Download Lecture 9 PP

LECTURE 9
Extensions of
Mendel’s First Law
(Chapter 4)
INTRODUCTION
• Mendel's First Law states that:
– Adults are diploid; gametes are haploid
– Each trait is controlled by a single gene
• For the traits Mendel studied in pea, the following were
also true:
–
–
–
–
–
–
Each gene has two alternative alleles
One allele is completely dominant over the other
All gametes are equally viable
Mating is random
All offspring are equally viable
Genotype always determines phenotype
• Hence Mendel observed a 3:1 phenotypic ratio when two
heterozygotes (dihybrids) are crossed
INTRODUCTION
• In this lecture we will examine traits that do not
result in a 3:1 phenotypic ratio when two dihybrids
(heterozygotes) are crossed
• Can be due to one of two reasons
– Extensions: Mendel's First Law is operating (adults
are diploid and gametes are haploid one gene controls
the trait) but some of the other assumptions underlying
the 3:1 phenotypic ratio are not met
– Violations: Mendel's First Law is NOT operating
• Adults are not always diploid; gametes are not always haploid
• More than one gene controls the trait
Mendel
Extension OR Violation
One allele is completely
dominant over the other
in all instances
EXTENSION: Alleles may be Incompletely
Dominant or Codominant; alleles can be
dominant in one sex and recessive in the other
(Sex Influenced); some alleles are expressed in
only one sex (Sex Limited)
All offspring from a cross
are equally viable
EXTENSION: Some genotypes survive better than
others; some genotypes may actually be fatal;
Overdominance, lethal alleles, semi-lethal alleles
Genotype does not always
determine phenotype
EXTENSION: AA does not always exhibit the
dominant phenotype, etc. Incomplete
penetrance, variable expressivity
Adults are diploid for all
genes and gametes are
haploid for all genes
VIOLATION: Males are haploid for X-linked genes;
gametes are nulliploid; Females are nulliploid for
Y (Sex linkage); Some adults have more than two
sets of genes (polyploidy); Some "adults" are
halpoid (haploidy - e.g. yeast); Some organisms
are both (Alternation of generations - e.g. moss)
More than one gene
controls a trait
VIOLATION: Often (in fact most of the time) traits
are controlled by more than one gene (Gene
interactions)
Complete Dominance
• In a simple dominant/recessive relationship, the
recessive allele does not affect the phenotype of the
heterozygote
• Usually, the mutant allele is recessive to the wild-type
because of one of the following:
– 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
Figure 4.2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
• But ... mutations are sometimes dominant
– Much less common than recessive
– Three explanations for most dominant mutations
• Gain-of-function
– Protein encoded by the mutant gene is changed so it gains a
new or abnormal function
• Dominant-negative
– Protein encoded by the mutant gene acts antagonistically to
the normal protein (also called a "poisonous allele")
• Haploinsufficiency
– mutant is loss-of-function
– heterozygote does not make enough product to give the wild
type phenotype
Incomplete Dominance
• In incomplete dominance the heterozygote exhibits a
phenotype that is intermediate between the
corresponding homozygotes and different from either one
• 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
– Note how the nomenclature has changed from Cc to
superscripts ("C" is still the gene)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Figure 4.3
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Red
White
P generation
CRCR
Gametes CR
1:2:1 phenotypic
ratio NOT the 3:1
ratio observed in
simple Mendelian
inheritance
CW CW
x
CW
Pink
F1 generation
CRCW
Gametes CR or CW
Self-fertilization
Sperm
F2 generation
CR
CW
CRCR
CRCW
CRCW
CW CW
CR
Egg
CW
In this case, 50% of
the CR protein is not
sufficient to produce
the red phenotype
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
Figure 4.4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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)
Codominance
• In Codominance, heterozygotes express the
phenotypes of both parents
• The ABO blood group provides an example
– Phenotype (A, B, AB or O) 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, adds antigen A to H antigen
– Allele IB, adds antigen B to H antigen
– Allele i, doesn't add anything to H antigen
• 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
Antigen A
Antigen B
H antigen
RBC
N-acetylgalactosamine
Galactose
RBC
Blood type:
O
A
B
AB
Genotype:
ii
IAIA or IAi
IBIB or IBi
IAIB
neither A or B
against A and B
A
against B
B
against A
Surface antigen:
Serum antibodies:
(a) ABO blood type
Figure 4.11a
A and B
none
Figure 4.11c
Antigen A
Glycosyl transferase
encoded by IA allele
Active
site
RBC
N-acetylgalactosamine
Glycosyl transferase
encoded by IB allele
RBC
Antigen B
Active
site
RBC
Galactose
(c) Formation of A and B antigen by glycosyl transferase
RBC
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
Sex-influenced Traits
• Example: Pattern baldness in humans
– Controlled by an autosomal gene with two alleles
• Allele B* is dominant in males, but recessive in
females
Genotype
Phenotype
in Males
Phenotype
in Females
B*B*
pattern-bald
late onset hair loss
BB*
pattern-bald
nonbald
BB
nonbald
nonbald
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)
Figure 4.15
© National Parks Service, Adams National Historical Park
b) John Quincy Adams (son)
© National Parks Service, Adams National Historical Park
(c) Charles Francis Adams
(grandson)
© Bettmann/Corbis
(d) Henry Adams
(great-grandson)
Sex-influenced Traits




