Incomplete Dominance
• Neither allele is
completely
dominant over the
other allele.
• The phenotype is
heterozygous
– A mixture or
blending of the two
What two colors do
you think blended to
make grey?
Incomplete Dominance
Ex: Four-o’ clock flowers
• Neither Red (R)
• or White (W) is dominant
• When a homozygous red flower (RR)
mixes with a homozygous white flower
(WW), the alleles blend in the hybrid
(RW) to produce pink flowers
How to Recognize Incomplete
Dominance…..
1. The offspring is showing a 3rd
phenotype (red flower, white flower,
and pink flower)
2. The trait in the offspring is a blend
(mixing) of the parental traits (red x
white = pink)
Incomplete Dominance Practice Problems
1.
A)
A cross between a blue
blahblah bird and a white
blahblah bird produces
offspring that are silver. The
color of blahblah birds is
determined by just two alleles.
What are the genotypes of the
parent blahblah birds in the
original cross?
B)
What is/are the genotypes of
the silver offspring?
C)
What would be the phenotypic
ratios of offspring produced
by two silver blahblah birds?
Incomplete Dominance Practice Problems
•
In northeast Kansas there is a
creature know as a wildcat. It
comes in three colors, blue, red,
and purple. This trait is
controlled by a single locus
gene with incomplete
dominance. A homozygous
(BB) individual is blue, a
homozygous (bb) individual is
red, and a heterozygous (Bb)
individual is purple. What would
be the genotypes and
phenotypes of the offspring if a
blue wildcat were crossed with
a red one?
Incomplete Dominance Practice Problems
Coat color in mice is
incompletely
dominant. Yellow and
white-colored mice are
homozygous, while
cream-colored mice
are heterozygous. If
two cream-colored
mice mate, what
phenotypic ratio can
we expect of their
offspring?
Codominance
• Two equally
dominant alleles are
expressed at the
same time.
• Heterozygous
phenotype will have
both phenotypes
visible
Codominance
• Ex; Short Horn Cattle
• Homozygous red (RR)
• Homozygous white (WW)
The offspring of will have both red and
white hairs (RW)
The offspring are heterozygous and
called “roan”
Codominance Practice Problems
A cross between a black
cat and a tan cat
produces a tabby
pattern (black & tan fur
together). What percent
of kittens would have
tan fur if a tabby cat is
crossed with a black
cat?
Codominance Practice Problems
Roan cattle are the
heterozygous hybrids of a
cross between a white
bull and a red cow. If a
roan bull were crossed
with a red cow, what
would be the possible
phenotypes of their
offspring?
Codominance
Human Blood types ABO are examples of
codominace. Blood types come from three
different alleles and therefore are considered
a multiple trait allele. The multiple trait alleles
are: I A, I B, and i. I A and I B both result in an
antigen on the surface of red blood cells
however; i is recessive and does not have an
antigen. An antigen stimulates the production of
antibodies.
Codominance
Genotype & Phenotype of Blood
Sickle- Cell Anemia
• Co- dominance
• Caused by an abnormal
Hemoglobin, the protein that
red blood cells use to carry
oxygen
• Normal hemoglobin is (RR)
• Sickle Cell shaped blood cells (SS)
• People who are carriers (heterozygous) for the
disease there is a mixture of both normal and
sickle cell (RS)
Problem: Codominance
• Show the cross between an individual
with sickle-cell anemia and another who
is a carrier but not sick. Remember:
RR = normal RS = carrier of sickle cell
SS = sickle cell
Problem: Codominance
Blood Types
•
Mrs. Clink is type “A” and Mr. Clink is type “O.” They have three children
named Matthew, Mark, and Luke. Mark is type “O,” Matthew is type “A,”
and Luke is type “AB.” Based on this information:
– Mr. Clink must have the genotype ______
– Mrs. Clink must have the genotype ______ because ___________ has
blood type ______
– Luke cannot be the child of these parents because neither parent
has the allele _____.
Problem: Codominance
Blood Types
•
Two parents think their baby was switched at the hospital. Its 1968, so DNA
fingerprinting technology does not exist yet. The mother has blood type “O,” the
father has blood type “AB,” and the baby has blood type “B.”
– Mother’s genotype: _______
– Father’s genotype: _______
– Baby’s genotype: ______ or ________
– Punnett square showing all possible genotypes for children produced by this
couple
– Was the baby switched?
Partner Work 
Find a friend or two and work the following genetic
problems. Have fun and focus on the task “at hand”!
PRACTICE QUESTIONS
1. In a certain case a woman’s blood type
was tested to be AB. She married and
her husband’s blood type was type A.
Their children have blood types A, AB,
and B. What are the genotypes of the
parents? What are the genotypic ratios of
the children?
2. In a certain breed of cow, the gene for
red fur, R, is codominant with that of
white fur, W. What would be the
phenotypic & genotypic ratios of the
offspring if you breed a red cow and a
white bull? What would they be if you
breed a red & white cow with a red & white
bull?
3. A rooster with grey feathers is mated
with a hen of the same phenotype. Among
their offspring 15 chicks are grey, 6 are
black and 8 are white.
a. What is the simplest explanation for the
inheritance of these colors in chickens?
b. What offspring would you expect from
the mating of a grey rooster and a black
hen?