Chapter 04 Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
INTRODUCTION
•  Mendelian inheritance describes
inheritance patterns that obey two laws
–  Law of segregation
–  Law of independent assortment
•  Simple Mendelian inheritance involves
–  A single gene with two different alleles
–  Alleles display a simple dominant/recessive
relationship
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4-2
INTRODUCTION
•  In this chapter we will examine traits that
deviate from the simple dominant/recessive
relationship
•  The inheritance patterns of these traits still
obey Mendelian laws
–  However, they are more complex and
interesting than Mendel had realized
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4-3
4.1 INHERITANCE PATTERNS
OF SINGLE GENES
•  There are many ways in which two alleles
of a single gene may govern the outcome of
a trait
•  Table 4.1 describes several different
patterns of Mendelian inheritance
–  These patterns are examined with two goals in
mind
•  1. Understanding the relationship between the
molecular expression of a gene and the trait itself
•  2. The outcome of crosses
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4-4
4-5
•  Prevalent alleles in a popula6on are termed wild-­‐
type alleles –  These typically encode proteins that •  Func6on normally •  Are made in the proper amounts •  Alleles that have been altered by muta6on are termed mutant alleles –  These tend to be rare in natural popula6ons –  They are likely to cause a reduc6on in the amount or func6on of the encoded protein –  Such mutant alleles are oCen inherited in a recessive fashion Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
4-6
•  Gene6c diseases are caused by mutant alleles •  In many human gene6c diseases , the recessive allele contains a muta6on –  This prevents the allele from producing a fully func6onal protein 4-7
•  In a simple dominant/recessive relationship, the
recessive allele does not affect the phenotype of the
heterozygote
–  So how can the wild-type phenotype of the heterozygote
be explained?
•  There are two possible explanations
–  1. 50% of the normal protein is enough to accomplish the
protein s cellular function
•  Refer to Figure 4.2
–  2. The heterozygote may actually produce more than 50%
of the functional protein
•  The normal gene is up-regulated to compensate for the lack of
function of the defective allele
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4-8
Figure 4.2
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Dominant (functional) allele: P (purple) "
Recessive (defective) allele: p (white)
Genotype
PP
Pp
pp
Amount of"
functional"
protein P
100%
50%
0%
Phenotype
Purple
Purple
White
Simple dominant/"
recessive"
relationship
4-9
•  Dominant Mutants –  Much less common than recessive –  Three explana6ons for most dominants •  Gain-­‐of-­‐func6on –  Protein encoded by the mutant gene is changed so it gains a new or abnormal func6on •  Dominant-­‐nega6ve –  Protein encoded by the mutant gene acts antagonis6cally to the normal protein •  Haploinsufficiency –  mutant is loss-­‐of-­‐func6on –  heterozygote does not make enough product to give the wild type phenotype Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
4-10
Incomplete Dominance
•  In incomplete dominance the heterozygote
exhibits a phenotype that is intermediate
between the corresponding homozygotes
•  Example:
–  Flower color in the four o clock plant
–  Two alleles
•  CR = wild-type allele for red flower color
•  CW = allele for white flower color
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4-11
Figure 4.3
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Red
White
P generation
C RC R
Gametes CR
1:2:1 phenotypic
ratio NOT the 3:1
ratio observed in
simple Mendelian
inheritance
CWCW
x
CW
In this case, 50% of
the CR protein is not
sufficient to produce
the red phenotype
Pink
F1 generation
C RC W
Gametes CR or CW
Self-fertilization
Sperm
F2 generation
CR
CW
C RC R
C RC W
C RC W
CWCW
CR
Egg
CW
4-12
Incomplete Dominance •  Whether a trait is dominant or incompletely
dominant may depend on how closely the trait is
examined
•  Take, for example, the characteristic of pea shape
–  Mendel visually concluded that
•  RR and Rr genotypes produced round peas
•  rr genotypes produced wrinkled peas
–  However, a microscopic examination of round peas
reveals that not all round peas are created equal
•  Refer to Figure 4.4
4-13
Figure 4.4
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Dominant (functional) allele: R (round)"
Recessive (defective) allele: r (wrinkled)
Genotype
RR
Rr
rr
Amount of functional"
(starch-producing)"
protein
100%
50%
0%
Phenotype
Round
Round
Wrinkled
With unaided eye"
(simple dominant/"
recessive relationship)
With microscope"
(incomplete"
dominance)
4-14
Incomplete Penetrance •  In some instances, a dominant allele does
not influence the outcome of a trait in a
heterozygote individual
•  Example = Polydactyly
–  Autosomal dominant trait
–  Affected individuals have additional fingers and/
or toes
•  Refer to Figure 4.5
–  A single copy of the polydactyly allele is usually
sufficient to cause this condition
–  In some cases, however, individuals carry the
