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Codominance
•When there are at least 2
usually 3 alleles that influence
a trait.
–1 is recessive.
–The other 2 are dominant to
that allele but not to each other.
Human
Blood Types
In the human population, there
are 3 major alleles for the surface
protein gene: A, B, and O.
A
B
O
A and B are
dominant to O but
not to each other
Recessive
Human Blood Types
Genotype
Phenotype
AA or AO
IAIA or IAi
BB or BO
IBIB or IBi
AB
IAIB
OO
ii
Type A
Type B
Type AB
Type O
Codominance
• Example:
IA
i
homozygous male B (IBIB)
x
heterozygous female A (IAi)
IB
IB
IA I B
IA IB
1/2 = IAIB
IB i
IB i
1/2 = IBi
Codominance
• Example: male O (ii) x female AB (IAIB)
IA
IB
i
IA i
IB i
i
IA i
IB i
1/2 = IAi
1/2 = IBi
Codominance
• In codominance, the
alleles are neither
dominant, nor
recessive. Neither
allele is masked by
the other.
Roan Cow
Is both white and red
Chestnut or Red Roan
Black or Blue Roan
Palonimo
Mystery Bull Experiment
• http://tepserver.ucsd.edu/peopl
e/Tracee-Parsons/Finalgenetics-project/mysterybull.html
Flav-R-Gro, Inc.
Through genetic engineering, you and
your partner, Maisie have developed a
new type of corn. You have named it
WonderCorn.
•It has a tasty flavor of butter and salt.
•The good thing about this new corn is
that you do not have to add butter and
salt to it once it is cooked.
•This makes it a much healthier dish.
You and Maisie suspect
that the taste trait is corn
in codominant. To find
out, you crossed two
other types of corn: a
homozygous salty corn
(SS) and a homozygous
buttery corn (BB). The
offspring were all
WonderCorn.
Create the Punnett
Square
S
S
B
SB
SB
B
SB
SB
What is the genotype of WonderCorn?
What percent of the offspring will have
this genotype?
You were correct.
Taste is a
codominant trait.
Make a Punnett
Square showing a
secondgeneration cross.
(Both parents are
heterozygous)
What percent of
the offspring will
be WonderCorn?
S
B
S
SS
SB
B
SB
BB
50 % will be WonderCorn.
1:2:1 Ratio
The demand for
WonderCorn is high.
Grover’s Grocery has
already ordered 50
bushels and wants
more. You are all out.
You are at the
beginning of a new
growing season.
Would you be better
off using homozygous
plants to grow or
heterozygous? Why?
Do the math!
• If a heterozygous
cross produces 736
offspring, how many
will be WonderCorn?
• How many of the 736
offspring will taste
salty but not buttery?
50% of 736 = 368
25% of 736 = 184
Quiz
Tomorrow