A dominant trait means an animal needs only one copy of the allele to display the trait. An example of a completely dominant trait would be black coat color. If an animal gets one copy of the black allele and one copy of the red allele, the animal will be black. A recessive trait means an animal needs to inherit two copies of that allele in order to display the trait (phenotype). This is only possible if both parents have at least one copy of this allele.
For example, red coat color is a recessive trait. A calf must have two copies of the red coat allele (ee) to display a red coat. That does not mean both parents had to be red, they just had to carry at least one copy of the red coat allele. Mating two heterozygous black parents (Ee) would have a red calf (ee) approximately 25% of the time.
Dominance is also important in polygenic traits (traits that involve multiple genes like growth). Dominance is the main reason for hybrid vigor or inbreeding depression as increasing genetic diversity leads to an increase in dominant gene expression and a resulting boost in production.
People often equate a dominant trait to mean “good” and recessive to mean “bad”. While this may frequently be the case (for instance, many lethal mutations are recessive), it is not always the rule. As mentioned about the true definition of dominant or recessive traits has nothing to do with good or bad alleles, just how many copies are needed to display the phenotype.
Varying Degrees of Dominance
Frequently when we talk of dominant traits, we automatically think of completely dominant traits where an animal with two copies of the dominant allele is phenotypically the same as an animal with just one copy of the dominant allele. This is the case with black coat color or the polled trait in cattle. However, many traits have varying degrees of dominance where the recessive allele is not completely masked by the presence of the dominant allele. Here we provide definitions of the varying degrees of dominance (complete, partial, no dominance, overdominance) and a figure to illustrate each form.
Complete dominance: The dominant allele completely masks the recessive alleles resulting in a heterozygote that is phenotypically the same as the dominant homozygotes.
Partial dominance: One allele is not completely dominant over the other allele. With partial dominance, the heterozygote has a phenotype between the two homozygotes but more similar to the homozygote dominant animals.
No dominance: The heterozygote exhibits a phenotype that is exactly in between the homozygotes.
Overdominance: The heterozygote exhibits a phenotype that is beyond the range of either of the homozygotes but is more similar to the dominant homozygotes
Caption: Illustration of varying degrees of dominance. The horizontal lines represent a phenotypic outcome of the genotype for a particular gene. “A” represents the dominant allele and “a” represents the recessive allele. Note that in all four examples the homozygotes have the same phenotypic outcome, what differs between various types of dominance is the phenotype of the heterozygotes.
Example: Consider a theoretical gene that influences growth up to weaning. Assume the dominant dominant homozygous animals have a 5 lb advantage at weaning and the homozygous recessive animals have 0 lb advantage from this one gene. With varying degrees of dominance, you could expect the following for the heterozygous animals:
Complete Dominance - 5 lb advantage (same as homozygous dominant)
Partial Dominance - 3 lb advantage (not the same as homozygous dominant but closer to it than average)
No Dominance - 2.5 lb advantage (average of the dominant and recessive homozygotes)
Over Dominance - 7 lb advantage (beyond the homozygote dominant animals)
Epistasis refers to the interaction among genes at different locations of the chromosome. The resulting phenotype from the gene at one location is determined by the gene at another location. The polled gene and phenotype is an example of epistasis where a second gene (scurred) affects the phenotype of the polled animal. The presence of scurs (typically small, movable, hollow pseudo horns) only occurs in heterozygous polled animals. All homozygous polled (PP) animals are phenotypically polled independent of the scurred gene. All homozygous horned (pp) animals have horns and are unaffected by the scurred gene. The heterozygous animals at the horned/polled gene (Pp) can either be polled or have scurs depending on their sex and the alleles for the scurred gene. Similar to the varying degrees of dominance, epistasis also contributes to inbreeding suppression and hybrid vigor.