dominant allele but do not exhibit the trait
4-15
Figure 4.5
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I-1
II-1
I-2
II-2
III-1
IV-1
IV-2
II-3
III-2
IV-3
II-4
III-3
II-5
III-4
III-5
Inherited the polydactyly allele from
his mother and passed it on to a
daughter and son
Does not exhibit the trait himself
even though he is a heterozygote
4-16
Incomplete Penetrance •  The term indicates that a dominant allele does not
always penetrate into the phenotype of the
individual
•  The measure of penetrance is described at the
population level
–  If 60% of heterozygotes carrying a dominant allele
exhibit the trait allele, the trait is 60% penetrant
•  Note:
–  In any particular individual, the trait is either penetrant or
not
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4-17
Expressivity •  Expressivity is the degree to which a trait is
expressed
•  In the case of polydactyly, the number of digits can
vary
–  A person with several extra digits has high expressivity
of this trait
–  A person with a single extra digit has low expressivity
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4-18
•  The molecular explanation of expressivity and
incomplete penetrance may not always be
understood
•  In most cases, the range of phenotypes is thought
to be due to influences of the
–  Environment
and/or
–  Other genes
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4-19
Environment •  Environmental conditions may have a great impact
on the phenotype of the individual
–  Some animals like the arctic fox change coat color
•  grayish brown in summer, white in winter
–  Humans affected by phenylketonuria (PKU) are unable
to metabolize phenylalanine
•  symptoms include mental retardation
–  When detected early, individuals can be fed a restricted
diet essentially free of phenylalanine and remain
symptom free
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4-20
Environment •  Geneticists often examine a range of conditions when studying
the effect of environment on phenotype
•  This allows them to see the norm of reaction of the
environmental influence on phenotypic range
•  Figure 4.6c shows the norm of reaction for eye facet number in
genetically identical flies raised at different temperatures
1000
Facet number
Facet
900
800
700
0
0
15
20
25
30
© PHOTOTAKE Inc/Alamy
Temperature (°C)
(c) Norm of reaction
Figure 4.6 c
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4-21
Overdominance •  Overdominance is the phenomenon in which a
heterozygote is more vigorous than both of the
corresponding homozygotes
–  It is also called heterozygote advantage
•  Example = Sickle-cell anemia
–  Autosomal recessive disorder
–  Affected individuals produce abnormal form of hemoglobin
–  Two alleles
•  HbA à Encodes the normal hemoglobin, hemoglobin A
•  HbS à Encodes the abnormal hemoglobin, hemoglobin S
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4-22
•  HbSHbS individuals have red blood cells that deform
into a sickle shape under conditions of low oxygen
tension
–  Refer to Figure 4.7
–  This has two major ramifications
•  1. Sickling phenomenon greatly shortens the life span
of the red blood cells
–  Anemia results
•  2. Odd-shaped cells clump
–  Partial or complete blocks in capillary circulation
–  Thus, affected individuals tend to have a shorter
life span
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4-23
Figure 4.7
© Stan Flegler/Visuals Unlimited
© Stan Flegler/Visuals Unlimited
7 mm"
(a) Normal red blood cell
7 mm"
(b) Sickled red blood cell
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4-24
•  The sickle cell allele has been found at a fairly high
frequency in parts of Africa where malaria is found
–  Why?
•  Malaria is caused by a protozoan, Plasmodium
–  This parasite undergoes its life cycle in two main parts
•  One inside the Anopheles mosquito
•  The other inside red blood cells
–  Red blood cells of heterozygotes, are likely to rupture
when infected by Plasmodium sp.
•  This prevents the propagation of the parasite
•  HbAHbS individuals have an advantage over
–  HbSHbS, because they do not suffer from sickle cell anemia
–  HbAHbA, because they are more resistant to malaria
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4-25
•  At the molecular level, overdominance is due to two
alleles that produce slightly different proteins
•  But how can these two protein variants produce a
favorable phenotype in the heterozygote
•  Well, there are three possible explanations for
overdominance at the molecular/cellular level
–  1. Disease resistance
–  2. Homodimer formation
–  3. Variation in functional activity
–  Refer to Figure 4.8
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4-26
Figure 4.8a
•  A microorganism will infect a cell if
certain cellular proteins function
optimally
n
Heterozygotes have one altered
copy of the gene
n
n
n
Therefore, they have slightly reduced
protein function
This reduced function is not enough to
cause serious side effects
n  But it is enough to prevent infections
Examples include
n
n
Sickle-cell anemia and malaria
Tay-Sachs disease
n  Heterozygotes are resistant to
tuberculosis
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Pathogen can"
successfully"
propagate.
