1. Unit 8: Mendelian Genetics 8
Unit 8: Mendelian Genetics 8.5 Other Patterns of Inheritance: Codominance

2. Phenotype produced when both alleles are clearly expressed.
Traits appear together
Examples:
Red and white spotted cattle (roan)
AB Blood types (the A and B proteins appear on the surface of your blood cells together)
Codominance

3. There are four major blood groups determined by the presence or absence of two antigens (proteins)-A and B-on the surface of red blood cells.
Group A-has only the A antigen on the red blood cells (B antibody in the plasma)
Group B-has only the B antigen on the red blood cells (A antibody in the plasma)
Group AB-has both A and B antigens on the red blood cells (neither A or B antibody in the plasma)
Group O-has neither A nor B antigens on the red blood cells (both A and B antibody in the plasma)
Blood Types

4. There are 3 alleles of the gene that controls blood type: IA, IB, I
The I stands for immunoglobin, or the type of white blood cell that would be triggered to attack
IA and IB are codominant genes, meaning when inherited together, they are both fully expressed, not blended like in incomplete dominance. i is the recessive form of the allele.
The following table shows the genotype and phenotype for the four blood types.
Blood Type Genotypes
Genotype
Phenotype
IAIA or IAi A
IBIB or IBi B
IAIB AB ii O

5. Using a Punnett Square for Codominance
Use only capital letters- since there is no dominant trait
Place the two traits onto the Punnett square, the same as you have before
Cross the traits in the square
Using a Punnett Square for Codominance

6. Example with Codominance
Jessica has B blood type. Jessica’s mom has blood type O. Jessica has a baby with John, who has type AB blood.
a. parent genotype
b. offspring genotype
c. offspring phenotype
Example with Codominance