Pattern baldness appears to be related to the
production of the male sex hormone testosterone
Pattern baldness results from overexpression of
a gene that converts testosterone to 5-adihydrotestosterone (DHT) which binds to
cellular receptors and alters expression of many
genes
In females, heterozygotes (Bb) are not bald
Women who are homozygous for the baldness
allele (BB) will develop the trait, characterized by
a significant thinning of the hair relatively late in
life

The autosomal nature of pattern baldness has
been revealed by analysis of human pedigrees

Refer to Figure 4.16
Bald fathers can pass
the trait to their sons
I-1
IV-1
IV-2
I-2
II-1
II-2
II-3
II-4
II-5
II-6
II-7
II-8
III-1
III-2
III-3
III-4
III-5
III-6
III-7
III-8
III-9
III-10
IV-3
IV-4
IV-5
IV-6
IV-7
IV-8
IV-9
IV-10
IV-11
IV-12
(a) A pedigree for human pattern baldness
Bb
×
Bb
Sperm
B
Figure 4.16
b
B
BB
Bald male
Bald female
Bb
Bald male
Nonbald female
b
bb
Bb
Nonbald male
Bald male
Nonbald female Nonbald female
(b) Example of an inheritance pattern involving baldness
I?V-13
IV-14
Sex-limited Traits
• Traits that are expressed in only one of the
two sexes
• Responsible for sexual dimorphism
– 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
• Usually due to presence/absence of a
hormone
Hereditary breast cancer
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© robert Maier/Animals, Animals
© robert Maier/Animals, Animals
(b) Rooster
Mendel
Extension OR Violation
One allele is completely
dominant over the other
in all instances
EXTENSION: Alleles may be Incompletely
Dominant or Codominant; alleles can be
dominant in one sex and recessive in the other
(Sex Influenced); some alleles are expressed in
only one sex (Sex Limited)
All offspring from a cross
are equally viable
EXTENSION: Some genotypes survive better than
others; some genotypes may actually be fatal;
Overdominance, lethal alleles, semi-lethal alleles
Genotype does not always
determine phenotype
EXTENSION: AA does not always exhibit the
dominant phenotype, etc. Incomplete
penetrance, variable expressivity
Adults are diploid for all
genes and gametes are
haploid for all genes
VIOLATION: Males are haploid for X-linked genes;
gametes are nulliploid; Females are nulliploid for
Y (Sex linkage); Some adults have more than two
sets of genes (polyploidy); Some "adults" are
halpoid (haploidy - e.g. yeast); Some organisms
are both (Alternation of generations - e.g. moss)
More than one gene
controls a trait
VIOLATION: Often (in fact most of the time) traits
are controlled by more than one gene (Gene
interactions)
Overdominance
• Overdominance is the phenomenon in which a
heterozygote is more vigorous than both of the
corresponding homozygotes
– It is also called heterozygote advantage
• Usually due to one of three reasons:
– Protection from microorganisms
– Homodimer formation
– Expansion of range of enzyme function
Figure 4.8a
• A microorganism will infect a cell if
certain cellular proteins function
optimally

