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Chapter 9
Patterns of
Inheritance
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 __________ occurs when genes are transmitted from
parent to offspring.
• The units of inheritance are genes, which are segments of
DNA of variable length.
 Genes are segments of DNA at specific locations on
chromosomes.
• A gene’s physical location on a chromosome is called its
_________.
• Each member of a pair of homologous chromosomes carries
the same genes, located at the same loci.
• Different versions of a gene at a given locus are called
_________.
Copyright © 2009 Pearson Education Inc.
9.1 What Is The Physical Basis Of Inheritance?
 The relationship among genes, alleles, and chromosomes
• Differences in alleles at a given locus are due to mutations at that gene.
• If a mutation occurs in the cells that become sperm or eggs, it can
be passed on from parent to offspring.
a pair of
homologous
chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism is
homozygous at this locus
gene loci
This locus contains another gene for which
the organism is homozygous
Each chromosome carries a different allele
of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
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the chromosome
from the female
parent
Fig. 9-1
9.1 What Is The Physical Basis Of Inheritance?
 An organism’s two alleles may be the same or
different.
• A diploid organism has pairs of homologous chromosomes
with two copies of each gene at a given locus.
• If both homologous chromosomes have the same allele at a
locus, the organism is said to be _______________.
• If two homologous chromosomes have different alleles at a
locus, the organism is ______________ at that locus.
• The gametes of a homozygous individual are all the
same at a particular locus, while gametes of a
heterozygous individual would contain half one allele
and half the other allele.
Copyright © 2009 Pearson Education Inc.
9.2 How Were The Principles Of Inheritance
Discovered?
 The patterns of inheritance were
discovered by an Austrian monk,
Gregor Mendel.
• Mendel was the first geneticist to
employ three key steps in his
experimentation:
• Choosing the right organism for the
work
• Designing and performing
experiments correctly
• Analyzing the data properly
Fig. 9-2
Copyright © 2009 Pearson Education Inc.
9.2 How Were The Principles Of Inheritance
Discovered?
 Mendel chose edible pea as the experimental subject for
his experiments in inheritance.
• Pea egg cells in a pea flower fertilized by sperm from the
same flower is called ______-fertilization.
• When sperm from one organism fertilizes eggs from a
different organism, the process is called ______fertilization.
• Mendel studied individual characteristics of pea plants, such
as flower color; these characteristics are called ______.
• He followed the inheritance of these traits for several
generations, counting the numbers of offspring with each
type of trait.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 True-breeding traits of organisms, such as purple
flower color, are always inherited by all of their
offspring that result from self-fertilization.
• In one experiment, Mendel cross-fertilized white-flowered
plants with purple-flowered plants.
• When he grew the resulting seeds, he found all the firstgeneration offspring, or the F1 generation, produced
purple flowers.
• What happened to the white color?
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 Cross of pea plants that are true-breeding for white or
purple flowers
pollen
Parental
generation (P)
pollen
cross-fertilize
true-breeding,
purple-flowered
plant
true-breeding,
white-flowered
plant
First-generation
offspring (F1)
all purple-flowered
plants
Fig. 9-4
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9.3 How Are Single Traits Inherited?
 The F2 generation
• Next, Mendel allowed the F1 flowers to self-fertilize,
collected the seeds, and grew the second generation,
called the F2 generation.
• Flowers in the F2 generation were three-fourths purple
and one-fourth white, in a ratio of 3 purple to 1 white.
• This showed that the gene for white flowers was
“hidden” in the F1 generation, but appeared again in
the F2 generation.
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9.3 How Are Single Traits Inherited?
 Cross of F1 plants with purple flowers
Firstgeneration
offspring (F1)
self-fertilize
Secondgeneration
offspring (F2)
3/4 purple
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1/4 white
Fig. 9-5
9.3 How Are Single Traits Inherited?
 All the white-flowered plants in the F2 generation only
produced additional white-flowered plants.
 Purple-flowered plants were of two types:
• About ⅔ were true-breeding for purple, while ⅔
produced both purple- and white-flowered offspring
(ratio 3 purple/1 white).
• Therefore, the F2 generation included ¼ true-breeding
purple plants, ½ hybrid purple, and ¼ true-breeding
white plants.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 The inheritance of dominant and recessive alleles on
homologous chromosomes can explain the results of
Mendel’s crosses.
• Mendel’s results allow us to develop a five-part hypothesis
to explain the inheritance of single traits.