A1A1
Normal homozygote"
(sensitive to infection)
Pathogen"
cannot"
successfully"
propagate.
A1A2
Heterozygote"
(resistant to infection)
(a) Disease resistance
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4-27
Figure 4.8b
•  Some proteins function as
homodimers
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–  Composed of two different subunits
–  Encoded by two alleles of the same gene
A1
A1
A2
A2
A1
A2
(b) Homodimer formation
•  A1A1 homozygotes
–  Make only A1A1 homodimers
n
A2A2 homozygotes
n
n
Make only A2A2 homodimers
A1A2 heterozygotes
n
n
n
Make A1A1 and A2A2 homodimers
AND A1A2 homodimers
For some proteins, the A1A2 homodimer
may have better functional activity
n  Giving the heterozygote superior
characteristics
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4-28
Figure 4.8c
•  A gene, E, encodes a metabolic
enzyme
•  Allele E1 encodes an enzyme that
functions better at lower
temperatures
n
n
Allele E2 encodes an enzyme that
functions better at higher
temperatures
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E1
E2
27°–32°C"
(optimum"
temperature"
range)
30°–37°C"
(optimum"
temperature"
range)
(c) Variation in functional activity
E1E2 heterozygotes produce both
enzymes
n
Therefore they have an advantage in
that they function over a wider
temperature range than either E1E1 or
E2E2 homozygotes
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4-29
Multiple Alleles •  Many genes have multiple alleles
–  Three or more different alleles
–  Multiple alleles are found within natural
populations
–  However, for genes present in a single copy/
haploid genome, a maximum of two alleles are
found in any particular diploid individual
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4-30
•  An interesting example is coat color in rabbits
–  Four different alleles
•  C (full coat color)
•  cch (chinchilla pattern of coat color)
–  Partial defect in pigmentation
•  ch (himalayan pattern of coat color)
–  Pigmentation in only certain parts of the body
•  c (albino)
–  Lack of pigmentation
–  The dominance hierarchy is as follows:
•  C > cch > ch > c
–  Figure 4.9 illustrates the relationship between
phenotype and genotype
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4-31
Figure 4.9
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© Zig Leszcynski/Animals, Animals
(a) Full coat color CC,"
Cch, Ccch, or Cc.
© P. Wegner/Peter Arnold
(c) Himalayan coat color chch or"
chc.
© John T. Fowler/Alamy
(b) Chinchilla coat color cchcch,"
cchch, or cchc.
© Gary Randall/Visuals Unlimited
(d) Albino coat color cc.
4-32
•  The Himalayan pattern of coat color is an
example of a temperature-sensitive
conditional allele
–  The enzyme encoded by this gene is functional
only at low temperatures
•  Therefore, dark fur will only occur in cooler areas of
the body
•  This is also the case in the Siamese pattern of coat
color in cats
Figure 4.9
Figure 4.10
© P. Wegner/Peter Arnold
(c) Himalayan coat color chch or"
chc.
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4-33
•  The ABO blood group provides another example of
multiple alleles
•  It is determined by the type of antigen present on
the surface of red blood cells
–  Antigens are substances that are recognized by
antibodies produced by the immune system
•  As shown in Figure 4.11, there are three different
alleles that determine which antigen(s) are present
on the surface of red blood cells
–  Allele IA, produces antigen A
–  Allele IB, produces antigen B
–  Allele i, no surface antigen is produced
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4-34
•  Allele i is recessive to both IA and IB
•  Alleles IA and IB are codominant
–  They are both expressed in a heterozygous individual
Antigen A
Antigen B
RBC
RBC
O
A
B
AB
ii
IAIA or IAi
IBIB or IBi
IAIB
neither A or B"
against A and B
A"
against B
B"
against A
A and B"
none
RBC
Blood type:"
Genotype:"
Surface antigen:"
Serum antibodies:
N-acetyl-"
galactosamine
Antigen A
Galactose
Antigen B
RBC
(a) ABO blood type
Figure 4.11a
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4-35
X-linked Genes •  Many species have males and females that
differ in their sex chromosome composition
–  Certain traits are governed by genes on the sex
chromosomes
–  A pedigree for an X-linked disease shows that it
is mostly males that are affected with their
mothers as carriers
–  Refer to Figure 4.12
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4-39
Figure 4.12
Affected "
with DMD
II-1
III-1
IV-1
III-2
IV-2
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I-1
I-2
II-2 II-3
III-3
IV-3
II-4
III-4
IV-4
Unaffected, "
presumed heterozygote
II-5
III-5
IV-5
II-6
III-6 III-7 III-8
IV-6
IV-7
Human Pedigree for Duchenne muscular dystrophy-Affected individuals are shown
with filled symbols. Female carriers are shown with half-filled symbols.
