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