Dominant allele

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In genetics, dominant allele refers to a genetic feature that hides the recessive allele. A dominant allele refers causes a phenotype that is seen in a heterozygous genotype. Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one gene (the dominant) will be found to effectively shut out the instructions from the other, recessive gene. For example, if a person has one gene for blue eyes and one for brown, that person will always have brown eyes because they are the dominant trait. For a person to have blue eyes, both their genes must be blue (recessive). When a person has two dominant alleles, they are referred to as homozygous dominant. If they have one dominant allele and one recessive allele, they are referred to as heterozygous.

A dominant allele when written in a genotype is always written before the recessive gene in a heterozygous pair. A heterozygous genotype is written Aa, not aA.

Usually, this masking effect is done by virtue of the fact that the recessive gene has a loss of some function that the dominant gene has. For example, in the case of ABO blood types, the O type is recessive because it does not produce any antigens or antibodies, whereas A and B types (which are codominant) do. Or, in the above case dealing with eye color, there is a complete loss of pigment in blue-eyed people, therefore to express the phenotype, both copies of the gene (after all, humans are diploid) must have that same loss of function.

Dominance/recessiveness refers to phenotype, not genotype.

1 Codominance and incomplete dominance
2 Dominant negative
3 References
4 See also

[edit] Codominance and incomplete dominance

In certain cases, a "blend" of genes will occur because neither of the two genes of a genotype are dominant over the other. As an example, in blood cells, the trait for blood type has three different alleles: type A, type B, or type O, with O being recessive. If a father passes a gamete with the allele of type A and the mother passes on type B, then codominance results, with the offspring being type AB since neither allele type dominates the other.

Incomplete dominance occurs when certain of the recessive gene appears within the phenotype of the organism, causing a blend in between both the dominant and recessive gene.

[edit] Dominant negative

A dominant negative mutation occurs when the gene product adversely affects the normal, wild-type gene product within the same cell. This usually occurs if the product can still interact with the same elements as the wild-type product, but block some aspect of its function.

Examples:

A mutation in a transcription factor that removes the activation domain, but still contains the DNA binding domain. This product can then block the wild-type transcription factor from binding the DNA site leading to reduced levels of gene activation.
A protein that is functional as a dimer. A mutation that removes the functional domain, but retains the dimerization domain would cause a dominate negative phenotype, because some fraction of protein dimers would be missing one of the functional domains.