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Chapter 15
The Chromosomal Basis of
Inheritance
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Locating Genes Along Chromosomes
Crash Course Genetics
• 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
Fig. 15-1
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-2a
Green-wrinkled
seeds ( yyrr)
Yellow-round
seeds (YYRR)
P Generation
Y
Y
R R
r

y
y
r
Meiosis
Fertilization
Gametes
R Y
y
r
All F1 plants produce
yellow-round seeds (YyRr)
Fig. 15-2b
All F1 plants produce
yellow-round seeds (YyRr)
0.5 mm
F1 Generation
R
R
y
r
Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
y
r
Y
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
r
R
Y
y
r
R
Metaphase I
Y
y
1
1
r
R
r
R
Y
y
Anaphase I
Y
y
r
R
Metaphase II
R
r
2
2
Gametes
y
Y
Y
R
R
1
4
YR
r
1
3
4
yr
Y
Y
y
r
y
Y
y
Y
r
r
14
Yr
y
y
R
R
14
yR
3
Fig. 15-2c
F2 Generation
An F1  F1 cross-fertilization
3
3
9
:3
:3
:1
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
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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
• He crossed these organisms for 2 years in
order to see naturally occurring mutants.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-3
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Where do genes come from?
• Morgan’s experiments suggested that a specific
gene is carried on a specific chromosome
• ALSO, genes located on a sex chromosome
exhibit unique inheritance patterns
Fig. 15-4a
EXPERIMENT
P
Generation
F1
Generation

All offspring
had red eyes
Fig. 15-4b
RESULTS
F2
Generation
Fig. 15-4c
CONCLUSION
P
Generation
w+
X
X

w+
X
Y
w
Eggs
F1
Generation
Sperm
w+
w+
w+
w
w+
Eggs
F2
Generation
w
w+
Sperm
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
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The Chromosomal Basis of Sex
• In humans and other
mammals, there are two
varieties of sex chromosomes:
a larger and far superior X
chromosome and a smaller and
weaker Y chromosome
• Only the ends of the Y
chromosome have regions that
are homologous with the X
chromosome (these areas will
pair during meiosis- increase
the variation)
• The SRY (sex determining
region on the Y) gene on the Y
chromosome codes for the
development of testes
• Remember:
female is the
default setting for
us, so in the
absence of a Y
cs, we survive
and do just fine
(almost)
• Without an X, we
die
Fig. 15-5
X
Y
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-6a
44 +
XY
44 +
XX
Parents
22 +
22 +
or Y
X
Sperm
44 +
XX
22 +
X
+
Egg
or
44 +
XY
Zygotes (offspring)
(a) The X-Y system
Fig. 15-6b
22 +
XX
22 +
X
(b) The X-0 system
Only one type of sex cs, the females are xx
And males are xo; so sex of offspring is determined
by whether the sperm has two X or one cs.
Fig. 15-6c
76 +
ZW
76 +
ZZ
(c) The Z-W system
The sex cs present in the EGG determine the sex; females
Are ZW, males ZZ
Fig. 15-6d
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system
There are no sex cs in most species of bees and ants.
Females develop from fertilized eggs (2n); males develop from
Unfertilized eggs and are haploid (n)
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 and more intelligent X
chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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 (why?)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• If a trait is sex
linked you must
diagram it in a
pedigree as
attached to the sex
chromosomes.
This way we know
it is “influenced” by
the sex of the
individual
Fig. 15-7
XNXN
Sperm Xn

