Exception #1:
Incomplete Dominance
Exceptions to the Dominance Rule:
Incomplete & Codominance
Biology I Notes
Exception #1:
Practice Problem
Incomplete Dominance
• If I cross 2 pink snapdragons, what will
be my resulting genotypes and
phenotypes?
• F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
– Example: snapdragons (flower)
• red (RR) x white (rr)
• All offspring (Rr) are PINK!?
r
r
R
Rr
Rr
R
Rr
Rr
• DEFINED:
– A form of intermediate inheritance
in which heterozygous alleles are
both expressed, resulting in a
combined phenotype.
•Genotype:
–1:2:1
•Phenotype:
–1:2:1
R
r
R
RR
Rr
r
Rr
rr
Exception #1: Codominance
• Two alleles are expressed (multiple alleles) in
heterozygous individuals.
– Neither one is recessive or dominant
– Both “show up”
up” in offspring
• Example: blood type
Exception #2: Codominance
• Blood Type
1. type A
2. type B
3. type AB
4. type O
Genotype
= IAIA or IAi
= IBIB or IBi
= IAIB
= ii
Phenotype
A
B
AB
OO
What type of blood can a person with type a
Receive in a blood transfusion?
– A or O
http://learn.genetics.utah.edu/units/basics/blood/types.cfm
Codominance Problem
• Example:
homozygous male Type B
x
heterozygous female Type A
IB
IB
IA
i
IAIB
IB i
IAIB
IB i
1/2 = IAIB
1/2 = IBi
Another Codominance
Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IB i
i
IAi
IB i
1/2 = IAi
1/2 = IBi
Codominance
Codominance
• Answer:
• Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents?
• boy - type O (ii)
• girl - type AB (IAIB)
IA
i
IB IAIB
i
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B