Download Extensions to Mendel`s Law

Extensions to Mendel
Lecture 3
Extensions to Mendel’s Law
Complexities in Relating
Genotype to Phenotype
ᯘ⩶ె ⪁ᖌ
http://lms.ls.ntou.edu.tw/course/136
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O tli off the
Outline
th class
l
Part 1
• Single-gene inheritance
• IIn which
hi h pairs
i off alleles
ll l show
h
d
deviations
i ti
ffrom
complete dominance and recessiveness
• In which different forms of the gene are not limited to
two alleles
• Where one gene may determine more than one trait
Part 2
• Multifactorial
M ltif t i l inheritance
i h it
in
i which
hi h th
the phenotype
h
t
arises from the interaction of one or more genes
with
ith th
the environment,
i
t chance,
h
and
d each
h other.
th
3
Part 1
Single-gene inheritance
1.
2.
3.
4.
Incomplete dominance
Codominance
Multiple alleles
Pleiotropy (ӭਏ‫) ܄‬
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D i
Dominance
iis nott always
l
complete
l t
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S
Summary
off d
dominance
i
relationships
l ti
hi
• C
Crosses b
between
t
ttrue-breeding
b di strains
t i
can produce hybrids with phenotypes
different from both parents.
• Incomplete dominance
• F1 hybrids that differ from both parents express an
intermediate phenotype. Neither allele is dominant
or recessive to the other
other.
• Phenotypic ratios are same as genotypic ratios
• Codominance
• F1hybrids express phenotype of both parents
equally.
• Phenotypic ratios are same as genotypic ratios.
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Fig. 3.2
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Incomplete dominance in snapdragons
Fig. 3.3
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C d
Codominant
i
t lentil
l til coatt patterns
tt
Spotted (CSCS) x dotted
(CDCD)
All F1 progeny are spotted
and dotted (CSCD)
F2 progeny ratios:
1 spotted (CSCS)
2 spotted
d and
dd
dotted
d
(CSCD)
1 dotted (CDCD)
Phenotype ratios reflect the
genotype ratios
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Fig. 3.4a
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Do variations
D
i ti
on dominance
d i
relations
l ti
negate Mendel’s law of segregation?
C d
Codominant
i
t blood
bl d group alleles
ll l
Gene I controls
the type of sugar polymer
on surface
f
off RBCs
RBC
Two alleles, IA and IB, result
i diff
in
differentt sugars
• IA IA individuals have A
sugar
• IB IB individuals have B
sugar
• IA IB individuals have
both A and B sugars
Fig. 3.4b
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• Dominance relations affect phenotype
y and
have no bearing on the segregation of
alleles.
• Alleles still segregate randomly.
• Gene products control expression of
phenotypes differently.
• Mendel’s
M d l’ law
l
off segregation
ti still
till applies.
li
• Interpretation
p
of p
phenotype/genotype
yp g
yp
relation is more complex.
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A gene can h
have more th
than ttwo alleles
ll l
• Genes may have multiple alleles that segregate in
populations.
• Although there may be many alleles in a population,
each individual carries only 2 of the alternatives.
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ABO blood types in humans are
determined by three alleles of one gene
IA allele Æ A type sugar
IB allele Æ B type sugar
• ABO blood group
• 3 alleles
• 6 possible ABO genotypes: IAIA, IBIB, IAIB, IAi, IBi, or ii
i allele Æ no sugar
• Dominance relations are unique to a pair of alleles.
• Dominance or recessiveness is always relative to a second allele.
• ABO blood group
• IA is completely dominant to i but codominant to IB.
• 6 genotypes generate 4 phenotypes.
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Six genotypes produce four blood types
Fig. 3.5 a
Dominance relations are relative to a second allele
• IA and IB are codominant
• IA and IB are dominant to i
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Medical and legal implications of ABO
blood group genetics
Antibodies are made against type A and type B sugars
• Successful blood transfusions occur only with matching
blood types
• Type AB are universal recipients, type O are universal
donors
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Seed coat patterns in lentils are
determined by a gene with five alleles
Five alleles for C gene: spotted (CS), dotted (CD),
clear (CC),
) marbled-1
marbled 1 (CM1),
) and marbled
marbled-2
2 (CM2)
Reciprocal crosses between pairs of pure-breeding
li
lines
iis used
d tto d
determine
t
i d
dominance
i
relations
l ti
Figure 3.5 b,c
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Fig. 3.6
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Human histocompatibility antigens are an
extreme example of multiple alleles
How do we establish
dominance relations
between multiple
alleles?