Xn Y
(a)
Sperm XN
Y
Eggs XN XNXn XNY
XN
XNXn

XNY
Xn
(b)
Sperm Xn
Y
Eggs XN XNXN XNY
XNXn XNY
XNXn

Xn Y
Y
Eggs XN XNXn XNY
Xn XN Xn Y
Xn
(c)
Xn Xn Xn Y
• Some disorders
caused by recessive
alleles on the X
chromosome in
humans:
– Color blindness
– Duchenne muscular
dystrophy- weakening
of the muscles and loss
of coordination; missing
a muscle protein called
dystrophin
– Hemophilia- proteins for
clotting factors are
missing
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
(not the same as polar bodies)
• If a female is heterozygous for a particular
gene located on the X chromosome, she will be
a mosaic for that character
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Barr body chromosomes are reactivated in cells
that give rise to gametes, so the female gamete
will have an active X
• So, we (girls) are a MOSAIC; some of our cells
have the X from dad active, others have from mom
• Once an X is inactivated in a cell, through mitosis
each cell will have a copy of the inactive X
• Women can show this in sweat glands, some
work, other “patches” don’t work
X inactivation and epigenetics
Bozeman: X inactivation
• Inactivation involves modification of DNA (chapter
18) or a gene called XIST (x-inactive specific
transcript) that is only on the barr body cs.;
• it is thought that multiple RNA copies of this gene
attach only to the X cs that is destined to be
inactivated, ultimately covering it
Fig. 15-8
X chromosomes
Early embryo:
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivation
Two cell
populations
in adult cat:
Active X
Active X
Inactive X
Black fur
What can you tell
Me about this cat?
Orange fur
Concept 15.3: Linked genes tend to be inherited
together because they are located near each other on
the same chromosome
• Each chromosome has hundreds or thousands
of genes
• Genes located on the same chromosome that
tend to be inherited together are called linked
genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Recombination frequency=
# of recombinants x 100= %
# of total offspring
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-UN1
b vg
b+ vg+
Parents
in testcross
Most
offspring

b vg
b vg
b+ vg+
b vg
or
b vg
b vg
Fig. 15-9-1
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
b+ b+ vg+ vg+

Double mutant
(black body,
vestigial wings)
b b vg vg
Fig. 15-9-2
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
b+ b vg+ vg
Double mutant
(black body,
vestigial wings)
TESTCROSS

Double mutant
b b vg vg
Fig. 15-9-3
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
Double mutant
TESTCROSS

b+ b vg+ vg
Testcross
offspring
b b vg vg
b vg
b+ vg
b vg+
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ b vg+ vg
b b vg vg b+ b vg vg b b vg+ vg
Eggs
b+ vg+
b vg
Sperm
Fig. 15-9-4
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
Double mutant
TESTCROSS