Heterozygotes have one altered
copy of the gene



Therefore, they have slightly reduced
protein function
This reduced function is not enough to
cause serious side effects
 But it is enough to prevent infections
Examples include


Sickle-cell anemia and malaria
Tay-Sachs disease
 Heterozygotes are resistant to
tuberculosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pathogen can
successfully
propagate.
A1A1
Normal homozygote
(sensitive to infection)
Pathogen
cannot
successfully
propagate.
A1A2
Heterozygote
(resistant to infection)
(a) Disease resistance
Figure 4.8b
• Some proteins function as
homodimers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
– Composed of two different subunits
– Encoded by two alleles of the same gene
• A1A1 homozygotes
– Make only A1A1 homodimers

A2A2 homozygotes


Make only A2A2 homodimers
A1A2 heterozygotes



Make A1A1 and A2A2 homodimers
AND A1A2 homodimers
For some proteins, the A1A2 homodimer
may have better functional activity
 Giving the heterozygote superior
characteristics
A1
A1
A2
A2
(b) Homodimer formation
A1
A2
Figure 4.8c
• A gene, E, encodes a metabolic
enzyme
• Allele E1 encodes an enzyme that
functions better at lower
temperatures
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
E1
E2
27°–32°C
(optimum
temperature
range)
30°–37°C
(optimum
temperature
range)
(c) Variation in functional activity