• Perform reciprocal
crosses between
pure breeding lines
of all phenotypes
Three major
Th
j genes (HLA-A,
(HLA A HLA-B,
HLA B and
d HLA-C)
HLA C)
encode histocompatibility antigens
• Cell surface molecules present on all cells except
RBCs and sperm
• Facilitates proper immune response to foreign
antigens (e
(e.g.
g virus or bacteria)
Each g
gene has 20-to-100 alleles each
• Each allele is codominant to every other allele
• Every
E
genotype
t
produces
d
a di
distinct
ti t phenotype
h
t
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Fig. 3.6
• Enormous phenotypic variation
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M t ti
Mutations
are the
th source off new alleles
ll l
• M
Multiple
lti l alleles
ll l arise
i spontaneously
t
l iin nature
t
d
due tto
chance alterations in genetic material – mutations.
• Mutation rate varies from 1 in 10
10,000
000 to 1 in
1,000,000 per gamete per generation.
• Allele frequency is the percentage of the total
number of gene copies represented by one allele.
yp – allele whose frequency
q
y is more than 1%
• Wild-type
• Mutant allele – allele whose frequency is less than
1%
• Monomorphic – gene with only one wild-type allele
• Polymorphic – gene with more than one wild-type
allele
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The mouse agouti gene controls hair color:
One wild
wild--type allele, many mutant alleles
Wild-type agouti allele (A)
produces yellow and black
pigment
i
t iin h
hair
i
14 different agouti alleles in lab
mice but only A allele in wild
mice,
mice
e g mutant alleles a and at
e.g.
• a recessive to A
aa has black only
• at dominant to a but
recessive to A
atat mouse has black on
back and yellow on belly
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One gene may contribute to several
visible characteristics
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Fig. 3.7c
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L th lit
Lethality
With some pleiotropic genes
• Pleiotropy – single gene determines
more than one distinct and seemingly
unrelated characteristics
• Heterozygotes can have a visible
p
phenotype
yp
• Homozygotes can be inviable
• Alleles that affect viability often produce
deviations from a 1:2:1 genoptypic and 3:1
phenotypic ratio predicted by Mendel’s
Mendel s
Laws
• e.g. Many aboriginal Maori men have
respiratory problems and are sterile
ƒ Defects due to mutations in a gene required for
functions of cilia (failure to clear lungs) and
flagella (immotile sperm)
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Pleiotropy – inheritance of coat color in
mice
a. IInbred
b d agouti
ti X
yellow yields 1:1
agouti:yellow
S
Summary
off Part
P t1
Agouti
• Yellow must be AYA
and AY is dominant
to A
b. Yellow x yellow
mice do not breed
true.
• AY is a recessive
l th l AYAY die
lethal.
di in
i
utero and do not
show up as progeny
Fig. 3.9
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Si kl
Sickle-cell
ll syndrome
d
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Pl i t
Pleiotropy
off Sickle-cell
Si kl
ll syndrome
d
• Multiple alleles
• N
Normall wild-type
ild t
is
i HbEA
• More than 400 mutant alleles identified so far
• HbES allele specifies abnormal peptide causing
sickling among red blood cells
• Pleitropy
• HbES affects more than one trait
•
•
•
•
Sickling
g
Resistance to malaria
Recessive lethality
Different dominance relations
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Fig. 3.10
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E d off P
End
Partt 1
Part II
Multifactorial Inheritance
• You should have learned…
• What is single-gene inheritance?
• What are the four extensions of the
Mendel’s Law here?
• How to explain the inheritance phenomena
of lentil’s coat color, snapdragon’s flower
color human blood types
color,
types, mouse hair color
and sickle-cell syndrome?