b+ b vg+ vg
Testcross
offspring
b b vg vg
b vg
b+ vg
b vg+
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ b vg+ vg
b b vg vg b+ b vg vg b b vg+ vg
Eggs
b+ vg+
b vg
Sperm
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
1
:
1
:
1
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
RESULTS
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Recombination and Linkage
Bozeman: gene recombination and gene mapping
• The genetic findings of Mendel and Morgan
relate to the chromosomal basis of
recombination
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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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A 50% frequency of recombination is observed for
any two genes on different chromosomes
• (from a test cross between parents a parent
hetero for two traits with a parent homo recessive
for two traits --- like AaBb x aabb
Fig. 15-UN2
Gametes from yellow-round
heterozygous parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent ( yyrr)
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
Recombinant
offspring
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-10
Gray body, normal wings
(F1 dihybrid)
Testcross
parents
Replication
of chromosomes
Meiosis I
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
b vg
b vg
Replication
of chromosomes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
Eggs
Testcross
offspring
b+ vg+
b vg
b+ vg
b vg+
965
944
206
185
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring Recombinant offspring
391 recombinants
Recombination
 100 = 17%
=
frequency
2,300 total offspring
b vg
Sperm
Fig. 15-10a
Testcross
parents
Black body, vestigial wings
(double mutant)
Gray body, normal wings
(F1 dihybrid)
Replication
of chromosomes
Meiosis I
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+
Meiosis I and II
vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
b+ vg+
b vg
Eggs
b+ vg
b vg+
b vg
Sperm
Replication
of chromosomes
Fig. 15-10b
Recombinant
chromosomes
Eggs
Testcross
offspring
b+ vg+
b vg
b+ vg
b vg+
944
Wild type
Black(gray-normal) vestigial
206
Grayvestigial
185
Blacknormal
965
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring
Recombinant offspring
391 recombinants
Recombination
 100 = 17%
=
frequency
2,300 total offspring
b vg
Sperm
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
(example I showed you earlier)
• Map units indicate relative distance and order,
not precise locations of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-11
RESULTS
Recombination
frequencies
9%
Chromosome
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-12
Short
aristae
0
Long aristae
(appendages
on head)
Mutant phenotypes
Black
body
48.5
Gray
body
Cinnabar Vestigial
eyes
wings
57.5
Red
eyes
67.0
Normal
wings
Wild-type phenotypes
Brown
eyes
104.5
Red
eyes
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
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Abnormal Chromosome Number
• In nondisjunction,
pairs of homologous
chromosomes do not
separate normally
during meiosis –
(anaphase I or II)
• As a result, one
gamete receives two
of the same type of
chromosome, and
another gamete
receives no copy
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-13-1
Meiosis I
Nondisjunction
One homologous pair
Doesn’t separate during
Anaphase 1
Therefore all gametes
Are affected
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
Fig. 15-13-2
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
The homologous pair
Separated (tetrad) into
Individual chromatids
And the sister
chromatid Separated
It has 2 copies of a cs
that codes for the
same information
Sister chromatid
Separated
But it is missing all
The genes that were coded
For by the homologous cs
That all went into the other gamete
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
• Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
– So the organism will either have more
copies or fewer than necessary
• Offspring with this condition have an abnormal
number of a particular chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A monosomic
zygote has only one
copy of a particular
chromosome –
usually lethal (only 1
viable form in
humans- Turner’s
syndrome)
• A trisomic zygote
has three copies of a
particular
chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
Down syndrome (trisomy 21): The result of an extra copy of
chromosome 21. People with Down syndrome are 47, 21+. Down
syndrome affects 1:700 children and alters the child's phenotype either
moderately or severely:
•
characteristic facial features, short stature; heart defects
•
susceptibility to respiratory disease, shorter lifespan
•
prone to developing early Alzheimer's and leukemia
•
often sexually underdeveloped and sterile, usually some degree of
mental retardation.
•
Down Syndrome is correlated with age of mother but can also be the
result of nondisjunction of the father's chromosome 21.
•
Fig. 15-16
• Patau syndrome (trisomy 13): serious eye,
brain, circulatory defects as well as cleft palate.
1:5000 live births. Children rarely live more than a
few months.
• Edward's syndrome (trisomy 18): almost every
organ system affected 1:10,000 live births.
Children with full Trisomy 18 generally do not live
more than a few months
• 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 (bananas and
wheat), but not animals
• Polyploids are more normal in appearance than
aneuploids
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-14
Alterations of Chromosome Structure
• Breakage of a chromosome can lead to four
types of changes in chromosome structure:
– Deletion removes a chromosomal segment
(more likely to occur during meiosis)
– Duplication repeats a segment (more likely to
occur during meiosis)
