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Dominant, Codominant, and Recessive Mutant Genes

A set of two alleles, a wild-type and a mutant allele, can be arranged into three gene pairs--two copies of the mutant allele, a mutant and a wild-type allele, and two copies of the wild-type allele. Compare the phenotypes of the three genotypes against table 1 below. The most appropriate row identifies the mutant allele as dominant, recessive, or codominant to the normal allele.
 
Table 1. Phenotypes From the Three Possible Gene Combinations Using Wild Type (Normal) as the Standard of Reference
  Two mutant genes A mutant gene paired with a normal gene Two normal genes Number of distinct phenotypes
Mutant gene is dominant to normal gene Full mutant phenotype Normal phenotype Two
Mutant gene is recessive to normal gene Full mutant phenotype Normal phenotype Two
Mutant gene is codominant* to normal gene Full mutant phenotype Phenotype different from the other two Normal phenotype Three

*Codominant is used as a catch-all term for all genetic conditions producing three distinct phenotypes from three combinations of two genes. Synonyms include incomplete dominant, less than dominant, partial dominant, semidominant, transdominant, and others.
 
Table 2 presents the same classification in a more graphic format.
 
Table 2. Phenotypes From the Three Possible Gene Combinations Using Wild Type (Normal) as the Standard of Reference
. Phenotype ranges
Red bar = a pair of mutant genes
Blue bar = a pair of normal genes
Green bar = a mutant gene and a normal gene
Number of distinct phenotypes
Mutant gene is dominant to normal gene heterozygous phenotype = homozygous mutant phenotype Two
Mutant gene is recessive to normal gene heterozygous phenotype = normal phenotype Two
Mutant gene is codominant to normal gene heterozygous phenotype = third phenotype Three

Some genetic loci have more than two alleles. It's useful to know the dominance relationships among the mutant alleles. Then table 3 is used.
 
Table 3. Phenotypes From the Three Possible Gene Combinations of Two Mutant Alleles
  Two mutant #1 genes A mutant #1 gene paired with a mutant #2 gene Two mutant #2 genes Number of distinct phenotypes
Mutant #1 is dominant to mutant #2, and mutant #2 is recessive to mutant #1 Mutant #1 phenotype Mutant #2 phenotype Two
Mutant #1 is codominant to mutant #2, and mutant #2 is codominant to mutant #1 Mutant #1 phenotype Phenotype different from the other two Mutant #2 phenotype Three

It's easy to get confused when discussing relationships among multiple alleles. One mutant allele is often recessive to the normal allele but dominant or codominant to another mutant allele. For example, the motley mutant gene is recessive to the normal allele but dominant to the stripe mutant allele. And the stripe mutant allele is recessive to both the normal allele and to the motley allele. To minimize the confusion, it is important to specify the comparisons, as in the stripe and motley example. When no comparison is actually specified, the comparison is always to the normal allele.
 
Some mutant alleles express a variable phenotype. The comparison of the heterozygous phenotype with the normal phenotype and the homozygous mutant phenotype do not perfectly fit the tables above.
 
One possible solution to variable expressivity is to multiply categories beyond the dominant/codominant/recessive trio. But this solution makes more problems. There are always borderline cases, which tempts still further subdivision. And newbies have enough trouble making sense out of just three categories. Further subdivision just magnifies their difficulties.
 
The best solution seems to be to retain the dominant, codominant, and recessive categories. Instead of requiring an exact fit in one category, we would use the category with the best fit, imperfect though it might be, as in table 4.
 
Table 4. Phenotypes From the Three Possible Gene Combinations Using Wild Type (Normal) as the Standard of Reference. Best fit rather than exact fit.
. Phenotype ranges
Red bar = a pair of mutant genes
Blue bar = a pair of normal genes
Green bar = a mutant gene and a normal gene
Number of distinct phenotypes
Mutant gene is dominant to normal gene heterozygous phenotype usually = homozygous mutant phenotype Two
Mutant gene is recessive to normal gene heterozygous phenotype usually = normal phenotype Two
Mutant gene is codominant to normal gene heterozygous phenotype almost always = third phenotype Three