Recessive

In genetics, the term "recessive gene" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). Simply put, an organism shows a recessive characteristic only if it has two recessive genes; otherwise, the characteristic of the recessive gene is concealed by the expression of the dominant 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.

Nomenclature

Technically, the term "recessive gene" is imprecise because it is not the gene that is recessive but the phenotype (or trait). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA and before molecular biology, thus mapping many newer concepts to "dominant" or "recessive" phenotypes is problematic. Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele.

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 gene is the one that shows up (becomes the phenotype).

Sickle-cell anemia example

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:

1. anemia (a recessive trait),
2. blood cell sickling (partially dominant),
3. altered beta-globin electrophoretic mobility (codominant), and
4. resistance to malaria (dominant).

This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.

See also

• Recessive trait
• Dominant gene
• Dominant trait
• Incomplete dominance
• Dominance relationship