1. 3/19/12 Objective: Understand codominance and incomplete dominance
Warm-Up: In pea plants, purple flowers are dominant to white flowers. Two plants are crossed and produce 800 offspring. Of the 800 offspring, 584 have purple flowers and 216 have white flowers. What are the most probable genotypes of the parents?

2. Codominance
So far, the traits we have worked with follow a complete dominance pattern
Traits with complete dominance are uncommon
Codominance is a more common inheritance pattern
In codominance, multiple alleles are distinctly expressed in the phenotype
This means that heterozygotes have a unique phenotype

3. Blood Type: Phenotypes
Human blood type is determined by the presence of surface proteins (called immunoglobins) on red blood cells
Possible phenotypes:
Type A: Individual has type A immunoglobin
Type B: Individual has type B immunoglobin
Type AB: Individual has both A and B immunoglobin
Type O: Individual has no immunoglobin

4. Blood Type: Genotypes
There are three alleles: A and B are codominant, and O is recessive
An individual can only be type O if he or she has two O alleles
Type A: AA, AO
Type B: BB, BO
Type AB: AB
Type O: OO
The heterozygote AB has a unique phneotype

5. Practice
A man with type A blood and a woman with type B blood have a child with type O blood. Explain how this happened, using a Punnett Square to support your explanation.
If a woman with type AB blood has a child with a man who is type O, will any of the children have the same blood type as one of the parents? Why or why not? You may support your answer with a Punnett Square.

6. Incomplete Dominance
In incomplete dominance, the heterozygote has a phenotype that is “in between” the two alleles
Example: In snapdragon flowers, flowers can be red, white, or pink. If a red-flowering plant is crossed with a white-flowering plant, all of the F1 will be pink.

7. R R RW RW W RW W RW
R=allele for red; W=allele for white RR = red flowers; RW = pink flowers; WW = white flowers

8. This isn’t blending, though…
If the F1 self-fertilizes: R W R RW RR RW W WW