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Ch. 9 Patterns of Inheritance
The science of genetics has ancient roots
 The blending hypothesis, was
suggested in the 19th century by
scientists studying plants but later
rejected because it did not explain
how traits that disappear in one
generation can reappear in later
generations.
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Experimental genetics began in an abbey
garden
 Heredity = the transmission of
traits from one generation to the
next.
 Genetics = the scientific study
of heredity.
 Gregor Mendel began the field of
genetics in the 1860s,deduced the
principles of genetics by breeding
garden peas, and relied upon a
background of mathematics, physics,
and chemistry.
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Experimental genetics began in an abbey
garden
 In 1866, Mendel correctly argued that parents pass
on to their offspring discrete “heritable factors” and
stressed that the heritable factors (today called
genes), retain their individuality generation after
generation.
 A heritable feature that varies among individuals, is
called a character (flower color)
 Each variant for a character, is a trait (purple or
white flowers)
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Experimental genetics began in an abbey
garden
 True-breeding varieties result when self-fertilization
produces offspring all identical to the parent.
 The offspring of two different varieties are hybrids.
 The cross-fertilization is a genetic cross
 True-breeding parental plants are the P generation
and their hybrid offspring are the F1 generation.
 A cross of F1 plants produces an F2 generation.
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Figure 9.2D_1
Traits
Character
Dominant
Recessive
Purple
White
Axial
Terminal
Yellow
Green
Round
Wrinkled
Flower color
Flower position
Seed color
Seed shape
Mendel’s law of segregation describes the
inheritance of a single character
 A monohybrid cross is a cross between two
individuals differing in a single trait
 Mendel performed a monohybrid cross between a
plant with purple flowers and a plant with white
flowers.
– The F1 generation produced plants with purple flowers.
– A cross of F1 plants with each other produced an F2
generation with ¾ purple and ¼ white flowers.
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Figure 9.3A_s3
The Experiment
P generation
(true-breeding
parents)

Purple
flowers
F1 generation
White
flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation
3
4
1 of plants
of plants
4
have purple flowers have white flowers
Mendel’s law of segregation describes the
inheritance of a single character
 The all-purple F1 generation did not produce light
purple flowers, as predicted by the blending
hypothesis.
 Mendel needed to explain why white color seemed to
disappear in the F1 generation and white color reappeared
in one quarter of the F2 offspring.
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Mendel’s law of segregation describes the
inheritance of a single character
 Mendel developed four hypotheses (using modern
terminology)
1. Alleles are alternative versions of genes
2. For each characteristic, an organism inherits two
alleles (on homologs), one from each parent. The
alleles can be the same or different.
– A homozygous genotype has identical alleles.
– A heterozygous genotype has two different alleles.
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Figure 9.4
Gene loci
P
a
B
P
a
b
Dominant
allele
Homologous
chromosomes
Genotype: PP
Homozygous
for the
dominant
allele
aa
Homozygous
for the
recessive
allele
Recessive
allele
Bb
Heterozygous,
with one dominant
and one recessive
allele
Mendel’s law of segregation describes the
inheritance of a single character
3. If the alleles of an inherited pair differ, then one determines
the organism’s appearance and is called the dominant allele.
The other has no noticeable effect on the organism’s
appearance and is called the recessive allele.
– The phenotype is the appearance or expression of a trait.
– The genotype is the genetic makeup of a trait.
*The same phenotype may be determined by more than one genotype.
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Mendel’s law of segregation describes the
inheritance of a single character
4. A sperm or egg carries only one allele for each inherited
character because allele pairs separate (segregate) from
each other during the production of gametes. This
statement is called the law of segregation.
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The law of independent assortment is revealed
by tracking two characters at once
 A dihybrid cross is a mating of parental varieties
that differ in two characters.
 Mendel performed the following dihybrid cross with
the following results:
– P generation: round yellow seeds  wrinkled green seeds
– F1 generation: all plants with round yellow seeds
– F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
The law of independent assortment is revealed
by tracking two characters at once
 Mendel needed to explain why the F2 offspring had
new nonparental combinations of traits and a 9:3:3:1
phenotypic ratio.
 Mendel suggested that the inheritance of one
character has no effect on the inheritance of another,
and that the dihybrid cross is the equivalent to two
monohybrid crosses, and called this the law of
independent assortment.
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rryy
P generation RRYY
Gametes RY
F1 generation