1. Genes interact to generate novel
phenotypes
2. Complementary gene action
3. Epistasis (΢Տ‫)܄‬
o e a e
effect
ec
4. Environmental
5. Heterogeneous trait
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Two genes can interact to determine
one trait
Novel phenotypes resulting from gene
interactions, e.g. seed coat in lentils
• Novel phenotypes can emerge from the
g
combination of alleles of two genes.
• Cross of tan and gray lentils produce
brown F1 generation
• Cross of F1 generation (brown X brown)
produces brown, tan, gray, and green
lentils
• Explanation emerges from experimental
cross ratios which are 9 brown:3 tan:3 g
gray:
y
1 green
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• Two genes, A and B
• aabb is green and will
breed pure
• AAbb and
d Aabb
A bb are tan
t
• aaBB and aaBb are
gray
• AABB, AABb, AaBB,
and AaBb are brown
Fig. 3.11a
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Results of self
self--crosses of F2 lentils
supports the twotwo-gene hypothesis
Sorting out the dominance relations by
select crosses of lentils
• Self pollination of F2 to produce F3 shows interaction
between two genes.
• F2 phenotypes from dihybrid crosses will be in 9:3:3:1 ratio
only when dominance of alleles at both genes is complete
• Further
F rther crosses ssupport
pport ttwo-gene
o gene h
hypothesis.
pothesis
• Each genotypic class determines a particular phenotype.
*This
This 1: 1: 2: 2: 1: 1: 2: 2: 4 F2 genotypic ratio corresponds
to a 9 brown: 3 tan: 3 gray: 1 green F2 phenotypic ratio29
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Complementary gene action generates
purple flower color in sweet peas
Complementary gene action in
sweet peas
Dihybrid cross
generates 9:7 ratio
in F2 progeny
Purple F1 progeny
are produced by
crosses of two
pure-breeding
b di
white lines
9/16 p
purple
p ((A—B—))
7/16 white (A— bb, aa
B—, aa bb)
B
Figure 3.12a
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Figure 3.12b
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Epistasis results from the effects of an
allele
ll l att one gene masking
ki the
th effects
ff t off
another gene
Possible biochemical explanation
p
for
complementary gene action for flower
color in sweet peas
One pathway has two
reactions catalyzed by
different enzymes
• At least one dominant
allele of both genes is
required
q
for p
purple
p
pigment
• Homozygous
recessive for either or
both genes results in
no pigment
Figure 3.13
The gene that does the masking is epistatic to the
other gene
The gene that is masked is hypostatic to the other
gene
Epistasis can be recessive or dominant
• Recessive – epistatic gene must be homozygous
recessive (e.g. ee)
• Dominant – epistatic gene must have at least one
dominant allele present (e.g. E—)
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Recessive epistasis in Golden
Labrador dogs
Recessive epistasis in Golden
Labrador dogs
• Labrador retriever example – recessive epistasis
• Coat color can be black, chocolate brown, or golden
yellow
• B allele is dominant and
determines black.
• b allele is recessive and
determines brown if
homozygous.
• E allele at second gene has
no affect on coat color.
• e allele is recessive and if
homozygous hides effects of
black or brown alleles.
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• BBEE (black pure
breeding) X bbee (golden
pure breeding) produce
BbEe black F1 offspring.
• BbEe X BbEe produce 9
black (B_E_) for every 3
brown (bbE
(bbE_),
) and 4 gold
(__ee).
• Genotype ee masks the
effect of all B genotypes
• 9:3:4 is a telltale ratio of Figure 3.14a
recessive epistasis.
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Molecular explanation for recessive
epistasis in human blood groups
• Two parents who
are apparently type
O have offspring
that is type A or B
on rare occasions.
• Bombay
phenotype –
mutant recessive
allele at second
gene (hh) masks
phenotype of ABO
alleles
D
Dominance
i
epistasis
i t i
• Presence of dominant allele at second
gene hides effects of alleles at a gene
• 12:3:1 and 13:3 are telltale ratios for
dominance epistasis.
