Chapter 15 chromosomes

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 15
The Chromosomal Basis of
Inheritance

Overview: Locating Genes Along Chromosomes
• Mendel’s “hereditary factors” were
genes, though this wasn’t known at the time
• Today we can show that genes are located on
chromosomes
• The location of a particular gene can be seen
by tagging isolated chromosomes with a
fluorescent dye that highlights the gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Concept 15.1: Mendelian inheritance has its
physical basis in the behavior of chromosomes
• Mitosis and meiosis were first described in the
late 1800s
• The chromosome theory of inheritance
states:
– Mendelian genes have specific loci (positions) on
chromosomes
– Chromosomes undergo segregation and independent
assortment
• The behavior of chromosomes during meiosis
was said to account for Mendel’s laws of
segregation and independent assortment

Fig. 15-2
P Generation Yellow-round
seeds (YYRR)
Y
F1 Generation
Y
R R
R Y
 r
r
r
y
y
y
Meiosis
Fertilization
Gametes
Green-wrinkled
seeds (yyrr)
All F1 plants produce
yellow-round seeds (YyRr)
R R
YY
r r
y y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R Rrr
y y
Anaphase I
r r
y Y
Metaphase IIR
Y
R
y
yyy
RR
YY
rrrr
yYY
R R
yRYryrYR1/4
1/4
1/4
1/4
F2 Generation
Gametes
An F1  F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
1
2
33
2
1

Fig. 15-2a
P Generation
Gametes
Meiosis
Fertilization
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds ( yyrr)
y
y
y
r
r
r
Y
Y
YR
RR


Fig. 15-2b
0.5 mm
Meiosis
Metaphase I
Anaphase I
Metaphase II
Gametes
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
1
4
yr 1 4
Yr1
4 YR
3 3
F1 Generation
1 4
yR
R
R
R
R
RR
R
R R R R
R
Y
Y
Y Y
Y
YY
Y
YY
YY
y
r r
rr
r r
rr
r r r r
y
y
y
y
y
y y
yyy
y
1
2 2
1

Fig. 15-2c
F2 Generation
An F1  F1 cross-fertilization
9 : 3 : 3 : 1
3 3

Morgan’s Experimental Evidence: Scientific
Inquiry
• The first solid evidence associating a specific
gene with a specific chromosome came from
Thomas Hunt Morgan, an embryologist
• Morgan’s experiments with fruit flies provided
convincing evidence that chromosomes are the
location of Mendel’s heritable factors

Morgan’s Choice of Experimental Organism
• Several characteristics make fruit flies a
convenient organism for genetic studies:
– They breed at a high rate
– A generation can be bred every two weeks
– They have only four pairs of chromosomes

• Morgan noted wild type, or
normal, phenotypes that were common in the
fly populations
• Traits alternative to the wild type are called
mutant phenotypes

Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
• In one experiment, Morgan mated male flies
with white eyes (mutant) with female flies with
red eyes (wild type)
– The F1 generation all had red eyes
– The F2 generation showed the 3:1 red:white
eye ratio, but only males had white eyes
• Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
• Morgan’s finding supported the chromosome
theory of inheritance

Fig. 15-4
P
Generation
Generation
Generation
Generation
Generation
Generation
F1
F2
All offspring
had red eyes
Sperm
Eggs
F1
F2
P
Sperm
Eggs


X
X
X
Y
CONCLUSION
EXPERIMENT
RESULTS
w
w
w
w
w
w
w w
+
+
+
+ +
w
w
w w
w
w
w
ww
+
+
+
+ +
+

Fig. 15-4a
EXPERIMENT
P
Generation
Generation
F1 All offspring
had red eyes


Fig. 15-4b
RESULTS
Generation
F2

Fig. 15-4c
Eggs
F1
CONCLUSION
Generation
Generation
P X
X
w
Sperm
X
Y
+
+
+
+ +
Eggs
Sperm
+
+
+ +
+
Generation
F2

w
w
w
ww
w
w
w
w
w
w
w
w
w
w

Concept 15.2: Sex-linked genes exhibit unique
patterns of inheritance
• In humans and some other animals, there is a
chromosomal basis of sex determination

