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Purebreds and Mutts — A Difference of Heredity
• Genetics is the science of heredity
• These black Labrador puppies are purebred—
their parents and grandparents were black Labs
with very similar genetic makeups
– Purebreds
often suffer
from serious
genetic defects
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The parents of these puppies were a mixture of
different breeds
– Their behavior
and appearance
is more varied
as a result of
their diverse
genetic
inheritance
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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
• He hypothesized
that there are
alternative forms
of genes
(although he did
not use that
term), the units
that determine
heredity
Figure 9.2D
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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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Homologous chromosomes bear the two alleles for
each characteristic
• Alternative forms of a gene (alleles) reside at
the same locus on homologous chromosomes
GENE LOCI
P
P
a
a
B
DOMINANT
allele
b
RECESSIVE
allele
GENOTYPE:
PP
aa
HOMOZYGOUS
for the
dominant allele
HOMOZYGOUS
for the
recessive allele
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Bb
HETEROZYGOUS
Figure 9.4
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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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
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– 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
• ABO blood types
Figure 9.13x
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Normal and sickle red blood cells
Figure 9.14x1
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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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Connection: Genetic testing can detect diseasecausing 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
A single characteristic may be influenced by many
genes
• This situation creates a continuum of
phenotypes
– Example: skin color
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
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THE CHROMOSOMAL BASIS OF
INHERITANCE
Chromosome behavior accounts for Mendel’s
principles
• Genes are located on chromosomes
– Their behavior during meiosis accounts for
inheritance patterns
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The chromosomal basis of Mendel’s principles
Figure 9.17
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9.18 Genes on the same chromosome tend to be
inherited together
• Certain genes are linked
– They tend to be inherited together because they
reside close together on the same chromosome
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.18
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Crossing over produces new combinations of alleles
• This produces gametes with recombinant
chromosomes
• The fruit fly Drosophila melanogaster was used
in the first experiments to demonstrate the
effects of crossing over
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
Figure 9.19A, B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.19C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Geneticists use crossover data to map genes
• Crossing over is more likely to occur between
genes that are farther apart
– Recombination frequencies can be used to map
the relative positions of genes on chromosomes
Chromosome
g
c
l
17%
9%
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9.5%
Figure 9.20B
• Alfred H. Sturtevant, seen here at a party with
T. H. Morgan and his students, used
recombination data from Morgan’s fruit fly
crosses to map genes
Figure 9.20A
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• A partial genetic map of a fruit fly chromosome
Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae
(appendages
on head)
Gray
body
(G)
Cinnabar
eyes
(c)
Red
eyes
(C)
Vestigial
wings
(l)
Brown
eyes
Normal
wings
(L)
Red
eyes
Wild-type phenotypes
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Figure 9.20C
SEX CHROMOSOMES AND SEX-LINKED
GENES
Chromosomes determine sex in many species
• A human male has one X chromosome and one
Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
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(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
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• Other systems of sex determination exist in
other animals and plants
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
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Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes are said to be
sex-linked
– In many organisms, the X chromosome carries
many genes unrelated to sex
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
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– Their inheritance pattern reflects the fact that
males have one X chromosome and females
have two
– These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
XRXR
Male
Xr Y
XR
Female
XRXr
Xr
XRXr
Male
XRY
XRY
Xr
XRXR
XrXR
XRY
XrY
R = red-eye allele
r = white-eye allele
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Male
XRXr
XR
XR
Y
Female
XrY
Xr
XR
Y
Xr
XRXr
Xr Xr
Y
XRY
XrY
Figure 9.22B-D
9.23 Connection: Sex-linked disorders affect
mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
Figure 9.23A
– A male receives a single X-linked allele from his
mother, and will have the disorder, while a
female has to receive the allele from both
parents to be affected
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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