Co-dominance & Incomplete
Dominance Notes
Co-dominance
Co-dominance – when both alleles of a
gene contribute to the phenotype
Examples:
Chickens – black feathers is codominant with white feathers, so
heterozygous chickens show up with
speckled black and white feathers
Human Blood Types
Human Blood Types
• Human Blood Types are
another example of codominance
• There are 3 alleles for
this gene (IA, IB & i) and
i is recessive, while IA
and IB are co-dominant,
so people who receive
one of each of these
alleles will have AB
blood
IAIA
IAi
IBIB
IBi
IAIB
Co-Dominance Sample Problems #1
 Use the chart to answer the following questions:
Write the genotype for each person based on the
description:
 Homozygous for the “B” allele
IBIB
 Heterozygous for the “A” allele
I Ai
 Type O
ii
 Type AB
 I AI B
Co-Dominance Sample Problem #2
 Pretend that Brad Pitt is
homozygous for the type B
allele, and Angelina Jolie is
type “O.” Do a Punnett
square to determine all the
possible blood types of
their baby.
 All their offspring, will be
IBi, so will have Type B
blood
IB
i
i
IB
IBi
IBi
IBi
IBi
Co-Dominance Sample Problem #3
 Two parents think their baby was switched at the
hospital. The mother has blood type “A,” the father
has blood type “B,” and the baby has blood type
“AB.”
IA
 Mother’s genotype
IAIA or IAi
 Father’s genotype
IBIB or IBi
 Baby’s genotype
IAIB
IB
i
I AI B
IBi
I Ai
ii
 Do a Punnett square that shows the baby’s
genotype as a possibility
 Was the baby switched?
 NO
i
Co-Dominance Sample Problem #4
 Based on the
information in this
table, which man
could not be the
father of the baby?
Justify your answer
with a Punnett
square.
 Sammy the player
Name
Blood Type
Mother
Type A (Heterozygous)
Baby
Type B
Sammy the player
Type O
George the sleeze
Type AB
The waiter
Type AB
The cable guy
Type B
IA
i
i
i
I Ai
ii
I Ai
ii
Probability
 What is probability?
 The likelihood a particular event will
occur
 How is it used in Biology?
 Used to predict phenotypes and genotypes
of offspring (diseases & disorders for
people planning families)
Additional Information
 Probability = # of times an event occurs
# of trials
 A previous event does not affect future
outcomes. Each event is separate and
independent
Ex. Gender of baby – 50/50 chance of
each sex each birth
Incomplete Dominance
 Incomplete Dominance – when one allele is not
completely dominant over the other allele, so it produces
offspring with a third phenotype that is a blending of the
parental traits.
 It's like mixing paints, red + white will make pink. Red
doesn't totally block (dominate) the white, instead there
is incomplete dominance, and we end up with something
in-between.
 Instead of using a capital letter for the dominant trait & a
lowercase letter for the recessive trait, the letters we use
are both going to be capital (because neither trait
dominates the other).
Incomplete Dominance
 Four o’clock flowers show this
type of inheritance
 Red flowers are RR, white are
WW and the heterozygous
genotype RW, gets you pink
flowers
 So looking at the following
Punnett Square, what percentage
of pink flowers would be
produced?
 100%
Incomplete Dominance Sample Problem #1
 Suppose you cross 2 pink flowers that show
incomplete dominance (R=Red, W=W).
 What are the parent genotypes?
 RW; RW
 Do a Punnett Square.
 What is the probability of:
 Red
R
R
 25%
 White
 25%
 Pink
 50%
W
W
RR
RW
RW
WW
Incomplete Dominance Sample Problem #2
 In Elmo’s family red (R) and blue (B) show
incomplete dominance.
 What is the Genotype of:
 Red
 RR
 Blue
 BB
 Purple
 RB
Incomplete Dominance Sample
Problem #3
 If both of Elmo’s
parents were purple,
R
what is the probability R
RR
that his next sibling
will be blue (Do a
RB
B
Punnett square to
show your results)?
 25%
B
RB
BB