1. Incomplete Dominance Codominance Multiple Alleles
In a nutshell!

2. Review: Dominant/Recessive
One allele is dominant over the other (capable of masking the recessive allele)
PP = purple
Pp = purple
pp = white

3. Review Problem: Dominant/Recessive
In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.
P p
GENOTYPES:
- PP (1); Pp (2); pp (1)
- ratio 1:2:1 P p PP Pp
PHENOTYPES: Pp pp
- purple (3); white (1)
- ratio 3:1

4. Incomplete Dominance
A third (new) phenotype appears in the heterozygous that is a blend of the two traits
Example are Flower Color in 4 o’clock flowers
R = red allele
W = white allele (other designations R’ and r)
RR = red
WW = white
RW = pink

5. Problem: Incomplete Dominance
Show the cross between a pink and a white flower.
GENOTYPES:
R W
- RW (2); WW (2)
- ratio 1:1 W WW RW
PHENOTYPES:
- pink (2); white (2)
- ratio 1:1

6. Examples of Incomplete Dominance

7. Codominance BW = checkered chicken (erminette) BB = Black Chicken
The heterozygous condition, both alleles are expressed equally
See both traits expressed (spots/strips)
BW = checkered chicken (erminette)
BB = Black Chicken
WW = white chicken

8. Codominant Human Example
Sickle Cell Anemia in humans
Cells are sickle-shaped (half-moon)
Do not carry oxygen well and can get stuck in veins
SS = sickle cells
NS = some of each
NN = normal cells

9. Problem: Codominance
Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick.
N S
GENOTYPES:
- NS (2) SS (2)
- ratio 1:1 S NS SS
PHENOTYPES:
- carrier (2); sick (2)
- ratio 1:1

10. Examples of Codominance in Animals
Appaloosa horse
Roan cow

11. Multiple Alleles
There are more than two alleles for a trait (not just H and h)
Hair color
HBr = brown, Hbl = blonde, hr = red, hBk = black
Eye Color
EBr = brown , EHz = hazel, eGr = green,
ebl = blue

12. An Example of Multiple Alleles – Human Blood Type
Blood Types?
Type A, Type B, Type AB, Type O
Blood Alleles – There are three
A, B, O (in book – IA, IB, i)
We all get a combination of two of these alleles

13. Rules for Blood Type
A and B are codominant and both are dominant over O
IA IA (homozygous form) and IA i (heterozygous form)= Type A
IB IB (homozygous form) and IB i (heterozygous form) = Type B
IAIB = Type AB
ii = Type O