Dominance (genetics)

The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while variants on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (see Sex linkage).

Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later.

He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.

In 1928, British population geneticist Ronald Fisher proposed that dominance acted based on natural selection through the contribution of modifier genes.

In 1929, American geneticist Sewall Wright responded by stating that dominance is simply a physiological consequence of metabolic pathways and the relative necessity of the gene involved.

Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively.

Incomplete dominance (also called partial dominance, semi-dominance, intermediate inheritance, or occasionally incorrectly co-dominance in reptile genetics[13]) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes.

A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed.

For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus.

[14] Another example occurs at the locus for the beta-globin component of hemoglobin, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all distinguishable by protein electrophoresis.

For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA.

[14] Dominance can be influenced by various genetic interactions and it is essential to evaluate them when determining phenotypic outcomes.

Multiple alleles, epistasis and pleiotropic genes are some factors that might influence the phenotypic outcome.

Polymorphism can have an effect on the dominance relationship and phenotype, which is observed in the ABO blood group system.

The dominance relationship between alleles involved in epistatic interactions can influence the observed phenotypic ratios in offspring.

Autosomal dominant and autosomal recessive inheritance, the two most common Mendelian inheritance patterns. An autosome is any chromosome other than a sex chromosome .
Inheritance of dwarfing in maize. Demonstrating the heights of plants from the two parent variations and their F1 heterozygous hybrid (centre)
Monohybrid cross between homozygote dominant (GG) and homozygote recessive (gg), always resulting in heterozygote genotype (Gg) and the phenotype associated with the dominant allele, in this case capital G.
Monohybrid cross between heterozygotes (Gg), resulting in genptypical ratio 1:2:1 (GG:Gg:gg) and phenotypical ratio 3:1 (G:g).
Dihybrid cross between homozygote dominant (GGRR) and homozygote recessive (ggrr) always resulting in heterozygotes (GgRr) with phenotype associated with the dominant alleles G and R.
Dihybrid cross between heterozygotes (GgRr), resulting in the phenotypical ratio 9:3:3:1 (G and R: G and r: g and R: g and r)
This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait (R) causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly.
Co-dominance in a Camellia cultivar
A and B blood types in humans show co-dominance, but the O type is recessive to A and B.
This Punnett square shows co-dominance. In this example a white bull (WW) mates with a red cow (RR), and their offspring exhibit co-dominance expressing both white and red hairs.