Dominance is the relationship between different alleles at the same locus on homologous chromosomes, in terms of how those alleles interact to produce a phenotype.

In general, a dominant allele is an allele that only needs one copy of itself in order to be expressed in the phenotype. Meanwhile, a recessive allele is only expressed in an organism that carries two copies of itself (i.e. when the allele is homozygous).

A simple dominance relationship might be in human eye colour: where allele B (for brown eyes) is dominant to allele b (for blue eyes); allele B is the dominant allele and allele b is the recessive allele. Because B is the dominant allele, it expresses the brown-eyed phenotype regardless of whether it appears in a homozygous dominant (BB) or heterozygous (Bb) genotype. Meanwhile blue eyes are only phenotypically expressed when they appear in a homozygous recessive (bb) genotype, because the dominant allele, B, is not present to silence the recessive allele, b.

In practice, however, such simplistic dominance relationships are rare and there is usually a more complex interaction of alleles at the molecular level. Some of the more complex dominance relationships are described below.

  • In the case of multiple alleles, a dominance hierarchy might form. For instance, in the case of blood group there are three alleles - I(A), I(B) and I(O) - each coding for A antigens, B antigens, or no antigens, respectively. The first two alleles are co-dominant (see below), and both are equally dominant to the last, which is completely recessive. For instance, I(A)I(B) produces the co-dominant AB blood-type phenotype, while I(A)I(O) and I(B)I(O) produce blood types A and B, respectively. Blood type O can only be expressed in individuals with the genotype I(O)I(O)

  • Co-dominance is the property where two different alleles are both fully expressed in the phenotype, in spite of the other allele's presence. So, for example, in an individual heterozygous for blood group - genotype I(A)I(B) - there is no dominant or recessive allele because both alleles express themselves fully in the phenotype to provide A and B antigens, and give an AB blood group.

  • Sometimes there may be a phenotype in the heterozygote that is intermediate between those of the homozygotes. For example, a red snapdragon flower and white snapdragon flower may be crossed to give one that is pink. This is called incomplete dominance because neither allele is completely dominating the phenotypic expression of the other; there is a 'compromise', almost. This is different from co-dominance, because in co-dominance the two alleles have full phenotypic expression, while in incomplete dominance neither allele is being fully expressed in the phenotype; they are both 'mixing' expression to give a different phenotype. When the intermediate phenotype generated by incompletely dominant alleles is exactly halfway between the two homozygous phenotypes, they are said to be in a state of semi-dominance.

  • Epistasis is an interaction between alleles at different loci. For instance, if there is a gene for pigment production (BB/Bb meaning pigment is produced and bb meaning it isn't), and a gene for pigment colour (AA/Aa meaning red and aa meaning white), then the genotype for pigment colour is only relevant if the genotype for pigment production is BB or Bb. If the genotype for pigment production is bb, and no pigment is produced, then even if the genotype for pigment colour is red (AA/Aa), it will appear white because no pigment production is in operation. The b allele is not dominant to the A allele, as they are at different loci, but the B locus is said to be in recessive epistasis to the A locus because when the B locus is homozygous recessive it overrides the phenotypic expression of the A locus.