Cornsnake Morph Guide ® | Genetics Tutorial | Charles Pritzel |
- In a dominant/recessive relationship, only the dominant allele is expressed. The recessive allele has no effect on the outward appearance.
- In a codominant/codominant relationship, both alleles will have some effect. Sometimes one allele will have more effect than the other. Sometimes their effects are equal. There are many subtle shades of codominance, and they can be subdivided into finer groups. Incomplete dominance is one such sub-group, and is discussed with the other advanced genetics topics.
For example, let's look again at the Circle locus. There are three alleles:
The "Normal" allele produces lots of black pigment. |
The "Albino" allele produces no black pigment. |
The "Semi" allele produces some black pigment, but not much. |
With these three alleles, a snake can have any of the following pairs:
Gene pair | Appearance |
| Normal |
The normal allele can produce enough for the whole snake by itself. Anything carrying even one copy looks normal. |
| Normal |
| Normal |
| Reduced black pigment (hypo) |
| Very reduced black pigment (extreme hypo) |
| No black pigment (albino) |
Notice that
is dominant to
and
. This is true because when
is present, the other two genes have no effect, the snake is normal. These are dominant/recessive relationships.
Also notice that
is codominant with
because
takes on a third appearance, between that of
and
.
Review:
- Dominant and Codominant alleles are expressed when present.
- Recessive alleles are not expressed unless they are homozygous.