1. Each trait is determined by pairs of distinct physical units
called genes. There are two alleles for each gene, one on
each homologous chromosome.
2. When two different alleles are present in an organism, the
dominant allele may mask the expression of the
recessive allele; however, the recessive allele is still
present.
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
3. The two alleles of a gene segregate (separate) from one
another during meiosis; this is known as Mendel’s law of
segregation.
4. Which allele ends up in any given gamete is determined by
chance.
5. True-breeding (homozygous) organisms have two copies of
the same allele for a given gene; hybrid (heterozygous)
organisms have two different alleles for a given gene.
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9.3 How Are Single Traits Inherited?
 The distribution of alleles in gametes
homozygous parent
A
A
gametes
A
A
(a) Gametes produced by a homozygous parent
heterozygous parent
A
a
gametes
A
a
(b) Gametes produced by a heterozygous parent
Fig. 9-6
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9.3 How Are Single Traits Inherited?
 In pea plants, purple is dominant to white.
• Letters can be used to describe the alleles (P is for the
dominant allele; p is for the recessive allele).
• Homozygous purple plants are PP; homozygous white
plants are pp.
purple parent
P
PP
+
P
all P sperm and eggs
white parent
pp
p
+
p
all p sperm and eggs
(a) Gametes produced by homozygous parents
Fig. 9-7a
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9.3 How Are Single Traits Inherited?
 F1 plants were produced by P and p gametes, making Pp
F1 hybrid offspring.
sperm
P
eggs
+
F1 offspring
p
Pp
P
Pp
or
p
+
(b) Fusion of gametes produces F1 offspring
Fig. 9-7b
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 Next, Mendel
crossed two
F1 hybrid
plants (Pp x
Pp).
gametes from F1 Pp plants
sperm
F2 offspring
eggs
P
+
P
PP
P
+
p
Pp
p
+
P
Pp
p
+
p
pp
• This cross
made three
types of F2
offspring,
with the
following
allele
composition:
¼ were PP;
½ were Pp;
¼ were pp.
(c) Fusion of gametes from the F1 generation produces F2 offspring
Fig. 9-7c
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9.3 How Are Single Traits Inherited?
 Mendel’s hypothesis was that two plants may look alike,
called its phenotype, but have a different allele
composition, called its genotype.
 In this case, purple plants had PP or Pp genotypes, but
their phenotype (purple color) was the same.
 The F2 generation could be described as having three
genotypes (¼ PP, ½ Pp, and ¼ pp) and two phenotypes
(¾ purple and ¼ white).
Copyright © 2009 Pearson Education Inc.
9.3 How Are Single Traits Inherited?
 Simple “genetic
bookkeeping” can predict
the genotypes and
phenotypes of offspring.
Pp
self-fertilize
P
1
2
1
2
1
2
1
2
p
P
sperm
• The Punnett square
method is a convenient
way to predict the
genotypes and
phenotypes of offspring.
eggs
1
4
PP
1
4
Pp
1
4
pP
1
4
pp
p
Fig. 9-8
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9.3 How Are Single Traits Inherited?
 Mendel’s hypothesis can
predict the outcome of new
types of single-trait crosses.
pollen
PP or Pp
pp
all eggs
sperm unknown
• Mendel predicted the outcome
of cross-fertilizing Pp plants
with homozygous recessive
plants (pp)—there should be
equal numbers of Pp (purple)
and pp (white) offspring.
if PP
if Pp
p
all
sperm
P
p
eggs
1
2
1
2
eggs
P
1
2 Pp
sperm
all Pp
 A Punnet square shows how
this “test cross” results in
the predicted offspring.
p
p
1
2 pp
Fig. 9-9
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Mendel next crossed pea
plants that differed in two
traits, such as seed color
(yellow or green) and seed
shape (smooth or wrinkled).
• He knew from previous crosses
that smooth and yellow were
both dominant traits in peas.
• His first cross was a truebreeding plant with smooth,
yellow seeds (SSYY) to a truebreeding plant with wrinkled,
green seeds (ssyy).
Trait
Dominant form
Recessive form
Seed
shape
smooth
wrinkled
Seed
color
yellow
green
Pod
shape
inflated
constricted
Pod
color
green
yellow
purple
white
Flower
color
Flower
locaat leaf
tion
junctions
Plant
size
tall
(about
6 feet)
at tips of
branches
dwarf
(about 8 to
16 inches)
Fig. 9-10
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9.4 How Are Multiple Traits Inherited?