4-40
•  Punnett squares of X-linked traits show
different results depending on which sex is
affected
–  An affected male and unaffected female have no
affected offspring, but females are carriers
–  An affected female and unaffected male have all
male offspring affected and females all carriers
–  Refer to figure 4.13
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4-41
Figure 4.13b
4-42
Sex Chromosomes and Traits
•  Sex-linked genes are those found on one of the
two types of sex chromosomes, but not both
•  X-linked
–  Hemizygous in males
•  Only one copy
•  Males are more likely to be affected
•  Y-linked
–  Relatively few genes in humans
–  Referred to as holandric genes
–  Transmitted from father to son
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4-43
Sex Chromosomes and Traits
•  Pseudoautosomal inheritance refers to the very
few genes found on both X and Y chromosomes
–  Found in homologous regions needed for chromosome
pairing
Mic2!
gene
X
Y
Figure 4.14
Mic2!
gene
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4-44
Sex-influenced Traits •  Traits where an allele is dominant in one sex but
recessive in the opposite sex
–  Thus, sex influence is a phenomenon of heterozygotes
•  Sex-influenced does not mean sex-linked
–  Most sex-influenced traits are autosomal
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4-45
Sex-influenced Traits •  Example: Pattern baldness in humans
–  Caused by an autosomal gene
•  Allele B is dominant in males, but recessive in females
Genotype
Phenotype
in Males
Phenotype
in Females
BB
bald
bald
Bb
bald
nonbald
bb
nonbald
nonbald
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4-46
Sex-influenced Traits •  Pattern baldness in humans
–  In males, this trait is characterized by loss of hair on front
and top of head but not on the sides
© National Parks Service, Adams National Historical Park
(a) John Adams (father)
© National Parks Service, Adams National Historical Park
b) John Quincy Adams (son)
© Bettmann/Corbis
© National Parks Service, Adams National Historical Park
(c) Charles Francis Adams "
(grandson)
(d) Henry Adams"
(great-grandson)
Figure 4.15
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4-47
n
The autosomal nature of pattern baldness has
been revealed by analysis of human pedigrees
n
Refer to Figure 4.16
Bald fathers can pass
the trait to their sons
I-1
II-1
III-2
IV-4
III-1
IV-1
IV-2
IV-3
III-3
IV-5
II-2
II-3
III-5
IV-7
III-4
IV-6
II-4
III-6
IV-8
I-2
II-5
III-7
IV-9
II-6
II-7
III-8
IV-10
II-8
III-9
IV-11
III-10
IV-12
IV-13
IV-14
(a) A pedigree for human pattern baldness
Bb
×
Bb
Sperm
B
B
BB!
Bald male"
Bald female
b
bb!
Bb!
Nonbald male"
Bald male"
Nonbald female Nonbald female
Egg
Figure 4.16
b
Bb!
