Dominant gene
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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. Dominant traits have a 50% chance to pass from parent to child.
That is 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: normal=GAG (glu), sickle=GTG (val). There are several phenotypes associated with the sickle genotype: 1) anemia (a recessive trait), 2) blood cell sickling (partially dominant), 3) altered beta-globin electrophoretic mobility (codominant), and 4) resistance to malaria (dominant). This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
Dominant negative
Most loss-of-function mutations are recessive. However, some are dominant and are called "dominant negative" mutations. Typically, a dominant negative mutation results in a protein that is structurally similar to the wild-type protein, but which has lost the normal function. Such proteins may be competitive inhibitors of the normal protein function.