Understanding
dominance/semi-dominance/co-dominance/recessive relationships
between alleles.
Point: The relationship between alleles can be determined by examining the
phenotype of the heterozygote in comparison to the phenotype of the two
homozygotes.
Key: The phenotype of the heterozygote:
1- is the same as one or the other homozygote, in which case one of the alleles is
dominant to the other (recessive).
2- is an intermediate between the two homozygotes, in which case it shows a semidominant (incomplete dominance) relationship
3- shows both homozygote phenotypes, in which case it is a co-dominant
relationship.
1 - Simple dominant/recessive with wildtype
a- Mutation is recessive to wild type
+/+
-both have the same phenotype – wild type
+/m
m/m
-mutant phenotype
a- Mutation is dominant to wild type
+/+
- wild type phenotype
+/m
- both have the same phenotype - mutant
m/m
2 - Semi-dominance (incomplete) e.g Four-o’clock flower colour
r/r
r/w
w/w
red/red - red colour flower
red/white - pink colour flower
white/white – white flower colour
3 - Co-dominance e.g human Hemoglobin “S” vs. “A”
A
A
one band on electrophoretic gel
A
S
see both - two bands on electrophoretic gel
S
S
one band on electrophoretic gel
Hb / Hb
Hb / Hb
Hb / Hb
Notes:
1- The description (dominant, etc) deals with the relationship between two alleles.
2- Some alleles may be both dominant and recessive. That is, an allele may be
dominant to one allele, but recessive to another.
3- Dominant/recessive relationships are descriptions that are separate from the
various classifications of Muller’s morphs.
Dominant_recessive_understanding.doc
11/1/06
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