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Chapter 4
Extensions of Mendelian Genetics
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2009 Pearson
Education,
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Mutations are the source of
variation
• Wild-type vs. Dominant
• Loss-of-function mutation
• Leads to reduction or loss of gene product
• Gain-of-function mutation
• Enhances the function of the wild-type
• Neutral mutation
• No detectable changes in function
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Symbols
• Mendel used upper/lowercase
• Most common other system is used by fruit fly
geneticists
• Use the initial letter of the mutant
phenotype
• Lowercase if recessive
• Wild-type gets same letters with a “+”
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Incomplete
Dominance
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Figure 4.1
Codominance
• Two alleles are responsible for producing two
distinct, detectable gene products
• Both alleles show up in the phenotype
• MN blood group in humans
• LMLM = type M
• LMLN = type MN
• LNLN = type N
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Codominance
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Genotype
Phenotype
LMLM
M
LMLN
MN
L NL N
N
Multiple Alleles
• The number of alleles in a POPULATION
doesn’t have to be just 2
• Each individual still only has 2
• ABO blood group in humans
• Three alleles
• IA  Produces A antigen
• IB  Produces B antigen
• IO  no antigen
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ABO Blood Groups
Genotype
Antigen
Phenotype
IAIA
A
A
I AI O
A
A
IBIB
B
B
IBIO
B
B
IAIB
A&B
AB
IOIO
None
O
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Table 4.1
Lethal Alleles
• Recessive lethal mutations
• Homozygous recessives do not survive
• May have a mutant phenotype in the
heterozygote
• Dominant lethal mutations
• Homozygous dominant and heterozygous
do not survive
• Affected individual must survive until
adulthood
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Recessive Lethal Mutations
• Usually a result of loss-of-function mutation
• One copy of a functional gene is usually
enough to permit survival
• Heterozygotes are usually OK but may have
a distinct phenotype
• Example: Yellow Mice
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Yellow Mice
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Genotype
Phenotype
AA
Agouti (w.t.)
AAY
Yellow
AYAY
Lethal
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Figure 4.4
Dominant Lethal Mutations
• One copy of the allele results in death
• The one working allele may not be enough
to compensate
• Mutant gene may override the functioning
wild-type product
• Example: Huntington Disease
• Typical onset around age 40
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Gene pairs with two modes of
inheritance
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Figure 4.5
Phenotypes are often affected by
more than one gene
• Gene interaction
• Several genes interact to influence a
phenotype
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Recessive Epistasis
• Recessive allele at one locus masks the expression of
another set of alleles at a second locus
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Genotype
Phenotype
AA
agouti
Aa
agouti
aa
black
Genotype
Phenotype
BB
color
Bb
color
bb
albino
AaBb x AaBb =
F2 Ratio
Genotype
Phenotype
9/16
3/16
3/16
1/16
A-BA-bb
aaBaabb
agouti
albino
black
albino
Phenotypic
Ratio
9/16 agouti
4/16 albino
3/16 black
Modified phenotypic ratio = 9:4:3 (still adds up to 16)
Precursor molecule (colorless)
Black pigment
Gene A
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Agouti pattern
Gene B
Dominant Epistasis
• Dominant allele at one locus masks the expression of
the alleles at a second locus
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Genotype
Phenotype
AA
White
Aa
White
aa
Whatever locus B is
Genotype
Phenotype
BB
Yellow
Bb
Yellow
bb
Green
AaBb x AaBb =
F2 Ratio
Genotype
Phenotype
9/16
3/16
3/16
1/16
A-BA-bb
aaBaabb
white
white
yellow
green
Phenotypic
Ratio
12/16 white
3/16 yellow
1/16 green
Modified phenotypic ratio = 12:3:1 (still adds up to 16)
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Complementary Gene Interaction
• Dominant allele at two different loci is necessary to
produce a phenotype
F2 Ratio
Genotype
Phenotype
9/16
3/16
3/16
1/16
A-BA-bb
aaBaabb
purple
white
white
white
Phenotypic
Ratio
9/16 purple
7/16 white
Modified phenotypic ratio = 9:7 (still adds up to 16)
Precursor molecule (colorless)
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Intermediate
(colorless)
Gene A
purple
Gene B
Summer Squash
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Novel phenotypes
• Gene interaction results in new phenotypes in the F2
generation
F2 Ratio
Genotype
Phenotype
9/16
3/16
3/16
1/16
A-BA-bb
aaBaabb
disc
sphere
sphere
long
Phenotypic
Ratio
9/16 disc
6/16 sphere
1/16 long
Modified phenotypic ratio = 9:6:1 (still adds up to 16)
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Complementation Analysis
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Figure 4.11
Pleiotropy
• A single gene has multiple phenotypic effects
• Two examples
• Marfan Syndrome
• Porphyria Variegata
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X-Linkage
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Figure 4.12
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Figure 4.13
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Table 4.3
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Sex-Limited
Genotype
Phenotype
FEMALE
MALE
HH
Hen-feathered
Hen-feathered
Hh
Hen-feathered
Hen-feathered
hh
Hen-feathered Cock-feathered
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Sex-Influenced
Genotype
Phenotype
FEMALE
MALE
BB
Bald (less pronounced)
Bald
Bb
Not bald
Bald
bb
Not bald
Not bald
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Penetrance
• Percentage of individuals that show
at least some degree of expression
of a mutant genotype
Expressivity
• Range of expression of the mutant
phenotype
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Conditional Mutation – Temperature
Effect
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Nutritional Effects
• Auxotroph
• Microorganism that has a mutation that
prevents the synthesis of a nutrient
• In humans, ingestion of certain dietary
substances can harm individuals with abnormal
genetic makeups
• Phenylketonuria
• Galactosemia
• Lactose Intolerance
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