Bald male"
Nonbald female
(b) Example of an inheritance pattern involving baldness
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4-49
Sex-limited Traits •  Traits that occur in only one of the two sexes
•  For example in humans
–  Breast development is normally limited to females
–  Beard growth is normally limited to males
•  In birds
–  males have more ornate plumage
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4-50
•  Another example of a sex-limited trait
–  Feather plumage in chickens (Figure 4.17)
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© robert Maier/Animals, Animals
(a) Hen
© robert Maier/Animals, Animals
(b) Rooster
Figure 4.17
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4-51
Sex-limited Traits
n
n
The pattern of hen-feathering depends on the
production of sex hormones
If the single ovary is surgically removed from a
newly hatched hh female
n
n
She will develop cock-feathering and look
indistinguishable from a male
Sex-limited traits are responsible for sexual
dimorphism
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4-53
Lethal Alleles
•  Essential genes are those that are absolutely
required for survival
–  The absence of their protein product leads to a lethal
phenotype
•  It is estimated that about 1/3 of all genes are essential for
survival
•  Nonessential genes are those not absolutely
required for survival
•  A lethal allele is one that has the potential to
cause the death of an organism
–  These alleles are typically the result of mutations in
essential genes
–  They are usually inherited in a recessive manner
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4-54
•  Many lethal alleles prevent cell division
–  These will kill an organism at an early age
•  Some lethal alleles exert their effect later in life
–  Huntington disease
•  Characterized by progressive degeneration of the nervous
system, dementia and early death
•  The age of onset of the disease is usually between 30 and 50
•  Conditional lethal alleles may kill an organism only
when certain environmental conditions prevail
–  Temperature-sensitive (ts) lethals
•  A developing Drosophila larva may be killed at 30° C
•  But it will survive if grown at 22° C
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4-55
Pleiotropic Effects
•  Most genes actually have multiple effects
•  Multiple effects of a single gene on the
phenotype of an organism is called pleiotropy
•  Can be caused because
–  The gene product can affect cell function in more
than one way
–  The gene may be expressed in different cell types
–  The gene may be expressed at different stages of
development
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4-58
Pleiotropic Effects
•  Cystic fibrosis as an example
–  Normal allele encodes the cystic fibrosis
transmembrane conductance regulator (CFTR)
–  Regulates ionic balance by transporting Cl- ions
–  Mutant does not transport Chloride effectively
•  In lungs, this causes very thick mucus
•  On the skin, causes salty sweat
•  Males are often sterile because Cl- transport is needed
for proper development of the vas deferens
–  Defect in CFTR can have multiple effects
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4-59
4.2 GENE INTERACTIONS
•  Gene interactions occur when two or more
different genes influence the outcome of a
single trait
•  Indeed, morphological traits such as height
weight and pigmentation are affected by many
different genes in combination with
environmental factors
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4-60
4.2 GENE INTERACTIONS
Mendelian Inheritance Patterns Involving Two genes
4-61
•  There can be dozens of genes affecting complex
traits like height. To make our analysis
understandable, we will look at different cases, all
involving two genes that exist in two alleles
•  The crosses we will perform can be illustrated in this
general pattern
–  AaBb X AaBb
•  Where A is dominant to a and B is dominant to b
•  If these two genes govern two different traits
–  A 9:3:3:1 ratio is predicted among the offspring
•  However, the two genes in this section do affect the
same trait
–  The 9:3:3:1 ratio may be affected
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4-62
•  Epistasis is when the alleles of one gene mask the
phenotypic effects of the alleles of another
–  Epistasis is considered relative to a particular phenotype
–  The case of walnut comb is recessive epistasis because
the homozygote or r or p is required to mask the walnut
phenotype.
•  Note:
–  Mendel s laws of segregation and independent assortment
still hold!
–  Epistasis may mask the phenotypes, but the 9:3:3:1 ratio
of genotypes is still present
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4-66
•  Note that Figure 4.20 showed the cross of two true-breeding
mutant lines with the same phenotype producing wild type
offspring
•  This is called complementation
–  Complementation shows that the two mutant lines had the
same phenotype caused by mutations in different genes
Mutation that knocks out
either enzyme
Colorless
precursor
Enzyme C
Colorless
intermediate
Enzyme P
Purple
pigment
Result in the same phenotype
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4-71
A Cross Involving a Two-Gene Interaction Can
Produce three distinct phenotypes due to
Epistasis •  Inheritance of coat color in rodents
–  A true-breeding black rat crossed to true-breeding
albino results in agouti coat color
•  Agouti have black at the tips of each hair that changes
to orange near the root
–  If two F1 agouti animals are crossed, they produce
offspring in the following ratios
•  9 agouti
•  3 black
•  4 albino
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4-72
Gene Redundancy
n
Geneticists have developed techniques to
directly generate loss-of-function alleles
This is called a gene knockout
n  Allows scientists to understand the affects of the
gene on structure or function of the organism
n
n
Interestingly, many knockouts have no
obvious phenotype
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4-84
Gene Redundancy
n
This may be due to gene redundancy where
one gene can compensate for the loss of
function of another
n
n
May be due to gene duplication
Duplicate genes are called paralogs
n
They are not identical because of accumulated
mutations during evolution
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Suppressor Mutations
n
A suppressor mutation is a second mutation
that reverses the phenotypic effect of a first
mutation
n
When it is in a different gene it is called an
intergenic or extragenic suppressor
Example-Hairless is a dominant mutant (H) which
causes flies to not have any sensory bristles
n  Suppressor of Hairless is a dominant (SoH)
mutation which produces a normal number of
bristles in Hairless mutant flies
n
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