1. Punnett Squares:
Codominance and
Incomplete Dominance

2. Expressed means…
The gene is “on” or “activated” and the protein will be produced.
We observe expressed genes as phenotypes.
Hair Color Expressed
Eye Color Expressed

3. Phenotype for Eye Color
Subject A
Subject B
Subject C
Genotype for Eye Color BB Bb bb
Phenotype for Eye Color
What eye color gene is expressed?
What eye color is unexpressed?
What is Ryan Reynolds’ phenotype for eye color?

4. Phenotype for Eye Color
Subject A
Subject B
Subject C
Genotype for Eye Color BB Bb bb
Phenotype for Eye Color
The examples we have seen so far are simple: you either have brown eyes or blue eyes.
It is often more complicated than just one gene being dominant over another. Sometimes, genes are considered codominant. This means both alleles are expressed equally; the phenotype will be a combination of both alleles.

5. Blood types are a great example of co-dominance!
Red blood cell
Blood types are a great example of co-dominance!
Your blood type results from a protein that is found in the membrane of your red blood cells.
If you have gene IA, then you make protein A.
If you have gene IB, then you make protein B.
If you have gene i, then you do not make a protein.
If you have both gene IA and gene IB, then you make BOTH proteins.
These alleles are co-dominant because they are
both always expressed! 5

6. Blood types are Codominant
If a person has genotype IAIA, their blood type (phenotype) will be ______
If a person has genotype IAi, their blood type (phenotype) will be ______
If a person has genotype IBIB, their blood type (phenotype) will be ______
If a person has genotype IBi, their blood type (phenotype) will be ______
If a person has genotype ii, their blood type (phenotype) will be ______
If a person has genotype IAIB, their blood type (phenotype) will be ______ A A B B O AB 6

7. Codominance Examples % A = _____ % O = _____ 100% % A = _____
IA i
IA i
IA IA
IA i IA IB i IA IB IB
IAIA IAi
IAIB IBi
IAIA IAIA
IAIB IBi
IAIA IAi
IAi ii
IAIB IAIB
IAIB IBi
% A = _____
% O = _____
100%
% A = _____
% B = _____
% O = _____
% AB = _____
25%
% A = _____
% B = _____
% AB = _____
50%
% A = _____
% B = _____
% O = _____
% AB = _____ 0% 0%
25% 0%
50%
25%
50% 0%
25%
50% 7

8. More Codominance Examples
IB i
i i
IA IA
IA i IB i IA IB
IAi IAi i IA IB
IBIB IBi
IAi IAi
IAIA IAi
IBi ii
IBi IBi
IAi IAi
IAIB IBi
% B = _____
% O = _____
75%
% A = _____
% B = _____
% O = _____
% AB = _____
50%
% A = _____
% O = _____
100%
% A = _____
% B = _____
% O = _____
% AB = _____
50%
25% 0%
50%
25% 0% 0% 0%
25% 8

9. Incomplete Dominance R R r R r → RR rr Rr
As we have seen, not all gene combinations are as simple as having one dominant gene and one recessive gene.
Based on the diagram, how would you define incomplete dominance?
R R r
R r → X RR rr Rr

10. R r R r R R R r R r r r Incomplete dominance
Occurs when a combination of genes produces an intermediate of the two traits.
One trait is not completely dominant, so the heterozygous phenotype is a mixture.
R r R r
R R
R r
R r
r r

11. Animals, like these horses, often exhibit incomplete dominance in the color of their hair or skin. This produces a range of shades from light to dark.
Photo by David Blaikie

12. Dark
Brown
D d D d DD Dd
Palomino Dd dd
Cream

13. Dark
Brown
d d D d Dd Dd
Palomino dd dd
Cream

14. R R r R r R r R r R r 100% pink flowers Examining Incomplete Dominance
1.A species of flowers exhibits incomplete dominance between the allele for red flowers and the allele for white flowers.
If a plant homozygous for red flowers is crossed with a plant that is homozygous for white flowers, what percentage of offspring will have pink flowers?
R r
R r
KEY:
100% pink flowers

15. R r R r R R R r R r r r 25% red flowers 50% pink flowers
Examining
Incomplete Dominance
R r
2. If two heterozygous, pink plants are crossed, what percentage of each color of flower will appear in the offspring? R r
R R
R r
R r
r r
25% red flowers
50% pink flowers
25% white flowers
KEY:

16. Genotype for Flower Color Phenotype for Flower Color
Complete the following Punnett squares about incomplete dominance and answer the questions go with each cross.
Subject A
Subject B
Subject C
Genotype for Flower Color RR Rr rr
Phenotype for Flower Color
Red
Pink
White
r r
R r
R r
R R R r
Rr Rr R r
RR Rr r
Rr rr R r
RR RR
rr rr
Rr rr
Rr rr
Rr Rr
50% 3
50%
50%

17. Codominance vs. Incomplete Dominance
Assume each chicken below is the results of a cross between a parent with black feathers and a parent with white feathers.
Based on the definitions we have learned, which breed shows codominance between feather color and which shows incomplete dominance.
Chicken with Gray Feathers
Chicken with Black & White Feathers
Incomplete Dominance
An intermediate trait appears in the offspring.
Codominance
Both traits appear in the offspring.