Dominant gene
In genetics, the term dominant gene refers to the allele that causes a phenotype that is seen in a heterozygous genotype. Every person has two copies of every gene, one from mother and one from father. If a genetic trait is dominant, a person only needs to inherit one copy of the gene for the trait to be expressed.
Dominance/recessiveness refers to phenotype, not genotype. Consider sickle cell anemia as an example. The sickle cell genotype is caused by a single base pair change in the beta-globin gene. There are several phenotypes associated with the sickle genotype:
- anemia (a recessive trait)
- blood cell sickling (partially dominant)
- altered beta-globin electrophoretic mobility (codominant)
- resistance to malaria (dominant).
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
A dominant gene when written in a genotype is always written before the recessive gene in a heterozygous pair. A heterozygous genotype is written Aa, not aA.
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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.
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.
See also
Categories
Articles to be merged since July 2006 | Classical genetics