Recessive gene

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 (if one is present) 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.

Contents

1 Nomenclature

2 Examples

2.1 Pea Plant
2.2 Sickle cell anemia

3 See also

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.

	T	t
T	TT	Tt
t	Tt	tt

Recessive genes are usually represented by a lowercase letter in a Punnett square, as opposed to the uppercase letters of dominant genes (see example at right). Using the letter "T" as an example, only in "tt" (the homozygous recessive genotype, indicated by blue) would the recessive physical trait appear. The form "Tt" is called heterozygous (indicated by magenta with a red border), and, even though a recessive allele is present, the dominant gene is the one that appears (becomes the phenotype). The homozygous dominant genotype is "TT" (indicated by red).

Examples

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Pea Plant

Gregor Mendel performed many experiments on pea plants while researching traits, chosen because of the simple and low variety of characteristics, as well as the short period of germination. He experimented with color (green vs. yellow), size (short vs. tall), pea texture (smooth vs. wrinkled), and many others.

For example, when testing the color of the pea plants, he chose two green plants, since green was more common than yellow. He mated them, and examined the offspring. He continued to mate only those that appeared green, and eventually, the yellow ones would stop being produced. He also mated the yellow ones together and determined that only yellow ones were produced.

He determined that this was because yellow was a recessive trait which only appeared when green, the dominant trait, was not present. Also, he determined that the dominant trait would be displayed whether or not the recessive trait was present.

Sickle cell anemia

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.