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Chapter 9
Patterns of Inheritance
BIOLOGY: Today and Tomorrow, 4e
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9.1 Menacing Mucus
 Cystic fibrosis, the most common fatal genetic disorder in the
US, is caused by a deletion in the CFTR gene
 The CF allele persists at high frequency despite devastating
effects
 Only those homozygous for the CF allele have the disorder
Victims of Cystic Fibrosis
ANIMATED FIGURE: Crossing garden pea
plants
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9.2 Tracking Traits
 Mid-1800s: Genes and
chromosomes were
unknown; Gregor
Mendel’s experiments
with pea plants
established principles of
inheritance
Breeding Garden Peas
carpel
anther
A) The flowers of garden
pea plants have
reproductive parts called
anthers and carpels. Pollen
grains that form in anthers
produce male gametes;
female gametes form in
carpels.
Breeding Garden Peas
B) Experimenters
can control the transfer
of hereditary material
from one pea plant to
another by snipping off
a flower’s anthers (to
prevent the flower from
self-fertilizing), and then
brushing pollen from
another flower onto its
carpel. In this example,
pollen from a plant that
has purple flowers is
brushed onto the carpel
of a white-flowered plant.
Breeding Garden Peas
C) Later, seeds develop
inside pods of the crossfertilized plant. An embryo
in each seed develops into
a mature pea plant.
Breeding Garden Peas
D) Every plant that arises from
the cross has purple flowers.
Predictable patterns such as
this are evidence of how
inheritance works.
Inheritance in Modern Terms
 Organisms breed true for a trait because they carry identical
alleles of genes governing that trait
 Homozygous
 Having identical alleles of a gene
 Heterozygous
 Having two different alleles of a gene
Inheritance in Modern Terms
 The particular set of alleles an individual carries is the
individual’s genotype
 Gene expression results in phenotype – an individual’s
observable traits
 An allele is dominant when its effect masks that of a
recessive allele paired with it
Genotype gives
rise to phenotype
genotype
PP (homozygous for
dominant allele P)
pp (homozygous for
recessive allele p)
Pp (heterozygous for
alleles P and p)
phenotype
9.3 Mendelian Inheritance Patterns
 A cross (mating) between heterozygous individuals can reveal
dominance relationships among the alleles under study
 Monohybrid cross
 Cross in which individuals with different alleles of a gene
are crossed
 Dominant trait will have a 3:1 phenotype ratio
Segregation of Genes
 When homologous chromosomes separate during meiosis,
the gene pairs on those chromosomes separate
 Each gamete that forms carries only one of the two genes of
a pair
Segregation
of Genes
1
2
DNA replication
meiosis I
meiosis II
gametes (P)
gametes (p)
zygote (Pp)
3
DNA replication
meiosis I
2
1
meiosis II
3
gametes (P)
gametes (p)
zygote (Pp)
Stepped Art
Figure 9-4 p153
Punnett Squares
 Punnett squares are used to calculate the probability of the
genotype and phenotype of offspring of crosses
 In a testcross, an individual with a dominant trait (but an
unknown genotype) is crossed with an individual known to be
homozygous for the recessive allele
 The pattern of traits among offspring can reveal whether the
tested individual is heterozygous or homozygous
Punnett Squares: A Monohybrid Cross
Monohybrid cross:
First generation
parent plant
homozygous
for purple
flowers
parent plant
homozygous
for white
flowers
Pp
hybrid
two types of gametes
A) All of the F1 (first generation) offspring of a cross
between two plants that breed true for different forms of a
trait are identically heterozygous (Pp). These offspring
make two types of gametes: P and p.
Monohybrid cross:
Second generation
B) A cross between two of the identically heterozygous F1
offspring is a monohybrid cross. In this example, the
phenotype ratio among the F2 (second generation)
offspring is 3:1 (three purple to one white).
