Download (XX) express twice as many genes as males (XY)?

CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
15
The
Chromosomal
Basis of
Inheritance
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Where are Mendel’s hereditary factors located in the cell?
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
Figure 15.1
© 2014 Pearson Education, Inc.
Mendelian inheritance has its physical basis
in the behavior of chromosomes
• Mitosis and meiosis were first described in the
late 1800s
- Chromosomes and genes are both present
in pairs in diploid cells.
– Homologous chromosomes separate and
alleles segregate during meiosis.
– Fertilization restores the paired condition
for both chromosomes and genes.
© 2014 Pearson Education, Inc.
Chromosome Theory of Inheritance
– Around 1902 a chromosome theory of inheritance began
to take form:
– Genes occupy specific loci on chromosomes.
– Chromosomes undergo segregation during meiosis.
– Chromosomes undergo independent assortment during
meiosis.
– The behavior of homologous chromosomes during meiosis
can account for the segregation of the alleles at each
genetic locus to different gametes.
– The behavior of nonhomologous chromosomes can account
for the independent assortment of alleles for two or more
genes located on different chromosomes.
• Mendelian genes have specific loci (positions) on chromosomes
• The behavior of chromosomes during meiosis was said to
account for Mendel’s laws of segregation and independent
assortment
© 2014 Pearson Education, Inc.
Law of Segregation:
Chromosomes during F1 Meiosis
•
•
•
The two alleles for
each gene separate
during gamete
formation.
R and r alleles
segregate at
anaphase I, yielding
two types of daughter
cells.
Each gamete gets
either the R or r allele.
During fertilization, the
R and r alleles
recombine randomly.
Figure15.2
Law of Independent Assortment:
Chromosomes during F1 Meiosis
1.
2.
3.
Alleles on nonhomologous
chromosomes assort
independently during gamete
formation.
Alleles at both loci segregate
during anaphase I giving rise
to 4 different daughter cells,
depending on how they
arranged during metaphase I.
Each gamete gets a
chromosome in one of 4
allele combinations.
Fertilization results in a F2
9:3:3:1 phenotypic ratio.
Fig.15.2
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
© 2014 Pearson Education, Inc.
Morgan used fruit flies for his
experiments
• Several characteristics make
fruit flies a convenient
organism for genetic studies:
– They breed at a high rate
and have more offspring
– A generation can be bred
every two weeks
– They have only four pairs of
chromosomes
– Still, Morgan spent a year
looking for variant individuals
among the flies he was
breeding!
– Finally he discovered a
single male fly with white
eyes instead of the usual
red.
© 2014 Pearson Education, Inc.
Figure 15.3
Wild and Mutant
• The normal character phenotype is called the
wild type.
• For a given character in flies, the gene’s symbol
is chosen from the first mutant discovered.
– The allele for white eyes in Drosophila is symbolized
by w.
• A + superscript identifies the wild-type (red-eye)
allele (w+).
• Alternative traits are called mutant phenotypes
because they are due to alleles that originate as
mutations in the wild-type allele.
© 2014 Pearson Education, Inc.
Correlating Behavior of a Gene’s Alleles
with Behavior of a Chromosome Pair
• 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
• (All the F2 females and half the F2 males had red eyes)
• Morgan concluded that a fly’s eye color was linked
to its sex
• Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
• Morgan’s finding supported the chromosome
theory of inheritance
Morgan’s Experiment
1. Mate w+ female with w male.
2. All F1 offspring had red eyes,
suggesting that the red eye allele is
dominant.
3. Breed w+ F1 female to w+ F1male.
4. All F1 females had w+, only male flies
had w (3:1 ratio)
Conclusion: the gene with the
white-eyed mutation is on the X
chromosome, with no
corresponding allele present on
the Y chromosome.
Females (XX) may have two redeyed alleles and have red eyes or
may be heterozygous and have red
eyes.
Males (XY) have only a single allele.
They will have red eyes if they have
a red-eyed allele or white eyes if they
have a white-eyed allele.
Figure 15.4
Sex-linked genes exhibit unique patterns
of inheritance
• In humans and some other animals, there
is a chromosomal basis of sex
determination
© 2014 Pearson Education, Inc.