ry
RrYy
RrYy
F1 generation
Sperm
1
4
1
4
RY
1
4
rY
RY
RRYY
RrYY
Eggs
1
4
1
4
1
4
rY
RrYY
rrYY
1
4
Ry
RRYy
RrYy
1
4
ry
RrYy
rrYy
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
9
16
Yellow
round
3
16
Green
round
3
16
Yellow
wrinkled
1
16
Green
wrinkled
The hypothesis of independent assortment
Actual results; hypothesis supported
Geneticists can use the testcross to determine
unknown genotypes
 A testcross is the mating between an individual of
unknown genotype and a homozygous recessive
individual.
 A testcross can show whether the unknown
genotype includes a recessive allele.
 Mendel used testcrosses to verify that he had truebreeding genotypes.
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What is the genotype of the black dog?

Testcross
Genotypes
B_?
bb
Two possibilities for the black dog:
BB
Gametes
B
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
Mendel’s laws reflect the rules of probability
 The probability of a specific event is the number of
ways that event can occur out of the total possible
outcomes.
 Determining the probability of two independent
events uses the rule of multiplication, where the
probability is the product of the probabilities for
each event.
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Figure 9.7
F1 genotypes
Bb male
Bb female
Formation
of eggs
Formation
of sperm
1
2
1
2
B
b
Sperm
1
1
(2  2 )
1
2
F2 genotypes
B
b
b
1
4
B
1
4
b
B
1
4
Eggs
1
2
B
B
b
b
1
4
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Genetic traits in humans can be tracked through
family pedigrees
 The inheritance of human traits follows Mendel’s
laws.
 A pedigree shows the inheritance of a trait in a family
through multiple generations, demonstrates dominant or
recessive inheritance, and can also be used to deduce
genotypes of family members.
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First generation
(grandparents)
Second generation
(parents, aunts,
FF
and uncles)
or
Ff
Third generation
(two sisters)
Female
Male
Attached
Free
Ff
ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Inherited human disorders show either recessive
inheritance or dominant inheritance
 Recessive inheritance: two recessive alleles are
needed to show disease, (heterozygous parents are
carriers of the disease-causing allele)
 Dominant inheritance: one dominant allele is
needed to show disease and dominant lethal alleles
are usually eliminated from the population.
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Normal
Dd
Parents
D
D
Offspring
Normal
Dd

Sperm
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Many inherited disorders in humans are controlled
by a single gene
 The most common fatal genetic
disease in the United States is
cystic fibrosis (CF), and an
example of recessive inheritance
(carried by about 1 in 31 Americans)
 Dominant human disorders
include achondroplasia, (results in
short stature), and Huntington’s
disease (degenerative disorder of the
nervous system)
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VARIATIONS ON
MENDEL’S LAWS
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Incomplete dominance results in intermediate
phenotypes
 Mendel’s pea crosses always looked like one of the
parental varieties, called complete dominance.
 In incomplete dominance, the appearance of F1
hybrids falls between the phenotypes of the two
parental varieties.
 Incomplete dominance: neither allele is dominant
over the other and expression of both alleles occurs.
Figure 9.11A_3
F2 generation
Sperm
1
2
R
1
2
r
1
R
2
RR
rR
1
r
2
Rr
rr
Eggs
Many genes have more than two alleles in the
population
 In codominance, neither allele is dominant over
the other and expression of both alleles is
observed as a distinct phenotype in the
heterozygous individual.
– AB blood type is an example of codominance.
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Many genes have more than two alleles in the
population
 Although an individual can at most carry two
different alleles for a particular gene, more than two
alleles often exist in the wider population.
 Human ABO blood group phenotypes involve three
alleles for a single gene.
 The four human blood groups, A, B, AB, and O,
result from combinations of these three alleles.
 The A and B alleles are both expressed in
heterozygous individuals, a condition known as
codominance.
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Figure 9.12_1
Blood
Group
(Phenotype)
Genotypes
Carbohydrates Present
on Red Blood Cells
A
IA IA
or
IA i
Carbohydrate A
Carbohydrate B
B
IB IB
or
IB i
AB
IA IB
O
ii
Carbohydrate A
and
Carbohydrate B
Neither
A single gene may affect many phenotypic
characters
 Pleiotropy occurs when one gene influences many
characteristics.
 Sickle-cell disease is a human example of pleiotropy.
Because it
– affects the type of hemoglobin produced and the shape of
red blood cells and
– causes anemia and organ damage, etc.
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Figure 9.13B
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Damage to organs
Other effects
Kidney failure
Heart failure
Spleen damage
Brain damage (impaired
mental function,
paralysis)
Pain and fever
Joint problems
Physical weakness
Anemia
Pneumonia and other
infections
A single character may be influenced by many
genes
 Polygenic inheritance -a single phenotypic
character results from the additive effects of two or
more genes.
 Human skin color is an example of polygenic
inheritance.
© 2012 Pearson Education, Inc.
Figure 9.14_1
P generation