Allele E2 encodes an enzyme that
functions better at higher
temperatures

E1E2 heterozygotes produce both
enzymes

Therefore they have an advantage in
that they function over a wider
temperature range than either E1E1 or
E2E2 homozygotes
Lethal and Semilethal 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
• Many lethal alleles prevent cell division
– These will kill an organism at an early age
• A lethal allele will produce ratios that deviate from
Mendelian ratios
• If recessive, the mutant phenotype will never be observed!
– Though may lead to more miscarriages in the mother
• If dominant and the homozygous dominant offspring do not
survive (the usual case), the ratio will be 2 mutant: 1 normal
–
–
–
–
–
An example is the Manx cat
Carries a dominant mutation that affects the spine
This mutation shortens the tail
This allele is lethal as a homozygote for the dominant allele
Refer to Figure 4.18b
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mm ×
(Manx)
Mm
(Manx)
Sperm
M
DEATH
M
m
Mm
(Manx)
2 Manx for every one normal kitten
Egg
m
Mm
(Manx)
mm
(non-Manx)
(b) Example of a Manx
inheritance pattern
Figure 4.18b
Example of Manx Inheritance patterns
Mendel's ratio has now changed to 2:1
• Semilethal alleles
– Kill some individuals in a population, not all of them
– If the allele is recessive, Mendel's ratio will be >3:1
• Many (but not all) in the "cc" category will die
• "CC" and "Cc" categories are enriched relative to "cc"
• E.g. 4:1, 15:1, 100:1, etc. depending on what fraction of "cc"
offspring die
– If the allele is dominant and the homozygous dominant
offspring do not survive very well, the ratio will be
somewhere between 2:1 (entirely lethal) and 3:1
(Mendelian - no lethality)
Mendel
Extension OR Violation
One allele is completely
dominant over the other
in all instances
EXTENSION: Alleles may be Incompletely
Dominant or Codominant; alleles can be
dominant in one sex and recessive in the other
(Sex Influenced); some alleles are expressed in
only one sex (Sex Limited)
All offspring from a cross
are equally viable
EXTENSION: Some genotypes survive better than
others; some genotypes may actually be fatal;
Overdominance, lethal alleles, semi-lethal alleles
Genotype does not always
determine phenotype
EXTENSION: AA does not always exhibit the
dominant phenotype, etc. Incomplete
penetrance, variable expressivity
Adults are diploid for all
genes and gametes are
haploid for all genes
VIOLATION: Males are haploid for X-linked genes;
gametes are nulliploid; Females are nulliploid for
Y (Sex linkage); Some adults have more than two
sets of genes (polyploidy); Some "adults" are
halpoid (haploidy - e.g. yeast); Some organisms
are both (Alternation of generations - e.g. moss)
More than one gene
controls a trait
VIOLATION: Often (in fact most of the time) traits
are controlled by more than one gene (Gene
interactions)
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
Figure 4.5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
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
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
• 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
• Please note: A mutant phenotype can be:
– BOTH incompletely penetrant and variably expressed
– ONLY incompletely penetrant (everybody who exhibits
the phenotype does so to the same degree
– ONLY variably expressed (everyone expresses the
phenotype associated with the genotype but to different
degrees
Mendel
Extension OR Violation
One allele is completely
dominant over the other
in all instances
EXTENSION: Alleles may be Incompletely
Dominant or Codominant; alleles can be
dominant in one sex and recessive in the other
(Sex Influenced); some alleles are expressed in
only one sex (Sex Limited)
All offspring from a cross
are equally viable
EXTENSION: Some genotypes survive better than
others; some genotypes may actually be fatal;
Overdominance, lethal alleles, semi-lethal alleles
Genotype does not always
determine phenotype
EXTENSION: AA does not always exhibit the
dominant phenotype, etc. Incomplete
penetrance, variable expressivity
Adults are diploid for all
genes and gametes are
haploid for all genes
VIOLATION: Males are haploid for X-linked genes;
gametes are nulliploid; Females are nulliploid for
Y (Sex linkage); Some adults have more than two
sets of genes (polyploidy); Some "adults" are
halpoid (haploidy - e.g. yeast); Some organisms
are both (Alternation of generations - e.g. moss)
More than one gene
controls a trait
VIOLATION: Often (in fact most of the time) traits
are controlled by more than one gene (Gene
interactions)
• Humans have 46 chromosomes
– 44 autosomes
– 2 sex chromosomes
• Males contain one X and one Y chromosome
– They are termed heterogametic
• Females have two X chromosomes
– They are termed homogametic
• The Y chromosome determines maleness
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
44 +
XY
(a) X–Y system in mammals
44 +
XX
• In some insects,
– Males are X0 and females are XX
• In other insects (fruit fly, for example)
– Males are XY and females are XX
• The Y chromosome does not determines maleness
• Rather, it is the ratio between the X chromosomes
and the number of sets of autosomes (X/A)
– If X/A = 0.5, the fly becomes a male
– If X/A = 1.0, the fly becomes a female
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 +
X
(b) The X–0 system in certain insects
22 +
XX
• The sex chromosomes are designated Z and W to
distinguish them from the X and Y chromosomes of
mammals
• Males contain two Z chromosomes
– Hence, they are homogametic
• Females have one Z and one W chromosome
– Hence, they are heterogametic
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
76 +
ZZ
(c) The Z–W system in birds
76 +
ZW
and some fish
• Males are known as the drones
– They are haploid
– Produced from unfertilized haploid eggs
• Females include the worker bees and queen bees
– They are diploid
– Produced from fertilized eggs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
16
haploid
(d) The haplodiploid system in bees
32
diploid
X-inactivation in Female Mammals
• Evens out gene dosage between males and
females
– Both have only one functional X per cell
– “All but one” rule
– Barr body
• In humans, wave of X-inactivation early in
embryogenesis
– Each cell makes an independent “decision” to
turn off the paternal or maternal X
– Females are “mosaic” if heterozygous for genes
on the X chromosome
X-Inactivation
Tribble: B_D_S_XOXo
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
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 during prophase of MI
Mic2
gene
X
Y
Figure 4.14
Mic2
gene
Mendel
Extension OR Violation
One allele is completely
dominant over the other
in all instances
EXTENSION: Alleles may be Incompletely
Dominant or Codominant; alleles can be
dominant in one sex and recessive in the other
(Sex Influenced); some alleles are expressed in
only one sex (Sex Limited)
All offspring from a cross
are equally viable
EXTENSION: Some genotypes survive better than
others; some genotypes may actually be fatal;
Overdominance, lethal alleles, semi-lethal alleles
Genotype does not always
determine phenotype
EXTENSION: AA does not always exhibit the
dominant phenotype, etc. Incomplete
penetrance, variable expressivity
Adults are diploid for all
genes and gametes are
haploid for all genes
VIOLATION: Males are haploid for X-linked genes;
gametes are nulliploid; Females are nulliploid for
Y (Sex linkage); Some adults have more than two
sets of genes (polyploidy); Some "adults" are
halpoid (haploidy - e.g. yeast); Some organisms
are both (Alternation of generations - e.g. moss)
More than one gene
controls a trait
VIOLATION: Often (in fact most of the time) traits
are controlled by more than one gene (Gene
interactions)
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
• We will look at three different cases, all
involving two genes that exist in two alleles
– NOTICE THAT TWO GENES ARE CONTOLLING A
SINGLE TRAIT
• Mendel would have predicted a 3:1 ratio
• But when two genes control the same trait, you
are really doing a dihybrid cross, not a
monohybrid one! You just don't know it -- until
you look at the cross data.
A Cross Involving a Two-Gene Interaction Can
Produce two distinct phenotypes
• Inheritance of flower color in the sweet pea
– Lathyrus odoratus normally has purple flowers
• Bateson and Punnett obtained several true-breeding
varieties with white flowers
• They carried out the following cross
– P: True-breeding purple X true-breeding white
– F1: Purple flowered plants
– F2: Purple- and white-flowered in a 3:1 ratio
• These results were not surprising
CCPP (purple)
x
ccpp (white)
F1 generation
Why would Mendel
have expected to see
a 3:1 ratio?
All purple
(CcPp)
Self-fertilization
F2 generation
CP
F2 generation
Cp
cP
cp
CP
CCPP
Purple
CCPp
Purple
CcPP
Purple
CcPp
Purple
Cp
CCPp
Purple
CCpp
White
CcPp
Purple
Ccpp
White
cP
CcPP
Purple
CcPp
Purple
ccPP
White
ccPp
White
cp
CcPp
Purple
Ccpp
White
ccPp
White
ccpp
White
Epistasis: Homozygosity
for the recessive allele
of either gene results in
a white phenotype, thereby
masking the purple
(wild-type) phenotype.
Both gene products
encoded by the wild-type
alleles (C and P) are
needed for a purple
phenotype.
A Cross Involving a Two-Gene Interaction Can
Produce three distinct phenotypes
• Inheritance of coat color in laborador
retrievers
– A true-breeding black lab is crossed to a purebreeding yellow lab
• F1 labs are all black
– If two F1 animals are crossed, they produce
offspring in the following ratios
• 9 black
• 3 chocolate
• 4 albino
• 9 B_C_ = black: 3 B_cc = chocolate: 4 bb__ = yellow
BC
Bc
bC
bc
BC
BBCC
black
BBCc
black
BbCC
black
BbCc
black
Bc
BBCc
black
BBcc
chocolate
BbCc
black
Bbcc
chocolate
bC
BbCC
black
BbCc
black
bbCC
yellow
bbCc
yellow
bc
BbCc
black
Bbcc
chocolate
bbCc
yellow
bbcc
yellow
Gene Redundancy