 All the offspring of this
cross (F1 generation)
were SsYy and had
smooth, yellow seeds
(both dominant traits).
SsYy
self-fertilize
eggs
1 SY
4
sperm
 F1 plants were allowed
to self-fertilize and
produced F2 offspring
in the phenotypic ratio
9:3:3:1.
1 Sy
4
1 sY
4
1 sy
4
1 SY
4
1 Sy
4
1 sY
4
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSYy
1
16 ssYY
1
16 ssYy
1
16 sSyY
1
16 sSyy
1
16 ssyY
1
16 ssyy
1
4
sy
Fig. 9-11
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9.4 How Are Multiple Traits Inherited?
 Mendel concluded that multiple traits are inherited
independently.
• Mendel realized that these results could be explained if the
genes for seed color and seed shape were inherited
independently.
• The independent inheritance of two or more distinct traits is
called the law of ___________ ____________.
• Multiple traits are inherited independently because the
alleles of one gene are distributed to gametes
independently of the alleles of other genes.
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Independent
assortment of
alleles
S
pairs of alleles on
homologous chromosomes
in diploid cells
s
Y
y
chromosomes replicate
S
Y
s
y
replicated homologues
pair during metaphase
S
of meiosis I,
orienting like this
or like this
s
y
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
s
Y
Y
SY
S
s
y
y
sy
s
S
y
y
s
Y
Y
Sy
sY
independent assortment produces four equally
likely allele combinations during meiosis
Fig. 9-12
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9.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genetic linkage is the inheritance of genes as a
group because they are on the same chromosome.
• Genes that are located on the same chromosome are
inherited together, rather than sort independently.
• In peas, the gene for flower color and the gene for pollen
shape occur on the same chromosome and are inherited
together.
• Because the two genes are located on the same
chromosomes, they tend to end up in gametes together,
and are then expressed in the plants.
Copyright © 2009 Pearson Education Inc.
9.5 How Are Genes Located on the Same
Chromosome Inherited?
 Homologous chromosomes of the sweet pea, showing the
genes for flower color and pollen shape
flower color gene
pollen shape gene
purple
allele, P
long
allele, L
red
allele, p
round
allele, l
Fig. 9-13
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9.5 How Are Genes Located on the Same
Chromosome Inherited?
 Crossing over can create new combinations of linked
alleles.
• Genes on the same chromosome do not always stay
together.
• During __________ I of meiosis, homologous
chromosomes sometimes exchange parts in the process,
called ___________ ________.
• Crossing over produces a new allele combination on both
homologous chromosomes.
• Therefore, the chromosomes of each haploid daughter
cell receives different combinations of alleles from those
of the parent cell.
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9.6 How Is Sex Determined?
 Offspring sex is determined
by a special pair of
chromosomes called the
sex chromosomes.
• In mammals, females have
two ___ chromosomes and
males have an ___
chromosome and a ___
chromosome.
• While the X chromosomes
look alike, the
___chromosomes are much
smaller than the
___chromosomes.
Y chromosome
X chromosome
Fig. 9-14
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9.6 How Is Sex Determined?
 During gamete formation, the sex
chromosomes segregate so that
each female gamete gets one X,
but the male gametes get either
an X or a Y.
female parent
X1
X2
eggs
Xm
 A Punnett square cross shows
how offspring sex is determined
from the segregation of the sex
chromosomes.
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Y
X2
X1
X1
Xm
X2
Xm
Xm
female offspring
sperm
 All animals have one pair of sex
chromosomes and a variable
number of other chromosomes,
male parent
called ____________.
X1
Y
X2
Y
Y
male offspring
Fig. 9-15
9.7 How Are Sex-Linked Genes Inherited?
 Genes that are found on one sex chromosome but not on the
other are called ____-________.
• Because females have two X chromosomes, they can be either
homozygous or heterozygous for genes on the X chromosome.
• Males only have one X chromosome, and therefore express all
the alleles they have on their X chromosome.
 If a man inherits one defective, recessive allele on his X
chromosome, he will show the defective phenotype.
 A female, however, may be phenotypically normal because
one of her two X chromosomes may display a functional,
dominant allele.
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9.7 How Are Sex-Linked Genes Inherited?
 In humans, the most familiar genetic defects caused
by recessive alleles on X-chromosome genes are
_____________ and _____-_______ color blindness.
Fig. 9-16
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9.8 Do Mendelian Rules Of Inheritance Apply
To All Traits?
