Dominance (genetics)

Dominance is a relationship between alleles in which one allele’s effect masks another’s in a heterozygote. It describes a phenotype, not an allele’s prevalence or evolutionary advantage.

Haplosufficiency: A condition in which one functional gene copy produces enough gene product for a normal phenotype. One working allele can mask a loss-of-function allele when half the usual gene product is sufficient.
Allele: One of two or more alternative forms of a genetic sequence at a particular locus. Dominance describes how alternative alleles relate in a heterozygote.
Incomplete dominance: An inheritance pattern in which a heterozygote has an intermediate phenotype between the two homozygotes. Neither allele fully masks the other’s phenotypic effect.
ABO blood group system: A human blood-group system determined by A, B, or O alleles at the ABO gene. A and B are codominant, while each is dominant over O for red-cell antigens.
Pedigree analysis: The study of how traits and genetic variants are transmitted through family relationships. Dominant and recessive patterns help interpret which genotypes could explain a family history.
Haploinsufficiency: A condition in which one functional gene copy does not produce enough gene product for a normal phenotype. It explains why a loss-of-function allele can show a dominant effect rather than being masked.
Genotype: The genetic constitution of an organism or cell, often specified at particular loci. Dominance concerns the phenotype produced by a particular allele combination.
Codominance: An inheritance pattern in which both alleles’ effects are detectably expressed in a heterozygote. Both allele products appear in the phenotype instead of one masking the other.
Sickle-cell trait: The heterozygous state for the hemoglobin S allele, usually associated with both normal and sickle hemoglobin. Different molecular and clinical traits can show different dominance patterns for the same alleles.
Autosomal dominant inheritance: A pattern in which one pathogenic allele on a non-sex chromosome can cause a trait or disorder. It is a transmission pattern often produced by dominant effects at an autosomal locus.

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