Codominance (AQA A-Level Biology): Revision Notes
Codominance
What is codominance?
Codominance occurs when both alleles at a gene locus are equally expressed in the phenotype, rather than one being dominant over the other. Unlike typical dominant-recessive inheritance, both versions of the gene contribute to the observable characteristics of an organism.
In codominant inheritance, heterozygous individuals show a phenotype that displays both allelic expressions simultaneously, not a blended or intermediate form. This creates distinct patterns of inheritance that differ from classical Mendelian genetics.
Codominance in shorthorn cattle
The coat colour inheritance in shorthorn cattle provides a clear example of codominant alleles. The genetic mechanism involves an enzyme that catalyses red pigment formation in hair follicles.
The shorthorn cattle example demonstrates how codominance works at the molecular level through enzyme function, making it an excellent model for understanding this inheritance pattern.
The genetic mechanism
Two alleles control coat colour:
- One allele codes for an enzyme that produces red pigment
- The other allele codes for an altered enzyme that lacks catalytic activity and produces no pigment
Phenotypes and genotypes
Three distinct coat colours result from different allelic combinations:
Coat Colour Inheritance in Shorthorn Cattle
Red coat: Animals homozygous for the pigment-producing allele have both copies coding for the enzyme. This results in pigment production and red hair throughout the coat.
White coat: Animals homozygous for the non-functional allele produce no enzyme and therefore no pigment. These cattle have white hair and a white coat.
Roan coat: Heterozygous animals possess both alleles - some hair follicles produce the enzyme and develop red pigment, while others do not. This creates a mixed appearance called roan, where both red and white hairs are present across the coat.
Notation system for codominance
Standard genetic notation uses superscript letters to represent codominant alleles, avoiding the traditional dominant/recessive upper/lower case system.
For shorthorn cattle coat colour:
- = allele for red pigment production
- = allele for no pigment production (white)
The base letter C represents the gene (colour), while superscripts R and W distinguish the specific alleles.
Inheritance patterns in codominance
Red × white cross
When a red-coated shorthorn () crosses with a white-coated animal (), all offspring are heterozygous () and display the roan phenotype.
Roan × roan cross
Worked Example: Roan × Roan Cross
Crossing two roan animals () produces offspring in a 1:2:1 genotypic ratio:
- 25% red coat ()
- 50% roan coat ()
- 25% white coat ()
This demonstrates how codominant alleles segregate according to Mendelian principles while producing distinct phenotypic expressions.
Key Points to Remember:
- Codominance means both alleles are equally expressed in the phenotype, not one dominating the other
- Notation uses superscripts on a base letter to represent codominant alleles (avoid upper/lower case)
- Shorthorn cattle demonstrate codominance through enzyme activity affecting pigment production in individual hair follicles