 When a heterozygous phenotype is intermediate between
the two homozygous phenotypes, the pattern of
inheritance is called ___________ dominance.
• Human hair texture is influenced by a gene with two
incompletely dominant alleles, C1 and C2.
• A person with two copies of the C1 allele has curly hair; two
copies of the C2 allele produces straight hair; heterozygotes
with C1C2 genotype have wavy hair.
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
mother
 Two wavy-haired
people could have the
following children:
C1
father
C1C2
eggs
C2
C1
sperm
• ¼ curly (C1C1)
• ½ wavy (C1C2)
• ¼ straight (C2C2)
C1C2
C1C1
C1C2
C1C2
C2C2
C2
Fig. 9-17
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 A single gene may have multiple alleles.
• A single individual can have only two alleles for any
gene, one on each homologous chromosomes.
• However, within all the members of a species there
could be dozens of alleles for every gene.
• Human blood types A, B, and O arise as a result of three
different alleles of a single gene on chromosome 9; this
gene codes for an enzyme that adds sugar molecules to
recognition proteins on the surfaces of red blood cells.
• A person may have one of six genotypes: AA, BB, AB,
Ao, Bo, and oo.
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 Alleles A and B are dominant to o
 People with type AB blood have both A and B proteins and
have type AB blood.
• When heterozygotes express phenotypes with both of the
homozygotes, the pattern of inheritance is called
_______________.
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 Many physical traits are governed not by single genes, but
by interactions among two or more genes, a phenomenon
called ____________ inheritance.
 The more genes that contribute to a single trait, the greater
the number of phenotypes and the finer the distinctions
among them.
• Human eye color is controlled by at least three genes.
• At least three, and possibly dozens, of genes affect human skin
pigmentation, and exposure to sun further alters skin color.
Fig. 9-18
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 The environment influences the expression of genes.
• The Himalayan rabbit
has the genotype for
black fur everywhere
on this body.
• An enzyme that
produces the black
pigment is inactive
at temperatures
above 93°F (34°C).
• Most of its body is above 93°F and its fur is pale; however,
its ears, nose, tail, and feet are cooler, and these areas have
black fur.
Fig. 9-19
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9.9 How Are Human Genetic Disorders
Investigated?
 Medical geneticists are especially interested in genes
that influence our susceptibility to disease.
• Human geneticists study the inheritance of diseasecausing alleles by searching medical, historical, and family
records to study past crosses.
• __________ are diagrams showing the genetic
relationships among a set of related individuals.
• Careful analysis of pedigrees reveals whether a particular
trait is inherited in a dominant, recessive, or sex-linked
pattern.
Copyright © 2009 Pearson Education Inc.
I
II
III
(a) A pedigree for a dominant trait
I
II
?
?
?
?
III
IV
?
?
?
(b) A pedigree for a recessive trait
How to read pedigrees
I, II, III = generations
= male
= female
= parents
= offspring
?
or
= shows trait
or
= does not show trait
or
= known carrier (heterozygote) for
recessive trait
or
?
= cannot determine the genotype from this
pedigree
Fig. 9-20
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9.10 How Are Single-Gene Disorders
Inherited?
 Some human genetic disorders are caused by recessive
alleles.
• Many genes encode information to synthesize enzymes or
structural proteins in cells, and a defective allele in such a
gene may cause damaged or inactive protein.
• In some cases, a defective gene may be masked when one
normal allele is also present and makes enough functional
protein.
• A heterozygote will remain healthy, but people who inherit
two copies of a recessive, defective allele will have the
disorder.
• Muscular dystrophy is a fatal muscle degeneration of young
boys that occurs with two defective recessive alleles of such
a protein-coding gene.
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9.10 How Are Single-Gene Disorders
Inherited?
 A defective allele for ___________ synthesis causes
sickle-cell anemia.
• There is a substitution of one nucleotide that results in a
single, incorrect amino acid in the hemoglobin
molecule.
• People homozygous for sickle-cell allele synthesize only
defective hemoglobin; many of their red cells become
sickled.
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9.10 How Are Single-Gene Disorders Inherited?
 The hemoglobin cannot transport oxygen correctly and
tissues do not receive enough oxygen.
 The sickled cells are more fragile than normal red blood cells
and die before their time, causing anemia.
 Heterozygotes have half normal and half abnormal hemoglobin,
but have few sickled cells and are not seriously affected.
 Because only homozygous recessives usually show
symptoms, sickle-cell anemia is a recessive disorder.