Dihybrid Crosses
 Mendel’s dihybrid crosses showed inheritance of one trait did
not affect inheritance of other traits
 Dihybrid cross
 Experiment in which individuals with different alleles of two
genes are crossed (9:3:3:1 ratio)
 Independent assortment
 A gene tends to be distributed independently of how other
genes are distributed
Dihybrid Cross:
First generation
parent plant
homozygous
for purple
flowers and
long stems
parent plant
homozygous for
white flowers
and short stems
2
PpTt
dihybrid
3
four types of gametes
1
Dihybrid Cross: Second generation
4
The Contribution of Crossovers
 Two genes located close together on the same chromosome
tend to be inherited together
 When two genes on the same chromosome are far apart,
crossing over occurs more frequently between them; they
tend to assort independently
A Human Example: Skin Color
 Variations in skin color depend on the kinds and amounts of
melanins produced
 More than 100 gene products affect production and
deposition of melanins
 Independent assortment of these genes produces a wide
variety of phenotypes
Variation in Skin Color
ANIMATED FIGURE: Dihybrid cross
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ANIMATED FIGURE: Independent assortment
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ANIMATED FIGURE: Monohybrid cross
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ANIMATED FIGURE: Test Cross
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9.4 Beyond Simple Dominance
 Codominant
 Refers to two alleles that are both fully expressed in
heterozygous individuals
 Incomplete dominance
 Condition in which one allele is not fully dominant over
another, so the heterozygous phenotype is between the
two homozygous phenotypes
Codominance: Blood Type
Genotypes:
Phenotypes
(blood
type):
AA
or
AO
A
AB
A
B
BB
or
BO
OO
B
O
Incomplete Dominance
homozygous
parent (RR)
X
homozygous
parent (rr)
heterozygous
offspring (Rr)
A) Cross a red-flowered with a white-flowered snap-dragon, and all of the offspring will have pink
flowers.
Incomplete Dominance
B) If two of the pink-flowered snapdragons are crossed, the phenotypes
of their offspring will occur in a 1:2:1 ratio.
Epistasis
 Some traits are affected by multiple gene products, an effect
called polygenic inheritance or epistasis
 Epistasis
 Effect in which a trait is influenced by the products of
multiple genes
 Example: Labrador retriever coat color
Epistasis
Pleiotropy
 Products of pleiotropic
genes influence two or
more traits
 Mutations in pleiotropic
genes are associated
with sickle cell anemia,
cystic fibrosis, and
Marfan syndrome
INTERACTION: Incomplete dominance
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ANIMATION: Chicken combs
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ANIMATION: Dog color
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9.5 Complex Variations in Traits
 Mutations, interactions among genes, and environmental
conditions can affect one or more steps in a metabolic
pathway, and contribute to variation in phenotypes
 Example: Seasonal changes affect production of pigments
that color the skin and fur of many animals
 Example: Water flea phenotypes depend on whether the
aquatic insects that prey on them are present
 Example: Genetically identical yarrow plants grow to different
heights at different altitudes
Snowshoe Hare in Summer and Winter
A) The color of the snowshoe hare’s fur varies by season.
In summer, the fur is brown (left ); in winter, it is white
(right ). The variation offers seasonally appropriate
camouflage from predators.
Water flea, with and without predators
B) The body form of the water flea on the top develops in
environments with few predators. A longer tail spine and a
pointy head (bottom) develop in response to chemicals
emitted by predatory insects.
Environmental
Effects on Plant
Phenotypes
Height (centimeters)
60
0
3060
1400
30
Elevation (meters above sea level)
C) The height of a mature yarrow plant depends
on the elevation at which it grows.
Continuous Variation
 Continuous variation
 A range of small increments of phenotype in a trait that is
influenced by the products of multiple genes
 The more genes and other factors that influence a trait,
the more continuous the distribution of phenotype
 Bell curve
 Curve that results when range of variation in a continuous
trait is plotted against frequency in a population
Continuous Variation in Eye Color
Continuous Variation in Height
A) Male biology students at the University of Florida were divided into
categories of one-inch increments in height and counted.
Continuous Variation (Bell Curve)
Number of individuals
20
15
10
5
0
63
64
65
66
67
68
69
70
71
72
73
74
75
76
B) Graphing the resulting data produces a bell-shaped curve, an
indication that height varies continuously.
77
9.5 Human Genetic Analysis
 Inheritance patterns in humans are studied by following
inherited genetic disorders in a family through generations
and graphing results as a pedigree chart
 Pedigree analyses shows whether a trait is associated with a
dominant or recessive allele, and whether the allele is on an
autosome or a sex chromosome
Pedigree: Polydactyly
Types of Genetic Variation
 Single genes on autosomes or sex chromosomes govern
more than 6,000 genetic abnormalities and disorders
 Genetic abnormality
 An uncommon version of a heritable trait that does not
result in medical problems
 Genetic disorder
 A heritable condition that results in a syndrome of mild or
severe medical problems
ANIMATION: Coat color in the Himalayan rabbit
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ANIMATION: Height Graph
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9.6 Human Genetic Disorders
 Some dominant or recessive alleles on autosomes or the X
chromosome are associated with genetic abnormalities or
disorders
 An autosomal dominant allele is expressed in homozygotes
and heterozygotes
 An autosomal recessive allele is expressed only in
homozygotes
Some Autosomal Dominant Traits
Autosomal
Dominant
Inheritance
normal
mother
affected
father
X
meiosis
and gamete
formation
A) A dominant
allele on an
autosome (red )
is fully
expressed in
heterozygous
people
affected child
normal child
disorder-causing
allele (dominant)
normal
mother
affected
father
meiosis
and gamete
formation
affected child
normal child
disorder-causing
allele (dominant)
Stepped Art
Figure 9-17a p163
ANIMATED FIGURE: Pedigree diagrams
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Some Autosomal Recessive Traits
Autosomal Recessive
Inheritance
A) Only people
homozygous for a
recessive allele on an
autosome have the trait
associated with the allele.
carrier mother
carrier father
X
meiosis
and gamete
formation
In this example, both
parents are carriers (red).
