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Genetic recessiveness is linked to genetic dominance.
Every diploid organism, including humans, has two copies of every gene on autosomal chromosomes, one from the mother and one from the father. The dominant allele of a gene will always be expressed while the recessive allele of a gene will be expressed only if the organism has two recessive forms. Thus, if both parents are carriers of a recessive trait, there is a 25% chance with each child to show the recessive trait.
The term "recessive allele" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotypes (organisms that have two copies of the same allele) and never in heterozygous genotypes.
The term "recessive allele" is part of the laws of Mendelian inheritance formulated by Gregor Mendel. Examples of recessive traits in Mendel's famous pea plant experiments include the color and shape of seed pods and plant height.
Autosomal recessive alleleEdit
In opposition to autosomal dominant trait, a recessive trait only becomes phenotypically apparent when two similar alleles of a gene are present. In other words, the subject is homozygous for the trait.
The frequency of the carrier state can be calculated by the Hardy-Weinberg formula: (p is the frequency of one pair of alleles, and q = 1 − p is the frequency of the other pair of alleles.)
Recessive genetic disorders occur when both parents are carriers and each contributes an allele to the embryo, meaning these are not dominant genes. As both parents are heterozygous for the disorder, the chance of two disease alleles landing in one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous, the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.
Nomenclature of recessivenessEdit
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 alleles, and the presence of one dominant and one recessive allele creates some blending of both dominant and recessive traits.
Gregor Mendel performed many experiments on pea plant (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 yellow plants, since yellow was more common than green. He mated them, and examined the offspring. He continued to mate only those that appeared yellow, and eventually, the green ones would stop being produced. He also mated the green ones together and determined that only green ones were produced.
Mendel determined that this was because green was a recessive trait which only appeared when yellow, the dominant trait, was not present. Also, he determined that the dominant trait would be displayed whether or not the recessive trait was there.
Autosomal recessive disordersEdit
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 (co-dominant)
- altered beta-globin electrophoretic mobility (co-dominant)
- resistance to malaria (dominant)
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
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