The Chromosomal Basis of Sex
• In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
• Only the ends of the Y chromosome have
regions that are homologous with the X
chromosome
• The SRY gene on the Y chromosome codes for
the development of testes

• Females are XX, and males are XY
• Each ovum contains an X chromosome, while
a sperm may contain either an X or a Y
chromosome
• Other animals have different methods of sex
determination

Fig. 15-6
44 +
XY
Parents
44 +
XX
22 +
X
22 +
X
22 +
Yor +
44 +
XX or
Sperm Egg
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
22 +
X
(b) The X-0 system
76 +
ZW
76 +
ZZ
(c) The Z-W system
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system

Fig. 15-6a
(a) The X-Y system
44 +
XY
44 +
XX
Parents
44 +
XY
44 +
XX
22 +
X
22 +
X
22 +
Yor
or
Sperm Egg
+
Zygotes (offspring)

Fig. 15-6b
(b) The X-0 system
22 +
XX
22 +
X

Fig. 15-6c
(c) The Z-W system
76 +
ZW
76 +
ZZ

Fig. 15-6d
(d) The haplo-diploid system
32
(Diploid)
16
(Haploid)

Inheritance of Sex-Linked Genes
• The sex chromosomes have genes for many
characters unrelated to sex
• A gene located on either sex chromosome is
called a sex-linked gene
• In humans, sex-linked usually refers to a gene
on the larger X chromosome

• Sex-linked genes follow specific patterns of
inheritance
• For a recessive sex-linked trait to be expressed
– A female needs two copies of the allele
– A male needs only one copy of the allele
• Sex-linked recessive disorders are much more
common in males than in females

Fig. 15-7
(a) (b) (c)
XN
XN Xn
Y XN
Xn
  XN
Y XN
Xn
 Xn
Y
YXn
SpermYXN
SpermYXn
Sperm
XN
Xn
Eggs XN
XN
XN
Xn
XN
Y
XN
Y
Eggs XN
Xn
XN
XN
Xn
XN
XN
Y
Xn
Y
Eggs XN
Xn
XN
Xn
Xn
Xn
XN
Y
Xn
Y

• Some disorders caused by recessive alleles on
the X chromosome in humans:
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia

X Inactivation in Female Mammals
• In mammalian females, one of the two X
chromosomes in each cell is randomly
inactivated during embryonic development
• The inactive X condenses into a Barr body
• If a female is heterozygous for a particular
gene located on the X chromosome, she will be
a mosaic for that character

Fig. 15-8
X chromosomes
Early embryo:
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivationTwo cell
populations
in adult cat:
Active X
Active X
Inactive X
Black fur Orange fur

• Each chromosome has hundreds or thousands
of genes
• Genes located on the same chromosome that
tend to be inherited together are called linked
genes
Concept 15.3: Linked genes tend to be inherited
together because they are located near each other on
the same chromosome

How Linkage Affects Inheritance
• Morgan did other experiments with fruit flies to
see how linkage affects inheritance of two
characters
• Morgan crossed flies that differed in traits of
body color and wing size

Fig. 15-UN1
b+ vg+
Parents
in testcross
Most
offspring
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg

or

Fig. 15-9-1
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vgb+ b+ vg+ vg+

Fig. 15-9-2
EXPERIMENT
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b b vg vg
Double mutant
TESTCROSS

b+ b+ vg+ vg+
F1 dihybrid
(wild type)
b+ b vg+ vg

Fig. 15-9-3
EXPERIMENT
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b b vg vg
Double mutant
TESTCROSS

b+ b+ vg+ vg+
F1 dihybrid
(wild type)
b+ b vg+ vg
Testcross
offspring Eggs b+ vg+ b vg b+ vg b vg+
Black-
normal
Gray-
vestigial
Black-
vestigial
Wild type
(gray-normal)
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg

Fig. 15-9-4
EXPERIMENT
RESULTS
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b b vg vg
Double mutant
TESTCROSS

b+ b+ vg+ vg+
F1 dihybrid
(wild type)
b+ b vg+ vg
Testcross
offspring Eggs b+ vg+ b vg b+ vg b vg+
Black-
normal
Gray-
vestigial
Black-
vestigial
Wild type
(gray-normal)
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg
PREDICTED RATIOS
If genes are located on different chromosomes:
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
1
1
1
1 1
0 0
965 944 206 185
:
:
:
:
:
:
:
:
:

• Morgan found that body color and wing size
are usually inherited together in specific
combinations (parental phenotypes)
• He noted that these genes do not assort
independently, and reasoned that they were on
the same chromosome

• However, nonparental phenotypes were also
produced
• Understanding this result involves exploring
genetic recombination, the production of
offspring with combinations of traits differing
from either parent

Genetic Recombination and Linkage
• The genetic findings of Mendel and Morgan
relate to the chromosomal basis of
recombination

Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
• Mendel observed that combinations of traits in
some offspring differ from either parent
• Offspring with a phenotype matching one of the
parental phenotypes are called parental types
• Offspring with nonparental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
• A 50% frequency of recombination is observed
for any two genes on different chromosomes

Fig. 15-UN2
YyRr
Gametes from green-
wrinkled homozygous
recessive parent (yyrr)
Gametes from yellow-round
heterozygous parent (YyRr)
Parental-
type
offspring
Recombinant
offspring
yr
yyrr Yyrr yyRr
YR yr Yr yR

Recombination of Linked Genes: Crossing Over
• Morgan discovered that genes can be
linked, but the linkage was incomplete, as
evident from recombinant phenotypes
• Morgan proposed that some process must
sometimes break the physical connection
between genes on the same chromosome
• That mechanism was the crossing over of
homologous chromosomes
Animation: Crossing Over

Fig. 15-10
Testcross
parents
Replication
of chromo-
somes
Gray body, normal wings
(F1 dihybrid)
Black body, vestigial wings
(double mutant)
Replication
of chromo-
somes
b+ vg+
b+ vg+
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b+ vg+
b+ vg
b vg+
b vg
Recombinant
chromosomes
Meiosis I and II
Meiosis I
Meiosis II
b vg+b+ vgb vgb+ vg+
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
944
Black-
vestigial
206
Gray-
vestigial
185
Black-
normal
b+ vg+
b vg b vg
b vg b+ vg
b vg b vg
b vg+
Sperm
b vg
Parental-type offspring Recombinant offspring
Recombination
frequency =
391 recombinants
2,300 total offspring
 100 = 17%

Fig. 15-10a
Testcross
parents
Replication
of chromo-
somes
Gray body, normal wings
(F1 dihybrid)
Black body, vestigial wings
(double mutant)
Replication
of chromo-
somes
b+ vg+
b+ vg+
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b+ vg+
b+ vg
b vg+
b vg
Recombinant
chromosomes
Meiosis I and II
Meiosis I
Meiosis II
Eggs Sperm
b+ vg+
b vg b+ vg b vgb vg+

Fig. 15-10b
Testcross
offspring
965
Wild type
(gray-normal)
944
Black-
vestigial
206
Gray-
vestigial
185
Black-
normal
b+ vg+
b vg b vg
b vg b+ vg
b vg
b vg
b+ vg+
Spermb vg
Parental-type offspring Recombinant offspring
Recombination
frequency
=
391 recombinants
2,300 total offspring
 100 = 17%
b vg
b+ vg b vg+
b vg+
Eggs
Recombinant
chromosomes

Mapping the Distance Between Genes Using
Recombination Data: Scientific Inquiry
• Alfred Sturtevant, one of Morgan’s
students, constructed a genetic map, an
ordered list of the genetic loci along a particular
chromosome
• Sturtevant predicted that the farther apart two
genes are, the higher the probability that a
crossover will occur between them and
therefore the higher the recombination
frequency

• A linkage map is a genetic map of a
chromosome based on recombination
frequencies
• Distances between genes can be expressed as
map units; one map unit, or
centimorgan, represents a 1% recombination
frequency
• Map units indicate relative distance and
order, not precise locations of genes

Fig. 15-11
RESULTS
Recombination
frequencies
Chromosome
9% 9.5%
17%
b cn vg

• Genes that are far apart on the same
chromosome can have a recombination
frequency near 50%
• Such genes are physically linked, but
genetically unlinked, and behave as if found on
different chromosomes

• Sturtevant used recombination frequencies to
make linkage maps of fruit fly genes
• Using methods like chromosomal
banding, geneticists can develop cytogenetic
maps of chromosomes
• Cytogenetic maps indicate the positions of
genes with respect to chromosomal features

Fig. 15-12
Mutant phenotypes
Short
aristae
Black
body
Cinnabar
eyes
Vestigial
wings
Brown
eyes
Red
eyes
Normal
wings
Red
eyes
Gray
body
Long aristae
(appendages
on head)
Wild-type phenotypes
0 48.5 57.5 67.0 104.5

Concept 15.4: Alterations of chromosome number
or structure cause some genetic disorders
• Large-scale chromosomal alterations often
lead to spontaneous abortions (miscarriages)
or cause a variety of developmental disorders

Abnormal Chromosome Number
• In nondisjunction, pairs of homologous
chromosomes do not separate normally during
meiosis
• As a result, one gamete receives two of the
same type of chromosome, and another
gamete receives no copy

Fig. 15-13-1
Meiosis I
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
Nondisjunction

Fig. 15-13-2
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction

Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
Number of chromosomes
n + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n

• Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
• Offspring with this condition have an abnormal
number of a particular chromosome

• A monosomic zygote has only one copy of a
particular chromosome
• A trisomic zygote has three copies of a
particular chromosome

• Polyploidy is a condition in which an organism
has more than two complete sets of
chromosomes
– Triploidy (3n) is three sets of chromosomes
– Tetraploidy (4n) is four sets of chromosomes
• Polyploidy is common in plants, but not animals
• Polyploids are more normal in appearance than
aneuploids

Alterations of Chromosome Structure
• Breakage of a chromosome can lead to four
types of changes in chromosome structure:
– Deletion removes a chromosomal segment
– Duplication repeats a segment
– Inversion reverses a segment within a
chromosome
– Translocation moves a segment from one
chromosome to another

Fig. 15-15
Deletion
A B C D E F G H A B C E F G H
(a)
(b)
(c)
(d)
Duplication
Inversion
Reciprocal
translocation
A B C D E F G H
A B C D E F G H
A B C D E F G H
A B C B C D E F G H
A D C B E F G H
M N O C D E F G H
M N O P Q R A B P Q R

Human Disorders Due to Chromosomal
Alterations
• Alterations of chromosome number and
structure are associated with some serious
disorders
• Some types of aneuploidy appear to upset the
genetic balance less than others, resulting in
individuals surviving to birth and beyond
• These surviving individuals have a set of
symptoms, or syndrome, characteristic of the
type of aneuploidy

Down Syndrome (Trisomy 21)
• Down syndrome is an aneuploid condition that
results from three copies of chromosome 21
• It affects about one out of every 700 children
born in the United States
• The frequency of Down syndrome increases
with the age of the mother, a correlation that
has not been explained

Aneuploidy of Sex Chromosomes
• Nondisjunction of sex chromosomes produces
a variety of aneuploid conditions
• Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY
individuals
• Monosomy X, called Turner
syndrome, produces X0 females, who are
sterile; it is the only known viable monosomy in
humans

Disorders Caused by Structurally Altered
Chromosomes
• The syndrome cri du chat (“cry of the
cat”), results from a specific deletion in
chromosome 5
• A child born with this syndrome is mentally
retarded and has a catlike cry; individuals
usually die in infancy or early childhood
• Certain cancers, including chronic
myelogenous leukemia (CML), are caused by
translocations of chromosomes