Fig. 3.14b
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Dominant epistasis I in summer
squash
D
Dominant
i
t epistasis
i t i II in
i chickens
hi k
13:3
13
3 ratio
ti in
i F2 progeny off
dihybrid crosses indicates
d i
dominant
t epistasis
i t i II
12:3:1 ratio in F2 progeny of
dihybrid
y
crosses indicates
dominant epistasis I
13/16 white
hit
(A— B—, aa B—, aa bb)
3/16 colored (A— bb)
12/16 white (A— B—, aa B—)
3/16 yellow (A
(A— bb)
1/16 green (aa bb)
The dominant allele of one
gene masks both alleles of
another gene
The dominant allele of one
gene masks the dominant
allele of another gene
Figure 3.15a
Figure 3.15b
3 15b
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Summary of gene interactions
discussed in this chapter
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Heterogeneous traits and the
complementation test
Observing the F2 ratios below is diagnostic of the type
of gene interaction
• These
Th
F2 ratios
ti occur only
l in
i dihybrid
dih b id crosses
where there is complete dominance
Heterogeneous traits have the same phenotype
but are caused by mutations in different genes
• e.g. deafness in humans can be caused by
mutations
t ti
in
i ~ 50 different
diff
t genes
Complementation
C
l
t ti testing
t ti is
i used
d to
t determine
d t
i if a
particular phenotype arises from mutations in the
same or separate
t genes
• Can be applied only with recessive, not
dominant, phenotypes
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Table 3.2
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Genetic heterogeneity in humans: Mutations
in many genes can cause deafness
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Interaction of two incompletely dominant
genes can produce nine phenotypes
Example, two genes A and
B:
Limit of complementation test: dominant mutant
• Allele A is incompletely
dominant to allele a
• Allele B is incompletely
dominant to allele b
F1 (all
identical)
F2
For each gene, two alleles
generate
t three
th
phenotypes
Fig. 3.16
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• F2 progeny have 32
phenotypes
Fig. 3.17
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Breeding studies help determine
inheritance of a trait
Two hypotheses to explain phenotypes in F2
progeny of mice with different coat colors
How do we know if a trait is caused by one gene or
by two genes that interact?
Are these F2 progeny in a ratio of 9:3:4 or 1:2:1?
Example: dihybrid cross of pure-breeding parents
produces three phenotypes in F2 progeny
• If single gene with incomplete dominance
dominance, then F2
progeny should be in 1:2:1 ratio
• If two independently assorting genes and recessive
epistasis, then F2 progeny should be in 9:3:4 ratio
• Further breeding studies can reveal which hypothesis
is correct
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Fig. 3.18
(top)
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Specific breeding tests can help
decide between two hypotheses
Hypothesis 1 – two genes
with recessive epistasis
Pedigree analysis can be used to test trait
inheritance hypotheses in humans
Hypothesis 2 – one gene with
incomplete dominance
•OCA
OCA
(ocularcutaneous
albanism) produces
little or no
pigmentation in
skin,, hair,, and eyes.
y
•Can you determine
the inheritance from
pedigrees?
Figure 3.18 (bottom)
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Fig. 3.19
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Family pedigrees help unravel the genetic
basis of ocularocular-cutaneous albinism (OCA)
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The same genotype does not always
produce the same phenotype
In all of the traits discussed so far, the
relationship between a specific genotype and
its corresponding phenotype has been
absolute
What can we learn
from this pedigree?
1. OCA is recessive
2. OCA is heterogeneity
Phenotypic variation for some traits can occur
because of:
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Fig. 3.19 b,c
•
•
•
•
Differences in penetrance and/or expressivity
Effects of modifier genes
Effects of environment
Pure chance
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Phenotype often depends
on penetrance and/or expressivity
• Penetrance )Ѧᡉ౗* is the percentage of a
population
l ti with
ith a particular
ti l genotype
t
th
thatt
shows the expected phenotype
• Can be complete (100%) or incomplete (e.g.
