Chapter 16 Codominance, Sex Linkage, Epistasis
16.5 - Codominance
1. What codominance is
2. How codominance is represented in genetics
3. An example of codominance in flower colour
What is codominance?
Codominance occurs when two different alleles are equally expressed in an organism's phenotype.
Unlike typical dominant-recessive gene relationships, both alleles in codominance share equal
dominance.
This means that both alleles are visible in the organism's observable characteristics, leading to a
phenotype that is a mixture of both alleles rather than just one.
How to show codominance in genetic diagrams:
An uppercase letter is used to signify the gene (e.g. C for the colour of a particular trait in an
organism).
Superscript uppercase letters indicate the alleles (e.g. CR and CW for red or white
colouration).
Avoid lowercase letters to prevent the implication of recessiveness.
An example of codominance in flower colour
The flower colour in snapdragons demonstrates codominance.
The alleles responsible for snapdragon flower colour are:
CR - Responsible for a high production of red pigment.
CW - Leads to a low production of red pigment.
There are three potential flower colours based on these alleles:
Genotype Phenotype
CRCR Red flowers
CWCW White flowers
, Genotype Phenotype
CRCW Pink flowers
In pink snapdragons, the blended colour results from the combined influence of both alleles: the
CR allele generates a certain amount of red pigment, while the C W allele contributes minimally,
creating a pink hue.
Worked example - Identifying offspring phenotypes with codominant alleles
Shorthorn cattle may have red, white, or roan coloured coats. The alleles CR and CW are codominant
and produce the roan coat colour.
Identify the phenotypic ratio in the F2 generation in a cross between a cow with a white coat and a
bull with a red coat.
Step 1: State the parents’ genotypes and gametes
as the parents are not roan coloured, they must be homozygous for the coat colour alleles
white coat parent genotype - CWCW, gametes CW
red coat parent genotype - CRCR, gametes CR
Step 2: Perform a cross to identify the genotypes of the F1 generation
Step 3: Perform a cross between F1 offspring to identify the genotypes of F2 generation
16.5 - Codominance
1. What codominance is
2. How codominance is represented in genetics
3. An example of codominance in flower colour
What is codominance?
Codominance occurs when two different alleles are equally expressed in an organism's phenotype.
Unlike typical dominant-recessive gene relationships, both alleles in codominance share equal
dominance.
This means that both alleles are visible in the organism's observable characteristics, leading to a
phenotype that is a mixture of both alleles rather than just one.
How to show codominance in genetic diagrams:
An uppercase letter is used to signify the gene (e.g. C for the colour of a particular trait in an
organism).
Superscript uppercase letters indicate the alleles (e.g. CR and CW for red or white
colouration).
Avoid lowercase letters to prevent the implication of recessiveness.
An example of codominance in flower colour
The flower colour in snapdragons demonstrates codominance.
The alleles responsible for snapdragon flower colour are:
CR - Responsible for a high production of red pigment.
CW - Leads to a low production of red pigment.
There are three potential flower colours based on these alleles:
Genotype Phenotype
CRCR Red flowers
CWCW White flowers
, Genotype Phenotype
CRCW Pink flowers
In pink snapdragons, the blended colour results from the combined influence of both alleles: the
CR allele generates a certain amount of red pigment, while the C W allele contributes minimally,
creating a pink hue.
Worked example - Identifying offspring phenotypes with codominant alleles
Shorthorn cattle may have red, white, or roan coloured coats. The alleles CR and CW are codominant
and produce the roan coat colour.
Identify the phenotypic ratio in the F2 generation in a cross between a cow with a white coat and a
bull with a red coat.
Step 1: State the parents’ genotypes and gametes
as the parents are not roan coloured, they must be homozygous for the coat colour alleles
white coat parent genotype - CWCW, gametes CW
red coat parent genotype - CRCR, gametes CR
Step 2: Perform a cross to identify the genotypes of the F1 generation
Step 3: Perform a cross between F1 offspring to identify the genotypes of F2 generation