The Chromosomal Basis of Sex
• In humans and other mammals, there are two
varieties of sex chromosomes, X and Y.
Figure 15.5
– An individual who inherits two X chromosomes usually
develops as a female.
– An individual who inherits an X and a Y chromosome
usually develops as a male.
• Short segments at either end of the Y chromosome are
the only regions that are homologous with the
corresponding regions of the X.
• The SRY gene on the Y chromosome codes for a protein
that directs the development of male anatomical features
• These homologous regions allow the X and Y
chromosomes in males to pair and behave like
homologous chromosomes during meiosis in the testes.
© 2014 Pearson Education, Inc.
Each conception has about a fifty-fifty
chance of producing a particular sex
• In both testes (XY) and ovaries (XX), the two sex
chromosomes segregate during meiosis, and
each gamete receives one.
• Each ovum receives an X chromosome.
• Half the sperm cells receive an X chromosome,
and half receive a Y chromosome.
– If a sperm cell bearing an X chromosome fertilizes an
ovum, the resulting zygote is female (XX).
– If a sperm cell bearing a Y chromosome fertilizes an
ovum, the resulting zygote is male (XY).
• Therefore, each conception has about a fiftyfifty chance of producing a particular sex.
© 2014 Pearson Education, Inc.
Figure 15.6
Chromosomal Systems of Sex Determination
Animals have different methods of sex
determination.
The X-0 system is found in some
insects. Females are XX and males
44 +
44 +
Parents
are X.
XX
XY
In birds, some fishes, and some
insects, females are ZW and males
are ZZ.
22 +
22 + or 22 +
X
In bees and ants, females are diploid
X
Y
and males are haploid.
Sperm
Egg
44 +
XX
or
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
(b) The X-0 system
22 +
X
76 +
ZW
76 +
ZZ
(c) The Z-W system
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system
Anatomical signs of sex first appear when the
embryo is about two months old
• Before that, the gonads can develop into either testes or
ovaries.
• The SRY (sex-determining region of the Y chromosome)
gene on the Y chromosome required for the
development of testes.
• In individuals with the SRY gene, the generic embryonic
gonads develop into testes.
– The SRY gene codes for a protein that regulates many other
genes, triggering a cascade of biochemical, physiological, and
anatomical features.
• In individuals lacking the SRY gene, the generic
embryonic gonads develop into ovaries.
The sex chromosomes
•
•
Researchers have sequenced
the Y chromosome and
identified 78 genes coding for
about 25 proteins.
– Half of the genes are
expressed only in the
testes, and some are
required for normal
testicular function.
– Some genes on the Y
chromosome are necessary
for the production of
functional sperm.
– In the absence of these
genes, an XY individual is
male but does not produce
normal sperm.
In addition to their role in
determining sex, the sex
chromosomes, especially the X
chromosome, have genes for
many characters unrelated to
sex.
© 2014 Pearson Education, Inc.
Inheritance of Sex-Linked Genes
• A gene located on either sex chromosome is called a sex-linked
gene.
• Genes on the Y chromosome are called Y-linked genes; there are
few of these
• Genes on the X chromosome are called X-linked genes
• In humans, the term sex-linked gene refers to a gene on the X
chromosome. X chromosome have genes for many characters
unrelated to sex, whereas the Y chromosome mainly encodes
genes related to sex determination
• Human sex-linked genes follow the same pattern of inheritance as
Morgan’s white-eye locus in Drosophila.
– Fathers pass sex-linked alleles to all their daughters but none of
their sons.
– Mothers pass sex-linked alleles to both sons and daughters.
• If a sex-linked trait is due to a recessive allele, a female will express
this phenotype only if she is homozygous.
• Heterozygous females are carriers for the recessive trait.
• Because males have only one X chromosome (hemizygous),
any male who receives the recessive allele from his mother will
express the recessive trait.
Disorders caused by recessive alleles
on the X chromosome in humans
–
–
Color blindness (mostly X-linked)
Duchenne muscular dystrophy- a lethal muscular disorder
•
•
•
–
Affected individuals rarely live past their early 20s.
This disorder is due to the absence of an X-linked gene for a
key muscle protein called dystrophin.
The disease is characterized by a progressive weakening of the
muscles and a loss of coordination.
