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)
P > p
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. HOW TO DO RATIOS GENOTYPIC RATIOS ALWAYS LIST THEM IN THIS ORDER:
Homozygous Dominant (HD)
Heterozygous (Het)
Homozygous Recessive (HR)
AKA: HD:Het:HR
1:2:1 (example from previous slide)

5. Incomplete Dominance No dominant/recessive allele
A third (new) phenotype appears in the heterozygous condition-mix/blend of traits
Flower Color in 4 O’clock flowers
RR = red
WW = white
RW = pink

6. Problem: Incomplete Dominance
Show the cross between a pink and a white flower.
GENOTYPES:
R W
- RR (0); RW (2); WW (2)
- ratio 0:2:2 W WW RW
PHENOTYPES:
- Red (0); Pink (2); White (2)
- ratio 0:2:2

7. Codominance AS = some of each SS = sickle cells AA = normal cells sick
The heterozygous condition, both alleles are expressed equally (A = S)
Sickle Cell Anemia in Humans
AS = some of each
SS = sickle cells
AA = normal cells
sick

8. Problem: Codominance
Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick.
A S
GENOTYPES:
- AA (0); AS (2); SS (2)
- ratio 0:2:2 S AS SS
PHENOTYPES:
- All normal (0); carrier (2); sick (2)
- ratio 0:2:2

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

10. Rules for Blood Type A and B are codominant (equal)
AA = Type A
BB = Type B
AB = Type AB
A and B are both dominant over O
AO = type A
BO = type B
OO = type O

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

12. 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