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MENDEL’S PRINCIPLES
9.1 The science of genetics has ancient roots
• The science of heredity dates back to ancient
attempts at selective breeding
• Parents pass their traits (color of hair, color of
eyes etc.) to children- How?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.2 Experimental genetics began in an abbey
garden
• Modern genetics began with Gregor Mendel’s
quantitative experiments with pea plants
Stamen
Carpel
Figure 9.2A, B
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• Mendel crossed
pea plants that
differed in certain
characteristics and
traced the traits
from generation to
generation
• This illustration
shows his
technique for
cross-fertilization
White
1
Removed
stamens
from purple
flower
Stamens
Carpel
PARENTS
(P)
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
OFFSPRING
(F1)
Figure 9.2C
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Planted
seeds
from pod
• Mendel studied seven pea
characteristics/traits
• He hypothesized
that there are
alternative forms
of genes
(although he did
not use that
term), that is
traits were
governed by
some heredity
units
Figure 9.2D
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FLOWER
COLOR
Purple
White
Axial
Terminal
SEED
COLOR
Yellow
Green
SEED
SHAPE
Round
Wrinkled
POD
SHAPE
Inflated
Constricted
POD
COLOR
Green
Yellow
STEM
LENGTH
Tall
Dwarf
FLOWER
POSITION
9.3 Mendel’s principle of segregation describes the
inheritance of a single characteristic
• From his
experimental data,
Mendel deduced
that an organism
has two genes
(alleles) for each
inherited
characteristic
– One characteristic
comes from each
parent
Figure 9.3A
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P GENERATION
(true-breeding
parents)
Purple flowers
White flowers
All plants have
purple flowers
F1
generation
Fertilization
among F1
plants
(F1 x F1)
F2
generation
3/
of plants
have purple flowers
4
1/
4 of plants
have white flowers
GENETIC MAKEUP (ALLELES)
• A sperm or egg
carries only one
allele of each pair
P PLANTS
Gametes
– The pairs of alleles
separate when
gametes form
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
– This process
describes Mendel’s
law of segregation
All Pp
1/
2
1/
P
P
2
p
P
Eggs
Sperm
PP
F2 PLANTS
– Alleles can be
dominant or
recessive
Phenotypic ratio
3 purple : 1 white
p
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
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9.5 The principle of independent assortment is
revealed by tracking two characteristics at
once
• By looking at two characteristics at once,
Mendel found that the alleles of a pair
segregate independently of other allele pairs
during gamete formation
– This is known as the principle of independent
assortment
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
HYPOTHESIS:
DEPENDENT ASSORTMENT
RRYY
P
GENERATION
rryy
Gametes
RRYY
ry
RY
rryy
Gametes
ry
RY
RrYy
F1
GENERATION
Eggs
1/
HYPOTHESIS:
INDEPENDENT ASSORTMENT
2
1/
2
RY
1/
2
RrYy
RY
1/
ry
Sperm
2
1/
ry
1/
F2
GENERATION
1/
Eggs
1/
4
4
4
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RY
1/
4
RrYy
rY
1/
RrYY
rrYY
rrYy
Figure 9.5A
4
RRYY
RRYy
ACTUAL
RESULTS
SUPPORT
HYPOTHESIS
1/
RrYY
RrYy
Actual results
contradict
hypothesis
RY
rY
Ry
ry
4
RrYy
RrYy
RRyy
Rryy
rryy
Ry
1/
RrYy
rrYy
Rryy
4
4
ry
9/
16
3/
16
3/
16
1/
16
Yellow
round
Green
round
Yellow
wrinkled
Yellow
wrinkled
• Independent assortment of two genes in the
Labrador retriever
Blind
PHENOTYPES
GENOTYPES
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
MATING OF HETEROZYOTES
(black, normal vision)
PHENOTYPIC RATIO
OF OFFSPRING
9 black coat,
normal vision
BbNn
3 black coat,
blind (PRA)
Figure 9.5B
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Blind
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
BbNn
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
9.6 Geneticists use the testcross to determine
unknown genotypes
• The offspring of a testcross often reveal the
genotype of an individual when it is unknown
TESTCROSS:
GENOTYPES
B_
bb
Two possibilities for the black dog:
BB
b
OFFSPRING
Bb
B
GAMETES
Figure 9.6
or
Bb
All black
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B
b
Bb
b
bb
1 black : 1 chocolate
9.7 Mendel’s principles reflect the rules of
probability
• Inheritance follows
the rules of probability
– The rule of
multiplication and
the rule of addition
can be used to
determine the
probability of certain
events occurring
F1 GENOTYPES
Bb female
Bb male
Formation of eggs
Formation of sperm
1/
B
1/
2
B
2
B
B
1/
