tt" is called heterozygous, and even though a recessive allele is present, the dominant phenotype is the one that shows up.
Dominance/recessiveness refers to phenotype, not genotype. An example to prove the point is sickle cell anemia. The sickle cell genotype is caused by a single base pair change in the beta-globin gene: normal=GAG (glu), sickle=GTG (val). There are several phenotypes associated with the sickle genotype:
- anemia (a recessive trait),
- blood cell sickling (partially dominant),
- altered beta-globin electrophoretic mobility (codominant), and
- resistance to malaria (dominant).
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.