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The term "recessive gene" is part of the laws of Mendelian inheritance created by Gregor Mendel. Examples of recessive genes in Mendel's famous pea plant experiments include those that determine the color and shape of seed pods, and the plant height.
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. This suggests that the dominant phenotype is dependent upon having two dominant genes and the presence of one dominant and one recessive gene creates some blending of both dominant and recessive traits.
| 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).
Gregor Mendel performed many experiments on pea plants ( Pisum sativum) 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. By good fortune, the characteristics displayed by these plants clearly exhibited a dominant and recessive form. This is not true for many organisms.
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.
Mendel 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.
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 ( valValine is one of the 20 most common natural amino acids on Earth, and is coded for in DNA. Nutritionally, valine is also an essential amino acid. In Sickle Cell Anaemia, it subsitutes for the hydrophilic glutamic acid amino acid in haemoglobin, and becaus). There are several phenotypes associated with the sickle genotype:
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.