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Incomplete Dominance, Co-Dominance, & Multiple Alleles
March 23, 2009
 Sometimes the pairing of different alleles in the
heterozygous offspring does not result in a simple
dominant/recessive result.
 Rather, the heterozygous offspring have a trait that is
not exactly like the trait of either purebred parent.
 Examples:
 Incomplete dominance
 Codominance
 At the conclusion of this lesson students should be
able to:
 Identify and distinguish between in-complete
dominance and codominance inheritance patterns.
 Determine the probability of a specified incomplete
dominant or codominant inheritance using a Punnett
square.
 Interpret the inheritance of a trait which is controlled by
multiple alleles.
 Incomplete dominance occurs when the
heterozygous phenotype is an intermediate
between the two homozygous parent phenotypes.
 “Incomplete” means
“in between”
 Ex: flower color in
snapdragons
X
Figure 11.4 The color of snapdragon flowers is a result of incomplete dominance.
When a plant with white flowers is crossed with a plant with red flowers, the
offspring have pink flowers. Red, pink, and white offspring will result from self
fertilization of a plant with pink flowers.
PROBLEM:
A red snapdragon is crossed with a pink snapdragon. What percentage of the
offspring will be pink?
Practice Problem:
In radishes, when a plant homozygous for red radishes is crossed with a plant
homozygous for white radishes , plants bearing purple radishes are produced.
a. What would be the genotype and phenotype ratios of a cross between a
purple radish plant and a white radish plant?
b. A red and a white?
c. A red and a purple?
 In codominance, both alleles are shown in the
heterozygous phenotype.
 “co-” means “together”
 Ex: sickle cell disease
 Sickle cell disease is caused by the




allele that controls the formation of
the protein hemoglobin.
Hemoglobin is the part of the red
blood cell that carries oxygen.
The allele for normal hemoglobin (A)
results in red blood cells that are discshaped.
The sickle cell allele (S) changes the
hemoglobin and results in red blood
cells that are sickle -shaped.
People who are heterozygous for these
two alleles (AS) have both normal and
sickle-shaped red blood cells.
PROBLEM:
What is the risk that two people
heterozygous for the sickle-shaped allele
will have a child with sickle cell disease?
 Red blood cells have two
different carbohydrates (called
antigens) that coat their
surface.
 Allele for antigen “A”
results in type A blood.
 Antigen “B” results in type
B blood.
 Both antigens results in
type AB blood.
 If neither antigen is
present, then type O blood
results.
QUESTION?
Which alleles are codominant?
How can you account for type O
blood?
Human blood type is a trait
that is an example of
multiple alleles.
If three or more alleles are
found in the population,
these genes are said to have
multiple alleles.
Possible gametes from
female parent
For human blood type
there are nine possible
genotypes and four
possible phenotypes.
Possible gametes from male parent
IA
IB
i
IA
IAIA
IAIB
IAi
IB
IAIB
IBIB
IBi
i
IAi
IBi
ii
 In a cross between a parent with type A blood and
a parent with type B blood what are the possible
genotypes and phenotypes of the offspring?
 The type A parent can either be IA IA
or
 The type B parent can either be IB IB
or
IA i
IB i
 Complete the Punnett squares and determine the possible
genotypes and phenotypes.
 Here are the possible Punnett Squares with their
results:
IA
IA
IA
i
IA
IA
IA
i
I B IA IB IA IB
I B IA IB
IB i
IB IA IB IA IB
IB IA IB
IB i
I B IA IB IA IB
I B IA IB
IB i
i
i
ii
IA IA x IB IB
100% type AB
IA ix IB IB
50% type AB
50% type B
IA i
IA i
IA IA x IB i
50% type AB
50% type A
IA i
IA i x IB I
25% type AB
25% type A
25% type B
25% type O
Practice Problems:
1. A man with blood type AB marries a
woman with blood type O. Is it possible
for them to have children with the same
blood type as either of the parents? Why
or why not?
2. In some cattle the roan coloration (red
and white together) results in
heterozygous offspring. What would the
genotypes and phenotypes of the
parents be? If two roan cattle bred, what
is the chance that their calf will also be
roan?
O
AB
An inheritance pattern that involves ...
...a single gene
that affects...
...multiple traits
is...
...a single trait...
...in which neither
allele dominates the
other and both are
expressed is...
PLEIOTROPY
Examples:
Sickle cell disease
Melanin production
in cats
...multiple genes
that affect ...
CODOMINANCE
...in which both alleles
appear to "blend" in
some instances is...
INCOMPLETE
DOMINANCE
Examples:
Example:
ABO bood group
Flower color in
snapdragons
Sickle cell disease
... two or more traits...
...in which one allele
dominates the
expression of another
allele is...
SIMPLE
DOMINANCE
...that do not interact
is...
LINKED GENES
...a single trait is a...
...that interact so that
one controls the
expression of the
other is...
EPISTASIS
POLYGENIC TRAIT
Examples:
Examples:
Example:
Examples:
albinism
Colorblindness
Huntington's disease
Hemophilia
Eye color
alkaptonuria
Coat color in
Labrador retrievers
and other mammals
Muscular dystrophy
Rooster combs
achondroplasia
Tay Sachs disease
cystic fibrosis
dimples
freckles
polydactyly
Height
Skin color
Hair color
Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.
Schmutz, Shelia. "Mendelian Inheritance and Beyond." 13 Feb 2004. Department of Animal and Poultry Science: The
Unviersity of Saskatchewan. 13 Apr 2008 <http://homepage.usask.ca/~schmutz/Mendelian.html#codominant>.
"What is Sickle Cell Anemia?." National Heart Lung and Blood Institute Disease and Conditions Index. Nov 2007. U.S.
Department of Health and Human Services: National Institutes of Health. 13 Apr 2008
<http://www.nhlbi.nih.gov/health/dci/Diseases/Sca/SCA_WhatIs.html>.
"Why people choose bloodless medicine." About.com: Heatlh Topics A-Z. 2008. 13 Apr 2008
<http://adam.about.com/care/Step-3-Why-people-choose-bloodless-medicine.htm>.