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 condition.
Flower Color in 4 O’clocks
RR = red
rr = white
Rr = pink

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

6. Codominance NS = some of each SS = sickle cells NN = normal cells sick
The heterozygous condition, both alleles are expressed equally
Sickle Cell Anemia in Humans
NS = some of each
SS = sickle cells
NN = normal cells
sick

7. 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

8. Multiple Alleles There are more than two alleles for a trait
Blood type in humans
Blood Types?
Type A, Type B, Type AB, Type O
Blood Alleles?
A, B, O (in book – IA, IB, I)

9. Rules for Blood Type A and B are codominant
AA = Type A
BB = Type B
AB = Type AB
A and B are dominant over O
AO = type A
BO = type B
OO = type O

10. Problem: Multiple Alleles
Show the cross between a mother who has type O blood and a father who has type AB blood.
A B
GENOTYPES:
- AO (2) BO (2)
- ratio 1:1 O AO BO
PHENOTYPES:
- type A (2); type B (2)
- ratio 1:1

11. Problem: Multiple Alleles
Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood.
GENOTYPES:
A O
AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1 B O AB OO BO AO
PHENOTYPES:
type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1

12. Multiple Allele vs. Polygenic Traits
Alleles are different versions or forms of ONE gene. So in multiple allele traits, there are more than two alleles for that particular trait.
For example, there are 3 alleles (A, B, and O) that contribute to blood types in humans. Each person can have only two alleles in their genome at one time, but in the population there are 3 forms.
In fruit flies, there are many different versions of eye color in the population, but one fly can have only 2 alleles at a time.

13. Multiple Allele vs. Polygenic Traits
Polygenic traits are those where more than one GENE contributes to the phenotype.
For example, the current model of skin color is that there are 3 genes that contribute to skin color. So there are 6 total alleles, 3 from the mother and 3 from the father.

14. Multiple Allele vs. Polygenic Traits
So…..
Multiple alleles=more than two forms of the same gene in the population
Polygenic traits=more than one gene contributes to the phenotype