penetrance of retinoblastoma is 75%)
Genetics 2011
Some traits result from different genes that
do not contribute equally to the phenotype
Modifier genes alter the phenotypes produced by
alleles of other genes can have major effect or
more subtle effects
Example: T locus of mice
• Mutant T allele causes abnormally short tail
• Outbred mice with T mutant
• Expressivity )߄౜ࡋ*!is
)߄౜ࡋ* is the degree or
intensity with which a particular genotype is
expressed in a phenotype
• Can be variable or unvarying
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• Tail length different
• Not effected byy diet,, bedding,
g, cage
g temperature
p
• Inbred mice with T allele have tails same length (but some are 75%
the length of normal tails, some are 10%)
• Different inbred strains must carry alternative alleles of a modifier
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gene for the T mutant phenotype
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E i
Environmental
t l effects
ff t on phenotype
h
t
Temperature is a common element of the
environment that can affect phenotype
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A temperature sensitive mutation
affects coat color in Siamese cats
• Example 1: Coat color in Siamese cats
ƒ Extremities are darker than body because of a
temperature sensitive allele
• Example 2: Survivability of a Drosophila mutant
ƒ Shibire mutants develop
p normally
y at < 29oC but
are inviable at temperatures > 29oC
Conditional lethal mutations are lethal
only under some conditions
Fig. 3.20
• Permissive conditions - mutant allele has wild-type functions
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• Restrictive conditions - mutant allele has defective functions
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Other effects of environment on
phenotype
• Ph
Phenocopy - phenotype
h
t
arising
i i from
f
an environmental
i
t l
agent that mimics the effect of a mutant gene
• Not heritable
• Can be deleterious or beneficial
• Examples in humans
ƒ Thalidomide produced a phenocopy of phocomelia, a rare
dominant trait
ƒ Children with heritable PKU can receive a protective diet
ƒ Genetic predisposition to cardiovascular disease can be
influenced by diet and exercise
ƒ Genetic predisposition to lung cancer is strongly affected by
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cigarette smoking
Genetics 2011
Phenocopy
Ph
• Phocomelia ੇଃަ྾‫׎‬
• Rare dominant trait
• Uptake thalidomide by
pregnant woman
• During the late 1950s
and
d early
l 1960
1960s as an
antiemetic
• From 1956 to 1962
1962, about
10,000 children were born with severe
malformities
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Genetics 2011
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Continuous variation can be explained
by Mendelian analysis
Th lid
Thalidomide
id
• Blocked by FDA in
1962
• Approved
pp
by
y FDA in
1998
• Used in ࿂ᅭੰΓ‫ޑ‬࿂
ᅭ่࿯‫܄‬आඬ
(Erythema Nodosum
Leprosum; ENL)
• Previous examples of Mendelian inherited
traits were discontinuous, clear cut
phenotypes.
phenotypes
• Continuous traits such as height in humans
are determined by segregating alleles of
many genes interacting with one another and
the environment.
• Continuous traits are called quantitative traits
by
y geneticists
g
and are usuallyy p
polygenic.
yg
• The more genes that contribute to a trait, the
greater the number of p
g
possible p
phenotypic
yp
classes and the greater the similarity to
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continuous variation.
FDA inspector Frances Oldham Kelsey
receiving an award from President
John F
F. Kennedy in 1962 for blocking use
of the drug in the United States
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Mendelian explanation of
continuous variation
T
Two
continuous
ti
traits
t it iin h
human
• Height
g is a continuous
trait
• Skin color is a
continuous trait
The more genes or
alleles,
ll l
th
the more
possible phenotypic
classes
l
and
d th
the
greater the similarity
t continuous
to
ti
variation
In these examples, all of the
alleles are incompletely
dominant and have additive
effects
Fig 3.21
Fig.
3 21
59
Fig. 3.22 (partial)
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Mendelian explanation of continuous
variation (continued)
E d off P
End
Partt 2
• The more genes or
alleles,
ll l
th
the more
possible phenotypic
classes
l
and
d th
the
greater the similarity
t continuous
to
ti
variation
•
In these examples,
all of the alleles are
incompletely
dominant and have
additive effects
Genetics 2011
• You should have learned…
• What is multifactorial Inheritance? What is
the telltale ratio of F2?
• What is complementary gene action? What
is the telltale ratio?
• What is Epistasis? What is the telltale
ratio?
• Can you give an example of environment
effect?
