1. Codominance and
Incomplete Dominance

2. Types of Dominance Co-Dominance Incomplete Dominance
What you have been taught so far is a very simplified version of how genes interact. It is often more complicated than just one gene being completely dominant over another. Today we will learn what it means for alleles to co-dominant and for alleles to show incomplete dominance.
Co-Dominance
Incomplete Dominance

3. Co-Dominance
When alleles are co-dominant, it means that both alleles are always expressed. The phenotype will be a combination of both alleles. Blood types are a great example of co-dominance.

4. Red blood cell
Your blood type results from a protein found in the membrane of your red blood cells. If you have the
allele IA, then your red blood cells make a protein called antigen A. If you have allele IB, they make the protein antigen B. If you have gene i, then your red blood cells do NOT make either protein. 4

5. But if you have both alleles IA and IB, then you make BOTH proteins.
Red blood cell
But if you have both alleles IA and IB,
then you make BOTH proteins.
These alleles are co-dominant because
they are both always expressed! 5

6. % A = _____ % O = _____ 100% % A = _____ % B = _____ % O = _____
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% 6

7. % B = _____ % O = _____ 75% % A = _____ % B = _____ % O = _____
IB i
i i
IA IA
IA i IB i IA IB i IA IB
IBIB IBi
IAi IAi
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% 7

8. Incomplete Dominance
Incomplete dominance occurs when a combination of alleles produces an intermediate trait. One trait is not completely dominant, so the phenotype of the heterozygote is between the dominant and recessive phenotypes.

9. Incomplete Dominance
Incomplete Dominance is easiest to see and understand in terms of pigment proteins. Dominant alleles produce colorful pigments, and homozygous dominant organisms produce bright rich colors. Recessive alleles often produce little to no pigment at all, so homozygous recessive organism are very light in color.

10. Incomplete Dominance
When an organism is a heterozygote, it produces half the pigment since it only has one allele making pigment protein. This results in a color that is lighter than a homozygous dominant organism but darker than a homozygous recessive organism. It is an intermediate phenotype.

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 Incomplete Dominance 100% pink flowers
1. A species of flowers exhibits incomplete dominance between the allele for red flowers (R) and the allele for white flowers (r). If a plant that is 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
R r
R r
KEY:
100% pink flowers

15. R r R r R R R r R r r r Incomplete Dominance 25% red flowers
2. A species of flowers exhibits incomplete dominance between the allele for red flowers (R) and the allele for white flowers (r). 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. ½ Complete the following Punnett squares 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 r
Rr rr R r
RR RR
RR Rr
rr rr
Rr rr
Rr rr
Rr Rr
50% 3
50%

17. Two different types of chickens are shown below
Two different types of chickens are shown below. Assume each the chicken below is the result of a cross between a parent with black feathers and a parent with white feathers. Which breed seems to show co-dominance for feather color and which seems to show 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.