 Individuals who are heterozygous for the sickle-cell allele are
resistant to ___________.
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9.10 How Are Single-Gene Disorders
Inherited?
 Some human genetic disorders are caused by dominant
alleles.
• Many genetic diseases are caused by dominant alleles,
in which a single defective allele is enough to cause the
disorder.
• For dominant diseases to be inherited, at least one
parent must suffer from the disease but live long
enough to have children.
• Some diseases, like Huntington disease, do not appear until
after the affected person has reproduced.
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9.10 How Are Single-Gene Disorders
Inherited?
 How can a defective allele be dominant to a normal allele?
• Some dominant alleles may encode proteins that carry out
new, toxic reactions.
• Other dominant alleles may encode a protein that is
overactive, performing its function at inappropriate times and
places.
• Some dominant alleles may encode defective proteins that
interfere with the action of the normal protein.
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9.10 How Are Single-Gene Disorders
Inherited?
 Huntington disease in a dominant inherited disorder that
causes a slow, progressive deterioration of parts of the
brain, leading eventually to death.
• Symptoms do not typically appear until 30 to 50 years of
age.
• Geneticists isolated the Huntington gene in 1993 and
identified its protein product, which they called “huntington.”
• Mutant huntington seems to interfere with normal huntington,
and form large aggregates in nerve cells that eventually kill
the cells.
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9.11 How Do Errors In Chromosome Number
Affect Humans?
 Abnormal numbers of sex chromosomes cause some
disorders.
• In normal males, X and Y chromosomes pair up during
meiosis, and sperm normally carry either an X or a Y
chromosome.
• Nondisjunction of sex chromosomes in a male produces sperm
that have two sex chromosomes (XX, YY, or XY) or that have
no sex chromosomes at all (designated O).
• Nondisjunction in a female can produce XX or O eggs instead
of normal eggs with a single X.
• When these defective sperm or eggs fuse, the resulting offspring
have abnormal numbers of sex chromosomes.
• The most common abnormalities are XO, XXX, XXY, and XYY.
Copyright © 2009 Pearson Education Inc.
9.11 How Do Errors In Chromosome Number Affect
Humans?
 Turner syndrome (XO)
• A female baby born with only one X chromosome has a
condition called Turner syndrome.
• Such women fail to menstruate and have slow development of
secondary sexual characteristics.
• They also have more X-linked recessive disorders, such as
hemophilia and color blindness, than do XX women.
 Trisomy X (XXX)
• Most of these women have no detectable defects.
• There is a higher incidence of below-normal intelligence in such
women.
• Such women are fertile and can have normal XX female
children.
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9.11 How Do Errors In Chromosome
Number Affect Humans?
 Klinefelter syndrome (XXY)
• This disorder is in males born with two X chromosomes and
one Y chromosome.
• At puberty they show mixed secondary sexual
characteristics.
• The men are usually infertile because of a low sperm count.
 Jacob syndrome (XYY)
• Males with this disorder have a high level of testosterone,
severe acne, and are tall compared to normal male height.
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9.11 How Do Errors In Chromosome
Number Affect Humans?
 Abnormal numbers of autosomes cause some disorders.
• Nondisjunction of autosomes produces eggs or sperm
that are missing an autosome, or that have two copies
of an autosome.
• Embryos with only one copy of any autosome abort
early in development.
• A baby with three copies of chromosome 21 (trisomy) can
live to adulthood.
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 Trisomy 21 (Down syndrome)
• Inheritance of an extra copy of chromosome 21 is called
trisomy 21, or Down syndrome.
• Individuals with Down syndrome display low resistance to
infectious diseases, heart malformations, and varying
degrees of mental retardation.
• Individuals with Down syndrome have several distinctive
physical characteristics, which include weak muscle tone, a
small mouth, and distinctively shaped eyelids.
Fig. 9-22
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9.11 How Do Errors In Chromosome
Number Affect Humans?
 The frequency of nondisjunction in gametes increases
with age.
• There is some increase in defective sperm with increasing
age of the father.
• The mother’s age is more significant in the probability of
Down syndrome.
• However, because younger women tend to have more
children than older women, the younger women still have the
majority of Down syndrome babies.
Copyright © 2009 Pearson Education Inc.
9.11 How Do Errors In Chromosome
Number Affect Humans?
 Down syndrome frequency increases with
maternal age.
400
number per 10,000 births
300
200
100
0
10
20
30
age of mother (years)
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40
50
Fig. 9-23