Transcript Incomplete Dominance

NonMendelian
Genetics
Chapter 14: Mendel and the Gene Idea
Complex patterns of
inheritance
 The relationship between genotype and phenotype is
rarely as simple as in Mendelian inheritance (controlled
by dominant and recessive paired alleles)
 Principles of segregation and independent assortment
apply to more complex patterns of inheritance
 Inheritance may deviate from simple Mendelian
patterns in the following situations:
Alleles are
A gene has
A gene produces
Incomplete dominance
 In complete dominance,
heterozygous and
homozygous dominant
individuals have the
 With incomplete
dominance, the phenotype
of the heterozygous is
 This intermediate occurs
because neither allele of the
pair is completely dominant
Incomplete Dominance
If you cross a white flower with a red flower that
exhibit incomplete dominance the first
generation (heterozygotes) will be
If you cross two of those heterozygotes you will
get
Incomplete dominance
genetic problems
We can still use the Punnett Square to solve
problems involving incomplete dominance.
The trick is to recognize when you are dealing
with a question involving incomplete
dominance.
There are two steps to this:
1) Notice that the offspring is showing a 3rd
phenotype. The parents each have one, and the
offspring are different from the parents.
2) Notice that the trait in the offspring is a blend
(mixing) of the parental traits.
Incomplete Dominance
Questions
1. A cross between a black bird & a white bird produces
offspring that are grey. The color of birds is determined by
just two alleles.
a) What are the genotypes of the parent birds in the
original cross?

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b) What is/are the genotype(s) of the grey offspring?
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c) What would be the phenotypic ratios of offspring
produced by two grey birds?
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Incomplete Dominance
Questions
 2. The color of fruit for plant "X" is determined by two
alleles. When two plants with orange fruits are crossed the
following phenotypic ratios are present in the offspring: 25%
red fruit, 50% orange fruit, 25% yellow fruit.
What are the genotypes of the parent orange-fruited
plants?
Codominance
 In codominance, two
dominant alleles affect the
phenotype in separate,
distinguishable ways
 Codominant alleles cause the
phenotypes of
to be produced in
heterozygote individuals.
 In codominance
 For example, red cows crossed
with white will generate roan
cows. Roan refers to cows that
have
Codominance
 The genetic gist to codominance is pretty much the
same as incomplete dominance.
 A hybrid organism shows a
--- not
the usual "dominant" one & not the "recessive" one.
 With incomplete dominance we get a blending of the
dominant & recessive traits so that the third phenotype
is something in the middle (red x white = pink).
 In codominance, the "recessive" & "dominant" traits
in the phenotype of hybrid
organisms.
 red x white ---> red & white spotted
Codominance Punnett Squares
 Some texts use letters & superscripts
when dealing with codominance.
 Others use different letters, noting
the type of nonMendelian cross.
 Let’s use the second method for
our example
 R = allele for red flowers
 W = allele for white flowers
 red x white --> red & white
spotted flowers
 RR x WW ----> 100% RW
 The symbols you choose to use
don't matter, in the end you end
up with hybrid organisms, and
rather than one trait (allele)
dominating the other, both traits
appear together in the phenotype.
R
R
W
RW
RW
W
RW
RW
Codominance Questions
 1. Predict the phenotypic ratios of offspring when
a homozygous white cow is crossed with a roan
bull.
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 2. A cross between a black cat & a tan cat
produces a tabby pattern (black & tan fur
together).
 a) What pattern of inheritance does this illustrate? Why?
 b) What percent of kittens would have tan fur if a tabby
cat is crossed with a black cat?
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Multiple Alleles
It is common for
Traits controlled by more than two
alleles are said to have multiple alleles
A diploid individual can possess
of each gene
Multiple Alleles
The number of alleles for any particular trait is not
limited to four, there are instances in which more
than 100 alleles are known to exist for a single trait
Multiple Alleles & Blood Types
Multiple Alleles govern blood type
Human blood types are
determined by the presence or
absence of certain molecules
on the surfaces of red blood
cells called antigens
As the determinant of blood type
the gene I has three alleles: IA, IB,
and i

IA (or A) allele produces
 IB (or B) allele produces
i (or O) produces
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Importance of Blood Typing
Incompatible blood types could clump together,
causing death.
Disputed parentage
Example: If a child has type AB blood and its
mother has type A, a man with type O blood
could not be the father.
Why?
Blood Typing Practice
1.
A woman with Type O blood and a man who is Type AB
have are expecting a child. What are the possible blood
types of the kid?
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2. What are the possible blood types of a child who's parents
are both heterozygous for "B" blood type?
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3. What are the chances of a woman with Type AB and a man
with Type A having a child with Type O?
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4. A test was done to determine the biological father of a
child.The child's blood Type is A and the mother's is B.
Man #1 has a blood type of O, & Man #2 has blood type
AB. Which man is the biological father?
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Pleiotropy
Most genes have
a property called pleiotropy
For example, pleiotropic alleles are
responsible for the multiple
symptoms of certain hereditary
diseases, such as cystic fibrosis and
sickle-cell disease
In the garden pea, gene for flower
color also affects color of seed coat
Epistasis
 In epistasis, a gene at one locus
 For example, in mice and many
other mammals, coat color
depends on
 One gene determines the
pigment color (B for black and b
for brown)
 The other gene (C for color and c
for no color) determines whether
the pigment will be deposited in
the hair
 Dominance =
 Epistasis =
Polygenic inheritance
poly = “many” ; genic = “genes”

contributes to a
phenotype
Effects of dominant alleles are additive
More dominant genes =
Number of dominant determines phenotype

are
polygenic traits
Many disorders may be polygenic
Cleft palate, club foot, diabetes, schizophrenia,
allergies, cancer
Skin color example
 If skin color was related to 3 gene
pairs
 Dominant gene A, B or C produces
pigment
 Incompletely dominant to a, b or c
 So # of dominant genes
determines
 AABBCC =
 AaBbCc =
 aabbcc =
 2 heterozygotes (AaBbCc) could
have a child with any pigment
range
Environmental Influences
 Genes are also influenced by the
 Temperature and Siamese cats
 The darker colors on the
extremities are due to a
 Gene that codes for production
of the pigment in the Siamese
cat only functions
Many diseases, such as heart
disease and cancer, have both
genetic and environmental
components
Pedigree
 A
is a family tree
that describes the interrelationships
of parents and children across
generations
 Inheritance patterns of
particular traits can be traced
 Can also be used to make
predictions about future
offspring
 Many genetic disorders are
inherited in a
 Recessively inherited disorders
show up only in individuals

are heterozygous
individuals who carry the recessive
allele but are phenotypically normal
Pedigree Symbols
Albinism
 Albinism is a recessive condition
characterized by a
 If a recessive allele that causes a
disease is rare, then the chance of
two carriers meeting and mating is
low
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(i.e.,
matings between close relatives)
increase the chance of mating
between two carriers of the same
rare allele
 Most societies and cultures have
laws or taboos against marriages
between close relatives
Cystic Fibrosis
 Recessive condition
 Cystic fibrosis is the most
common lethal genetic disease
in the US, striking one out of
every
 The cystic fibrosis allele results
in defective or absent
 Symptoms include mucus
buildup in some internal organs
and abnormal absorption of
nutrients in the small intestine
Sickle-cell disease
 Recessive condition
 Sickle-cell disease affects
one out of
 The disease is caused by the
substitution of a single amino
acid in the hemoglobin
protein in red blood cells
 Symptoms include physical
weakness, pain, organ
damage, and even paralysis
Dominant Genetic Diseases
 Some human disorders are caused by
dominant alleles
 Dominant alleles that cause a lethal
disease are rare and arise by mutation

is a form of
dwarfism caused by a rare dominant
allele

is a
degenerative disease of the nervous
system caused by a dominant allele
The disease has no obvious
phenotypic effects until the
individual is about 35 to 40 years
of age
Genetic Tests
 There are many genetic diseases that exist (way beyond the
scope of what we will discuss)

can provide information to
prospective parents concerned about a family history for a
specific disease
 Using family histories, they help couples determine the
odds that their children will have genetic disorders
 For a growing number of diseases, tests are available that
identify carriers and help define the odds more accurately
Amniocentesis
In amniocentesis, a
long thin needle is
used to remove
The amniotic fluid
contains
,
which can be tested
for genetic diseases
The DNA from fetal
cells is
Chorionic Villus Sampling
In chorionic villus
sampling (CVS), a
sample of the chorionic
villus
is removed and tested
 The chorionic villus cells
contain the
,
making them fetal cells
 The DNA from fetal cells is
Karyotypes
 Karyotypes (picture of
chromosomes
arrested during
mitosis) are prepared,
which determines:
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Other Genetic Tests
 Other techniques, such as
and
,
allow fetal health to be
assessed visually in utero
 Some genetic disorders can
be detected at birth by
simple tests that are now
routinely performed in
most hospitals in the US
 Phenylketonuria (PKU)
 Congential Hypothyroidism
Review Questions
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Name 3 examples of when inheritance patterns may not follow
Mendelian rules.
Explain, identify, and solve genetics problems involving incomplete
dominance, codominance, & multiple alleles.
Complete genetics problems involving blood types.
Explain, differentiate between, and complete nontraditional genetics
problems involving pleiotropy, epistasis, and polygenic inheritance.
Explain the effect of the environment on the expression of our genes.
Define and analyze a pedigree in order to answer inheritance
questions.
Identify the most common pedigree symbols.
Identify the inheritance patterns and major characteristics of the
following genetic conditions: albinism, cystic fibrosis, sickle-cell
disease, achondroplasia, & Huntington’s disease.
Explain the purpose, benefits, and risks of genetic testing.
Differentiate between amniocentesis and chorionic villus sampling.
Explain the purpose and use of a karyotype.
List 3 pieces of information that can be obtained from a karyotype.