Geneticists have developed techniques to
directly generate loss-of-function alleles



This is called a gene knockout
Allows scientists to understand the affects of the
gene on structure or function of the organism
Interestingly, many knockouts have no
obvious phenotype
Gene Redundancy

This may be due to gene redundancy where
one gene can compensate for the loss of
function of another


May be due to gene duplication
Duplicate genes are called paralogs

They are not identical because of accumulated
mutations during evolution
Gene Redundancy

George Shull conducted the first studies on
gene redundancy




Studied a weed known as shepherd’s purse
Trait followed was the shape of the seed capsule which
is usually triangular
Strains producing small ovate capsules are due to lossof-function alleles in two genes (ttvv)
True breeding strains were crossed
 Triangular x Ovate
 F1 progeny were crossed to one another
 Refer to Figure 4.24 for results of the cross
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
x
TTVV
Triangular
ttvv
Ovate
F1 generation
TtVv
All triangular
F1 (TtVv) x F1 (TtVv)
F2 generation
TV
Tv
tV
tv
TTVV
TTVv
TtVV
TtVv
TTVv
TTvv
TtVv
Ttvv
TtVV
TtVv
ttVV
ttVv
TtVv
Ttvv
ttVv
ttvv
TV
Tv
tV
tv
Figure 4.24 A 15:1 ratio results from gene redundancy. Either dominant
allele, T or V, is sufficient to give a triangular seed