Fig. 15-17
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)

Concept 15.5: Some inheritance patterns are
exceptions to the standard chromosome theory
• There are two normal exceptions to Mendelian
genetics
• One exception involves genes located in the
nucleus, and the other exception involves
genes located outside the nucleus

Genomic Imprinting
• For a few mammalian traits, the phenotype
depends on which parent passed along the
alleles for those traits
• Such variation in phenotype is called genomic
imprinting
• Genomic imprinting involves the silencing of
certain genes that are “stamped” with an
imprint during gamete production

Fig. 15-18
Normal Igf2 allele
is expressed
Paternal
chromosome
Maternal
chromosome
Normal Igf2 allele
is not expressed
Mutant Igf2 allele
inherited from mother
(a) Homozygote
Wild-type mouse
(normal size)
Mutant Igf2 allele
inherited from father
Normal size mouse
(wild type)
Dwarf mouse
(mutant)
Normal Igf2 allele
is expressed
Mutant Igf2 allele
is expressed
Mutant Igf2 allele
is not expressed
Normal Igf2 allele
is not expressed
(b) Heterozygotes

Fig. 15-18a
Normal Igf2 allele
is expressed
Paternal
chromosome
Maternal
chromosome
(a) Homozygote
Wild-type mouse
(normal size)
Normal Igf2 allele
is not expressed

Fig. 15-18b
Mutant Igf2 allele
inherited from mother
Mutant Igf2 allele
inherited from father
Normal size mouse
(wild type)
Dwarf mouse
(mutant)
Normal Igf2 allele
is expressed
Mutant Igf2 allele
is expressed
Mutant Igf2 allele
is not expressed
Normal Igf2 allele
is not expressed
(b) Heterozygotes

• It appears that imprinting is the result of the
methylation (addition of –CH3) of DNA
• Genomic imprinting is thought to affect only a
small fraction of mammalian genes
• Most imprinted genes are critical for embryonic
development

Inheritance of Organelle Genes
• Extranuclear genes (or cytoplasmic genes) are
genes found in organelles in the cytoplasm
• Mitochondria, chloroplasts, and other plant
plastids carry small circular DNA molecules
• Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from
the egg
• The first evidence of extranuclear genes came
from studies on the inheritance of yellow or
white patches on leaves of an otherwise green
plant

• Some defects in mitochondrial genes prevent
cells from making enough ATP and result in
diseases that affect the muscular and nervous
systems
– For example, mitochondrial myopathy and
Leber’s hereditary optic neuropathy

Fig. 15-UN4
EggSperm
P generation
gametes
C
B
AD E
F
D
F E
A
B
C
e
d
f
c ba
d
f
e
c
b
a
This F1 cell has 2n = 6
chromosomes and is
heterozygous for all six
genes shown (AaBbCcDdEeFf).
Red = maternal; blue = paternal.
+
Each chromosome
has hundreds or
thousands of genes.
Four (A, B, C, F) are
shown on this one.
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e) or so
far apart on the same
chromosome (c and f)
that they assort
independently.
Genes on the same chromo-
some whose alleles are so
close together that they do
not assort independently
(such as a, b, and c) are said
to be linked.

You should now be able to:
1. Explain the chromosomal theory of
inheritance and its discovery
2. Explain why sex-linked diseases are more
common in human males than females
3. Distinguish between sex-linked genes and
linked genes
4. Explain how meiosis accounts for
recombinant phenotypes
5. Explain how linkage maps are constructed

6. Explain how nondisjunction can lead to
aneuploidy
7. Define trisomy, triploidy, and polyploidy
8. Distinguish among
deletions, duplications, inversions, and
translocations
9. Explain genomic imprinting
10.Explain why extranuclear genes are not
inherited in a Mendelian fashion

Chapter 15 chromosomes

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Chapter 15 chromosomes