b
1/
1/
2
b
B
b
1/
4
b
b
4
B
1/
2
4
b
F2 GENOTYPES
1/
4
Figure 9.7
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9.8 Connection: Genetic traits in humans can be
tracked through family pedigrees
• The inheritance of many
human traits follows
Mendel’s principles and
the rules of probability
Figure 9.8A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Family pedigrees are used to determine
patterns of inheritance and individual
genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
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Hearing
VARIATIONS ON MENDEL’S PRINCIPLES
9.11 The relationship of genotype to phenotype is
rarely simple
• Mendel’s principles are valid for all sexually
reproducing species
– However, often the genotype does not dictate the
phenotype in the simple way his principles
describe
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.12 Incomplete dominance results in intermediate
phenotypes
• When an offspring’s
phenotype—such
as flower color— is
in between the
phenotypes of its
parents, it exhibits
incomplete
dominance
P GENERATION
White
rr
Red
RR
Gametes
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
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R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
• Incomplete dominance in human
hypercholesterolemia
GENOTYPES:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
PHENOTYPES:
LDL
LDL
receptor
Cell
Normal
Mild disease
Figure 9.12B
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Severe disease
9.13 Many genes have more than two alleles in the
population
• In a population, multiple alleles often exist for a
characteristic
– The three alleles for ABO blood type in humans
is an example
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– The alleles for A and B blood types are
codominant, and both are expressed in the
phenotype
Blood
Group
(Phenotype)
Genotypes
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
O
ii
Anti-A
Anti-B
A
IA IA
or
IA i
Anti-B
B
IB IB
or
IB i
Anti-A
AB
IA IB
Figure 9.13
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A
B
AB
9.14 A single gene may affect many phenotypic
characteristics
• A single gene may affect phenotype in many
ways
– This is called pleiotropy
– The allele for sickle-cell disease is an example
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Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of red
blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Pain and
fever
Paralysis
Brain
damage
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Figure 9.14
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9.16 A single characteristic may be influenced by
many genes
• This situation creates a continuum of
phenotypes
– Example: skin color
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P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.9 Connection: Many inherited disorders in
humans are controlled by a single gene
• Most such
disorders are
caused by
autosomal
recessive alleles
– Examples:
cystic fibrosis,
sickle-cell
disease
Normal
Dd
PARENTS
Normal
Dd
D
D
Eggs
Sperm
DD
Normal
d
OFFSPRING
d
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Figure 9.9A
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• A few are caused by dominant alleles
– Examples: achondroplasia, Huntington’s disease
Figure 9.9B
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Table 9.9
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9.10 Connection: Fetal testing can spot many
inherited disorders early in pregnancy
• Karyotyping and biochemical tests of fetal cells
and molecules can help people make
reproductive decisions
– Fetal cells can be obtained through
amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
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Several
weeks later
Cell culture
Karyotyping
• Chorionic villus sampling is another procedure
that obtains fetal cells for karyotyping
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
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• Examination of the fetus with ultrasound is
another helpful technique
Figure 9.10C, D
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9.15 Connection: Genetic testing can detect
disease-causing alleles
• Genetic testing can be of
value to those at risk of
developing a genetic disorder
or of passing it on to offspring
Figure 9.15B
• Dr. David Satcher, former U.S.
surgeon general, pioneered
screening for sickle-cell disease
Figure 9.15A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings