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SBI3U
Co-Dominance and Multiple Alleles
CO-DOMINACE:
 Both alleles for a trait are dominant
 both alleles are expressed in heterozygous individuals
Example: feather colour in chickens
 If you cross a black rooster and white hen
 offspring are not gray BUT “checkered” black and white
MULTIPLE ALLELES:
 When a gene has more than two alleles
TRY THIS:
1. “Thinking Lab” see textbook p. 145
 examine inheritance of coat colour in rabbits
2. Practice Problems see textbook p. 144 #3
 examine fur colour in foxes
EXAMPLE: BLOOD TYPE
 3 alleles – A, B, O
 these 3 alleles can form 6 different genotypes
Note: We will use the letter “ I ” for blood type.
Let  capital I = dominant
 lowercase i = recessive
Blood Group
Phenotype
Blood Cell
Image
Genotype
Antibodies
present in serum
ii
O
anti A, anti B
IAIA
A
anti B
A
I i
IBIB
B
anti A
B
I i
IAIB
AB
 IA and IB are dominant to I
 IA and IB are both expressed, if present  Co-Dominant
none
Sample Problem:
A man of blood type A marries a woman of blood type B and they have a
child with blood type O.
a. What are the genotypes of these three individuals?
Let I = dominant, i = recessive
P:
IAi
F1:
x IBi
(both parent must have an recessive allele)
ii
b. What are the possible genotypes for the offspring of these parents? In
what frequencies would you expect these genotypes among the
children of such a marriage?
P:
IAi
x IBi
F1:
Genotypes:
25% heterozygous AB
25% heterozygous A
25% heterozygous B
25% homozygous O
PRACTICE PROBLEMS  complete CP page 32