Fig. 3.22 (partial)
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Mouse s coat and tail: example of multiple
Mouse’s
alleles and multifactorial traits
Genetics 2011
Mouse’s coat and tail: example
p of multiple
p
alleles and multifactorial traits
• Gene 2 – black or brown
• Second gene specifies whether dark hair from gene one
is black or brown.
• Gene 1 Agouti and other coat patterns
• Agouti
A
ti gene has
h multiple
lti l alleles
ll l
• Wild-type A allele specifies bands of yellow and
bl k (bl
black
(blended
d d iinto
t d
dark
k gray).
)
Y
• A gets rid of black and produces solid yellow.
• a gets
t rid
id off yellow
ll
and
d produces
d
solid
lid bl
black.
k
t
• a specifies black on the back and yellow on the
stomach
• Dominance series is AY > A > at > a.
• AY is however
however, recessive to all others for lethality
lethality.
63
• b is recessive and generates brown.
• B is dominant and generates black.
hair, and thus acts in
• AY alleles completely eliminates dark hair
a dominant epistatic manner to the B gene.
• A_B_ produces wild-type agouti with black and yellow
hairs.
hairs
• A_bb generates cinnamon (hairs with strips of brown and
yellow)
• aabb is all brown
brown.
• atatbb is brown on the back and yellow on the stomach.
produce a ratio of 8 yyellow
• AYaBb X AYaBb would p
(AY_,__), 3 black (aa, B_), 1 brown (aa, bb). The AYAY
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class of 4 would be missing because of lethality.
Genetics 2011
Mouse’s coat and tail: example of multiple
alleles and multifactorial traits
• Gene 3 – Albino or pigmented
Mouse’s coat and tail: example of multiple
alleles and multifactorial traits
• Gene 4 – Short and long tail
• A third gene c, abolishes the function of the
enzyme
y
that leads to formation of dark
pigment melanin when recessive. It is thus
recessive epistatic
p
to all others.
• cc is all white, Cc are agouti, black, brown,
yellow, or yellow and black, etc. depending
on the A and B genes.
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Essential Concepts
Genetics 2011
• Wild type (++) mice have long tapering tails.
• X
X-ray
ray induced T mice have short tails.
• TT mice during gestation making the
mutation recessive lethal
• T+ mutants have short tails.
• Variable expressivity in tail length is due to
modifier genes.
• May be viewed as a quantitative trait
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• The F1 phenotype generated by a pair of alleles
defines
de
es the
t e dominance
do
a ce relationship
e at o s p bet
between
ee tthese
ese
alleles.
• Incomplete dominance – F1 resembles neither parent
• Codominance
C
– F1 resembles components off each parent
• One gene can contribute to multiple traits. Dominance
relations for these genes can vary
vary.
• A single gene may have many alleles. New alleles
arise by mutation
mutation.
• Wild type alleles > 1% in the population.
• Mutant alleles < 1% in the population.
• Two or more wild-type alleles in the population
constitute a p
polymorphic
y
p
gene.
g
• A gene with only one wild-type allele is monomorphic.
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Genetics 2011
Essential Concepts
• Two or more genes may interact in several ways to
produce
d
a phenotype.
h
t
• Observation of deviations from traditional Mendelian
phenotypic ratios often generates an understanding of the
gene interactions.
• Epistasis is when the action of an allele at one gene
hid traits
hides
t it normally
ll caused
db
by alleles
ll l att another
th gene.
• Complementary gene action is when dominant alleles
of two or more genes are required to generate a trait
trait.
• With heterogeneity, mutant alleles at any one of two or
more genes are sufficient to elicit a phenotype
phenotype.
• Complementation tests can reveal whether a particular
phenotype
p
yp arises from mutations in the same or
separate genes.
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Genetics 2011
E
Essential
ti l Concepts
C
t
• E
Expression
i off phenotypes
h
t
can b
be modified
difi d b
by
the environment, chance, or other genes.
• Incomplete penetrance is when fewer
f
than 100%
%
of individuals with the same genotype express a
specific phenotype
phenotype.
• A phenotype may show variable expressivity when
it is expressed at a different level in different
individuals with the same genotype.
• Aq
quantitative,, or continuous trait can have any
y
value of expression between two extremes. Most
continuous traits are polygenic.
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