Hemophilia- absence of one or more proteins required for
blood clotting.
•
•
•
•
These proteins normally slow and then stop bleeding.
Individuals with hemophilia have prolonged bleeding because a
firm clot forms slowly.
Bleeding in muscles and joints can be painful and can lead to
serious damage.
Today, people with hemophilia can be treated with intravenous
injections of the missing protein.
© 2014 Pearson Education, Inc.
Sex-linked recessive traits
Figure 15.7
a)
b)
c)
A color-blind father (XnY) will pass the mutant allele to all daughters, but
not sons. If the mother is a normal dominant homozygote (XNXN) the
daughters will have a normal phenotype, but be carriers of the N
mutation (XNXn).
If one of the female carriers (XNXn) mate with a normal male (XNY) there is
a 50% chance that each daughter will be a carrier (XNXn) and a 50%
chance that a son will have the disease (XnY).
If one of the female carriers (XNXn) mate with a diseased male (XnY) there
is a 50% chance that each child (♀ or ♂) will have the disease. All normal
daughters will be carriers (XNXn) and normal sons will not carry the
recessive allele (XNY).
© 2014 Pearson Education, Inc.
Q-Do females (XX) express twice as
many genes as males (XY)?
Answer - NO
One of the X chromosomes condense in every cell during female
embryo development and becomes a Barr body
Most of the genes on the Barr-body chromosome are not
expressed.
Selection of which X chromosome will form the Barr body occurs
randomly and independently in embryonic cells at the time of X
inactivation.
As a consequence, females consist of a mosaic of two types of cells,
some with an active X chromosome from their fathers and others with
an active X chromosome from their mothers.
After an X chromosome is inactivated in a particular cell, all mitotic
descendants of that cell will have the same inactive X.
If a female is heterozygous for a sex-linked trait, approximately
half her cells will express one allele, and the other half will
express the other allele.
© 2014 Pearson Education, Inc.
X Inactivation in the tortoiseshell cat
The tortoiseshell gene is located
on the X chromosome.
The tortoiseshell phenotype
requires both the orange fur
and black fur alleles.
Only females can have both
alleles (XX).
Females heterozygous for the
tortoiseshell gene have orange
patches of fur where the
orange allele is active as well
as patches of black fur where
the black allele is active.
Figure 15.8 X
© 2014 Pearson Education, Inc.
inactivation and the tortoiseshell cat
What causes X chromosome
inactivation?
1. CH4 groups are added to DNA nucleotides.
2. XIST (X-inactive specific transcript)
–
–
–
This gene is active only on the Barr-body
chromosome and produces multiple copies of an
RNA molecule that attach to the X chromosome on
which they were made.
This initiates X inactivation.
The mechanism that connects XIST RNA and DNA
methylation is unknown.
© 2014 Pearson Education, Inc.
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
• The results of crosses with linked genes differ
from those expected according to the law of
independent assortment.
• Morgan observed this linkage and its deviations
when he followed the inheritance of characters for
body color and wing size in Drosophila.
© 2014 Pearson Education, Inc.
Testcross: body color and wing size
or
•
•
•
•
•
The wild-type body color is gray (b+), and the mutant is black (b).
The wild-type wing size is normal (vg+), and the mutant has vestigial wings
(vg).
– The mutant alleles are recessive to the wild-type alleles.
– Neither gene is on a sex chromosome.
Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous
recessive males (bbvgvg).
According to independent assortment, this should produce four phenotypes in
a 1:1:1:1 ratio.
Morgan observed that most F1 offspring resembled parents
Morgan’s Experiment
1.
2.
3.
4.
Mate true-breeding wild
type (b+b+vg+vg+) with
recessive (bbvgvg) to
obtain heterozygous F1.
All of the F1
heterozygotes have wild
type appearance.
Testcross: Mate dihybrid
F1 females (b+bvg+vg)
with black vestigialwinged males (bbvgvg).
Results: Most offspring
are gray body /large
wings or back body/small
wings.
Conclusion: Genes are
located on same
chromosome and are
inherited together
(usually).
Figure 15.9
© 2014 Pearson Education, Inc.
Some nonparentals ??
Genetic Recombination
• Genetic recombination- The production of offspring
with combinations of traits that differ from those found in
either parent.