aabbcc
AABBCC
(very light) (very dark)
F1 generation

AaBbCc
AaBbCc
Figure 9.14_2
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
Eggs
1
8
1
8
1
8
1
8
1
8
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Figure 9.14_3
Fraction of population
20
64
15
64
6
64
1
64
Skin color
THE CHROMOSOMAL BASIS
OF INHERITANCE
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Chromosome behavior accounts for Mendel’s
laws
 The chromosome theory of inheritance states that
genes occupy specific loci (positions) on
chromosomes and chromosomes undergo
segregation and independent assortment during
meiosis.
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Chromosome behavior accounts for Mendel’s
laws
 Mendel’s laws correlate with chromosome
separation in meiosis.
– The law of segregation depends on separation of
homologous chromosomes in anaphase I.
– The law of independent assortment depends on
alternative orientations of chromosomes in metaphase I.
© 2012 Pearson Education, Inc.
Figure 9.16_s3
F1 generation
R
r
All yellow round seeds
(RrYy)
y
Y
r
R
Y
R
y
Metaphase I
of meiosis
r
R
Y
y
r
r
R
Y
y
Anaphase I
Y
y
Metaphase II
R
r
r
R
Y
y
Y
y
Gametes
Y
Y
R
R
1
4
RY
y
y
r
r
1
4
Y
Y
r
r
ry
F2 generation 9
Fertilization
:3
:3
:1
1
4
rY
y
y
R
R
1
4
Ry
SEX CHROMOSOMES AND
SEX-LINKED GENES
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Chromosomes determine sex in many species
 Many animals have a pair
of sex chromosomes,
designated X and Y, that
determine an individual’s
sex.
 In mammals, males have
XY sex chromosomes,
females have XX sex
chromosomes
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X
Y
Chromosomes determine sex in many species
 In some animals, environmental temperature
determines the sex.
– For some species of reptiles, the temperature at which
the eggs are incubated during a specific period of
development determines whether the embryo will develop
into a male or female.
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Sex-linked genes exhibit a unique pattern of
inheritance
 Sex-linked genes are located on either of the sex
chromosomes.
 The X chromosome carries many genes unrelated
to sex.
 The inheritance of white eye color in the fruit fly
illustrates an X-linked recessive trait.
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Figure 9.21A
Figure 9.21B
Female
Male
XRXR
XrY
Sperm
Eggs XR
Xr
Y
XRXr
XRY
R  red-eye allele
r  white-eye allele
Figure 9.21C
Female
Male
XRXr
XRY
Sperm
Y
xR
XR
XRXR
XRY
XrXR
XrY
Eggs
Xr
R  red-eye allele
r  white-eye allele
Figure 9.21D
Female
Male
XRXr
XrY
Sperm
Xr
Y
XR
XRXr
XRY
Xr
XrXr
XrY
Eggs
R  red-eye allele
r  white-eye allele
CONNECTION: Human sex-linked disorders
affect mostly males
 A male receiving a single X-linked recessive allele
from his mother will have the disorder.
 A female must receive the allele from both parents
to be affected.
 Therefore, human sex-linked disorders affect
mostly males.
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CONNECTION: Human sex-linked disorders
affect mostly males
 Recessive and sex-linked human disorders
include
– hemophilia, red-green color blindness, and Duchenne
muscular dystrophy
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Figure 9.22
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Female Male
Hemophilia
Carrier
Normal