Genotype, phenotype, dominant, recessive, codominant, additive
An individual’s genotype is the totality of that individual’s hereditary material, whereas an individual’s phenotype is the individual’s appearance.
However, the terms genotype and phenotype are usually used in reference to a particular locus or set of loci, and to a particular trait or set of traits. Genotypes are not observed directly, but rather inferred from particular phenotypes. Thus, with respect to the ABO locus, the four blood types A, B, O and AB are (discrete) phenotypes; and the possible genotypes, formed by pairs of alleles, are AA, AO, BB, BO, BB and OO. With respect to the ABO blood group phenotypes, the A allele is dominant to the O allele and the O allele is recessive to the A allele. Similarly, the B allele is dominant to the O allele and the O allele is recessive to the B allele. The A and B alleles are codominant.
Note that for the words “dominant” and “recessive” to have any meaning, at least two alleles and two phenotypes must be specified. If a particular allele at a locus is dominant with respect to the presence of a disease, there must be at least one other allele at that locus that is recessive with respect to absence of that disease.
Geneticists loosely talk about a disease being dominant, meaning that, with respect to the phenotype “disease”, the underlying disease allele is dominant, i.e. the disease is present when either a single or two copies of the allele is present. Similarly, they talk of a disease being recessive, meaning that, with respect to the same phenotype, the underlying disease allele is recessive, i.e. the disease is present only when two disease alleles are present.
Alternatively, they may talk of an allele being dominant or recessive, the particular phenotype (often disease) being understood. The important thing to realize is that “dominance” and “recessivity” describe a relationship between one or more genotypes and a particular phenotype. This leads to the concept of phenosets: the genotypes AA and AO form the phenoset corresponding to the A blood type, and the genotypes BB and BO form the phenoset corresponding to the B blood type.
In the case of the ABO blood group, a person who has one A allele and one B allele has the blood type AB, which is a different phenotype from that of either of the corresponding homozygotes, AA and BB; this relationship is called codominance. In general, a locus is codominant with respect to the set of phenotypes it controls if the phenotypes of each heterozygote at that locus differ from that of each of the corresponding homozygotes.
We make a distinction between codominant and additive; the latter implies that the phenotype (or phenotypic distribution, see below under quantitative phenotypes) corresponding to the heterozygote is in some sense half-way between those of the two corresponding homozygotes. Whereas the term additive is only meaningful when a scale of measurement has been defined, codominance is a more general concept that does not require the definition of a scale of measurement.