– Inversion reverses a segment within a
chromosome
– Translocation moves a segment from one
chromosome to another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-15
(a)
(b)
(c)
(d)
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 D E
F G H
Deletion
Duplication
A B C E
F G H
A B C B C D E
Inversion
A D C B E
R
F G H
M N O C D E
Reciprocal
translocation
M N O P Q
F G H
A B P Q
R
F G H
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
– Individuals are somewhat taller than average
– often have below normal intelligence.
– At one time (~1970s), it was thought that these men
were likely to be
criminally aggressive, but this
hypothesis has been disproven over time.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Monosomy X, called Turner syndrome, produces
X0 females
• 1:5000 live births; the only viable monosomy in
humans - women with Turner's have only 45
chromosomes!!! XO individuals are genetically
female, however, they do not mature sexually
during puberty and are sterile. Short stature and
normal intelligence. (98% of these fetuses die
before birth)
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-17
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Fragile X: the most common form of mental retardation.
The X chromosome of some people is unusually fragile
at one tip - seen "hanging by a thread" under a
microscope. Most people have 29 "repeats" at this end of
their X-chromosome, those with Fragile X have over 700
repeats due to duplications. Affects 1:1500 males, 1:2500
females.
Translocation: a fragment of a chromosome is moved
("trans-located") from one chromosome to another - joins
a non-homologous chromosome. The balance of genes
is still normal (nothing has been gained or lost) but can
alter phenotype as it places genes in a new environment.
Can also cause difficulties in egg or sperm development
and normal development of a zygote. Acute
Myelogenous Leukemia is caused by this translocation:
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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 (most are on
the autosomes and not the sex cs.)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Normally, most genes get a copy of an allele from
both parents.
• With imprinted genes there is only ONE working
copy of the gene, one is turned off during gamete
formation
– Usually by the addition of a methyl group
Once it has been silenced, it will remain silent
until they become reprogrammed right before
gamete formation
• But that doesn’t always happen, sometimes the
imprinted genes don’t ever reset and begin their
life with epigenetic tags in place
What happens if imprinting goes wrong?
You can have two active alleles (when you only
need one) or you have two inactive alleles
this can lead to cancer, developmental
abnormalities and other problems
Prader-Willi and Angelman’s Syndrome
• Both on chromosomes 15
• Both have the same imprinted area on the cs
– Some of the genes on this cs are inactivated in
the egg, and atleast one of them is inactive in
the sperm
Someone who inherits this disorder on this cs
gets this because of the inactive allele from either
parent
Prader-Willi
• Symptoms include learning
difficulties, short stature,
and compulsive eating.
MOST individuals are missing
gene activity that normally
comes from dad.
Happens when dad's copy is
missing, or when there are
two maternal copies.
Angelman’s
• Symptoms include learning
difficulties, speech problems,
seizures, jerky movements, and
an unusually happy disposition.
• Individuals are missing gene
activity that normally comes
from mom.
• Happens when mom's copy is
defective or missing, or when
there are two paternal copies
So, YOU ALL ARE ASKING, what the heck is
epigenetics and why do I care
• “The development and maintenance of an
organism is orchestrated by a set of chemical
reactions that switch parts of the genome off and
on at strategic times and locations. Epigenetics is
the study of these reactions and the factors that
influence them.”
• Source:
• http://learn.genetics.utah.edu/content/epigenetics/
• This is important because it makes you the
wonderful being that you are!
• Neil Tyson DeGrasse
• https://www.youtube.com/watch?v=7WEHoCA1hp
o
• 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 (about 1%)
• Most imprinted genes are critical for embryonic
development
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
EVERYTHING is about EVOLUTION
• Genetic Conflict Theory: some organisms have
litters of babies that can be fathered by different
males; so the dad would “want” its own offspring to
have a better chance to survive (and taking more
of the nutrients offered by mother from other litter
mates); dad would prefer his offspring to be larger
• OR
• It would be better for all the babies to have the
maternal imprint to ensure survival of ALL the
babies, mom would prefer smaller babies in order
to be able to support them
Ligers and Tigons and Mules
The
differences
seen between
the different
combinations
is determined
by which
parental
genes are
being
imprinted
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from the
egg
• Remember: most of the paternal mitochondria are
located behind the head of the sperm, and fall off
• The first evidence of extranuclear genes came
from studies on the inheritance of yellow or white
patches on leaves of an otherwise green plant
Fig. 15-19
Variegations are
Due to mutations in
Plastids genes that
Control pigmentation
These pigment genes
Are distributed
Randomly to daughter
Cells; the plant
Pattern is determined
By the ratio of wild-type
To mutant plastids
In the tissue
• 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
– Blame it on your mother
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings