1. Codominance Notes
Mrs. Callan
2017

2. Codominance
Codominance - If offspring phenotype is heterozygous, the offspring will exhibit BOTH traits from the parents.
NO BLENDING! Both dominant and recessive traits expressed.
USE CAPITAL LETTERS FOR PUNNETT SQUARES.
Examples: Blood types in humans, sickle-cell disease, coat color in dogs, cattle and horses

3. ROAN Codominance in Cattle Red Cattle = RR (homozygous)
White Cattle = WW (homozygous)
Roan (Red & White Hair) = RW (Heterozygous)
ROAN

4. Example Problem #1 Cross a roan cow and a white cow.
Key:
RR = Red
WW = White
RW = Roan (Red & White) R W W RW WW
Genotypes:
2/4 50% RW
2/4 50% WW
Ratio 2:2
Phenotypes:
2/4 50% Roan
2/4 50% White W RW WW

5. Codominance in Chickens
Black Chicken = BB
White Chickens = WW
Andalusian (Black & White) = BW

6. B W B BB BW B BB BW Key: BB = Black WW = White
Example Problem #2 Cross a black & white Andalusian chicken with a black chicken.
Key:
BB = Black
WW = White
BW = Black & White (Andalusian) B W B BB BW
Genotypes:
2/4 50% BB
2/4 50% BW
Ratio 2:2
Phenotypes:
2/4 50% Black
2/4 50% Andalusian (B&W) B BB BW

7. Codominance in Human Blood
Human blood types are determined by genes that follow the codominance pattern of inheritance.
There are two dominant alleles A and B and one recessive allele O.
Possible blood types are:
Type O = OO
Type AB = AB
Type A = AA (Homozygous) or AO (Heterozygous)
Type B = BB (Homozygous) or BO (Heterozygous)

8. Example Problem #1
Create a Punnett Square showing all the possible blood types of the offspring produced by a “O” mother and a Type “AB” father. O O
Key:
Type O = OO
Type AB = AB
Type A = AA (Homozygous) or AO (Heterozygous)
Type B = BB (Homozygous) or BO (Heterozygous) AO AO A BO BO B

9. Example Problem #1
Create a Punnett Square showing all the possible blood types of the offspring produced by a “O” mother and a Type “AB” father. O O
Genotypes:
2/4 50% AO
2/4 50% BO
Phenotypes
2/4 50% Type A Blood
2/4 50% Type B Blood AO AO A BO BO B

10. Example Problem #2
Create a Punnett Square showing all the possible blood types of the offspring produced by a heterozygous Type A mother and a Homozygous Type B father. A O
Key:
Type O = OO
Type AB = AB
Type A = AA (Homozygous) or AO (Heterozygous)
Type B = BB (Homozygous) or BO (Heterozygous) AB BO B AB BO B

11. Example Problem #2
Create a Punnett Square showing all the possible blood types of the offspring produced by a heterozygous Type A mother and a Homozygous Type B father. A O
Genotypes:
2/4 50% AB
2/4 50% BO
Phenotypes:
2/4 50% Type AB Blood
2/4 50% Type B Blood AB BO B AB BO B