Each of their children has a
25 percent chance of
inheriting two alleles, and
being affected by the trait.
affected child
carrier child
normal child
B) The albino phenotype is associated
with autosomal recessive alleles that
cause a deficiency in melanin.
disorder-causing
allele (recessive)
carrier mother
carrier father
meiosis
and gamete
formation
affected child
carrier child
normal child
disorder-causing
allele (recessive)
Stepped Art
Figure 9-18a p163
Victim of Tay–Sachs disease
X-Linked Recessive Disorders
 Alleles on the X chromosome are inherited and expressed
differently in males and females
 Males cannot transmit a recessive X-linked allele to their sons
 Females pass X-linked alleles to male offspring
 Example: red-green color blindness
Some X-Linked Recessive Disorders
X-Linked Recessive
Inheritance
carrier mother
normal father
X
meiosis
and gamete
formation
A) In this example of
X-linked inheritance,
the mother carries a
recessive allele on
one of her two X
chromosomes (red ).
normal daughter or son
carrier daughter
affected son
recessive allele
on X chromosome
carrier mother
normal father
meiosis
and gamete
formation
normal daughter or son
carrier daughter
affected son
recessive allele
on X chromosome
Stepped Art
Figure 9-20a p165
Red–green color blindness
B) A view of color blindness. The image on the left shows how a person with red–
green color blindness sees the image on the right. The perception of blues and
yellows is normal; red and green appear similar.
Red–green
color
blindness
You may have one
form of red–green
color blindness if you
see a 7 instead of a 29
in this circle.
C) Part of a
standardized test for
color blindness. A set
of 38 of these circles
is commonly used to
diagnose deficiencies
in color perception.
You may have another
form of red–green
color blindness if you
see a 3 instead of an 8
in this circle.
INTERACTION: Autosomal-dominant inheritance
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INTERACTION: Autosomal-recessive inheritance
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INTERACTION: X-linked inheritance
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VIDEO: Genetics, Sociology, and Breast Cancer
9.8 Changes in Chromosome Number
 Many flowering plants, and some insects, fishes and other
animals are polyploid – having three or more of each type of
chromosome characteristic of the species
 Chromosome number can change permanently, usually
resulting from nondisjunction – the failure of chromosomes
to separate normally during meiosis or mitosis
Nondisjunction
During Meiosis
Metaphase I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Metaphase I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Stepped Art
Figure 9-21 p166
Aneuploidy
 Aneuploidy
 A chromosome abnormality in which a cell has too many
or too few copies of a particular chromosome (trisomy,
monosomy)
 The most common aneuploidy in humans, trisomy 21, causes
Down syndrome
Some Disorders Caused by Aneuploidy
Autosomal Change and Down Syndrome
 Trisomy 21 (Down syndrome)
 The only autosomal trisomy that allows humans to survive
to adulthood
 Affected individuals tend to have certain physical features
and impairments
 Nondisjunction leading to trisomy 21 increases with age of the
mother
Down Syndrome
Change in Sex Chromosome Number
 Usually associated with learning difficulties, speech delays,
and motor skill impairment
 Female sex chromosome abnormalities:
 Turner syndrome (XO)
 XXX syndrome
 Male sex chromosome abnormalities:
 Klinefelter syndrome (XXY)
 XYY syndrome
9.9 Genetic Screening
 Geneticists estimate the chance that a couple’s offspring will
inherit a genetic abnormality or disorder
 Potential parents who may be at risk of transmitting a harmful
allele to offspring have screening or treatment options
Prenatal Diagnosis
 Obstetric sonography may reveal defects associated with a
genetic disorder
 Other tests performed before birth carry risks of miscarriage
or injury to fetus
 Amniocentesis
 Chorionic villi sampling (CVS)
 Fetoscopy
Three ways of imaging a fetus
C) Fetoscopy
A) Conventional ultrasound
B) 4D ultrasound
The amniotic sac
amniotic sac
chorion
Preimplantation Diagnosis
 A single cell taken from
an embryo produced by
in vitro fertilization is
tested before
implantation
9.10 Menacing Mucus (revisited)
 The cystic fibrosis (CF) allele is very common in some
populations
 The CF allele is lethal in homozygotes, but offers
heterozygotes some protection against bacterial diseases
such as typhoid fever
Digging Into Data:
Cystic Fibrosis and Typhoid Fever