– Genetic recombination can result from independent
assortment of genes located on nonhomologous
chromosomes.
• 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
• Mendel also observed that combinations of traits in
some offspring differed from either parent
© 2014 Pearson Education, Inc.
Dihybrid Cross: YyRr x yyrr
Testcross:
X
F1
•
•
•
•
•
Mendel’s dihybrid cross experiments produced offspring that had a
combination of traits that did not match either parent in the P generation.
If the P generation consists of a yellow-round seed parent (YYRR) crossed with
a green-wrinkled seed parent (yyrr), all the F1 plants have yellow-round seeds
(YyRr).
A cross between an F1 plant and a homozygous recessive plant (a testcross)
produces four phenotypes.
Half are the parental types, with phenotypes that match the original P parents,
with either yellow-round seeds or green-wrinkled seeds.
Half are recombinant types or recombinants, new combinations of parental
traits, with yellow-wrinkled or green-round seeds.
Independent Assortment of alleles
results in genetic recombination
•
•
•
•
A 50% frequency of recombination is
observed for any two genes located on
different (nonhomologous)
chromosomes.
The physical basis of recombination
between unlinked genes is the random
orientation of homologous
chromosomes at metaphase I of
meiosis, which leads to the
independent assortment of alleles.
The F1 parent (YyRr) produces
gametes with four different
combinations of alleles: YR, Yr, yR,
and yr.
The orientation of the tetrad containing
the seed-color gene has no bearing on
the orientation of the tetrad with the
seed-shape gene.
Recombination of Linked Genes is a result
of Crossing Over
• Linked genes (genes located on the same chromosome)
tend to move together through meiosis and fertilization.
• Under normal Mendelian genetic rules we would not expect
linked genes to recombine into assortments of alleles not
found in the parents.
– 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
© 2014 Pearson Education, Inc.
Crossing Over produced recombinant
offspring in Morgan’s testcross experiments
1.
2.
3.
During Meiosis I crossing
over between b and vg loci
produces new allele
combinations in some (not
most) egg producing cells.
Most eggs will harbor the
maternal type chromosomes.
During Meiosis II separation
of the chromatids produces
recombinant gametes (eggs)
with the new allele
combinations. Note that no
new allele combinations are
produced in the bbvgvg
sperm during Meiosis I and II.
Fertilization of the eggs by the
bbvgvg sperm will give rise to
the recombinant offspring.
Black body, vestigial wings
(double mutant)
Gray body, normal wings
(F1 dihybrid)
Testcross
parents
b+ vg+
b vg
b vg
b vg
Replication
of chromosomes
Meiosis I
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
b+vg+
b vg
b+ vg
b vg+
944
Blackvestigial
206
Grayvestigial
185
Blacknormal
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
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
Figure 15.10
b vg
Sperm
New Combinations of Alleles: Variation for
Normal Selection
• Recombinant chromosomes bring alleles
together in new combinations in gametes
• Random fertilization increases even
further the number of variant combinations
that can be produced
• This abundance of genetic variation is the
raw material upon which natural selection
works
© 2011 Pearson Education, Inc.
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
• 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
© 2014 Pearson Education, Inc.
• The percentage of recombinant offspring, the
recombination frequency, is related to the distance
between linked genes.
• The farther apart two genes are, the higher the
probability that a crossover will occur between them and,
therefore, the higher the recombination frequency
– The greater the distance between two genes, the more points
there are between them where crossing over can occur
• A genetic map based on recombination frequencies is
called a linkage map
© 2014 Pearson Education, Inc.
Linkage Maps
Figure 15.12
Figure 15.11
Recombination frequencies from fruit fly crosses were used to map
the relative positions of the body color (b), wing size (vg), and eye
color (cn) genes along chromosomes.
•
By combining linkage maps with other methods like chromosomal
banding, geneticists can develop cytogenetic maps of chromosomes.
– These maps indicate the positions of genes with respect to
chromosomal features.
– Recent techniques show the physical distances between gene loci
in DNA nucleotides.
Alterations of chromosome number or
structure cause some genetic disorders
• Small-scale random mutations are the source of all new
alleles and lead to new phenotypic traits.
• Physical and chemical disturbances can also damage
chromosomes in major ways.
– Errors during meiosis can alter the number of
chromosomes in a cell.
– Plants tolerate genetic defects to a greater extent than
do animals.
• Large-scale chromosomal alterations often lead to
spontaneous abortions (miscarriages) or cause a variety of
developmental disorders
© 2014 Pearson Education, Inc.
Errors during meiosis can alter the
number of chromosomes in a cell
• Nondisjunction occurs when
problems with the meiotic
spindle cause errors in
daughter cells.
a) Nondisjunction may occur if
tetrad chromosomes do not
separate properly during
meiosis I.
b) Alternatively, sister chromatids
may fail to separate during
meiosis II.
• As a consequence of
nondisjunction, one gamete
receives two of the same type
of chromosome, and another
gamete receives no copy.
© 2014 Pearson Education, Inc.
Figure 15.13
Offspring produced by nondisjunction
• Offspring resulting from the fertilization of a normal
gamete with one produced by nondisjunction have an
abnormal chromosome number, a condition known as
aneuploidy.
• Trisomic cells have three copies of a particular
chromosome and have 2n + 1 total chromosomes.
– Down Syndrome is caused by trisomy
• Monosomic cells have only one copy of a particular
chromosome and have 2n − 1 chromosomes.
• If the organism survives, aneuploidy typically leads to a
distinct phenotype.
• Tetrasomic cells have two copies of a particular
chromosome and have 2n + 2 total chromosomes.
• Nullisomicsomic cells have no copies of a particular
chromosome and have 2n -2 total chromosomes.
© 2014 Pearson Education, Inc.
Polyploidy
• Polyploid organisms have more than two complete sets of
chromosomes in all somatic cells.
– Triploidy-3 sets (3n) of chromosomes
– Tetraploidy-4 sets (4n) of chromosomes
Tetraploid mammals of burrowing rodent has twice chromosome as
those of closely related species.
• Polyploidy is relatively common among plants and much less
common among animals, although it is known to occur in fishes and
amphibians.
• The spontaneous origin of polyploid individuals plays an important
role in the evolution of plants.
• Many crop plants are polyploid. For example, bananas are
triploid (3n) and wheat is hexaploid (6n).
• Polyploids are more nearly normal in phenotype than aneuploids.
– One extra or missing chromosome apparently upsets the genetic
balance during development more than does an entire extra set
of chromosomes.
Figure 15.14
Alterations of Chromosome Structure
(a) Deletion
A B C
D E
F G
H
A deletion removes a chromosomal segment.
A B C
E
F G H
(b) Duplication
A B C
D E
F G
H
A duplication repeats a segment.
A B C
B C
D E
F G H
(c) Inversion
A B C
D E
F G H
An inversion reverses a segment within a
chromosome.
A D C
B E
F G H
(d) Translocation
A B C
D E
F G H
M N O
P Q
R
A translocation moves a segment from one
chromosome to a nonhomologous chromosome.
M N O
C D E
F G H
A
B P Q
R
Breakage of a chromosome can lead to four types of changes in chromosome structure.
Human Disorders Due to Chromosomal
Alterations
• Alterations of chromosome number and structure
are associated with some serious disorders
– Most result in miscarriage of the fetus
• 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
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Down Syndrome -Trisomy 21
• Down syndrome is an aneuploid condition that results
from three copies of chromosome 21
– Although chromosome 21 is the smallest human
chromosome, trisomy 21 severely alters an individual’s
phenotype in specific ways.
• Most cases of Down syndrome result from nondisjunction
during gamete production in one parent.
• The frequency of Down syndrome increases with the age
of the mother, a correlation that has not been explained.
• Trisomy 21 may be linked to some age-dependent
abnormality in a meiosis I checkpoint that normally delays
anaphase until all the kinetochores are attached to the
spindle.
• It affects about one out of every 700 children born in the
United States
© 2014 Pearson Education, Inc.
Figure 15.15
Down’s Syndrome
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
–
–
These individuals have male sex organs but abnormally small
testes and are sterile.
Although the extra X is inactivated, some breast enlargement and
other female characteristics are common.
• Monosomy X, called Turner syndrome, produces X0
females, who are sterile; it is the only known viable
monosomy in humans
–
–
X0 individuals are phenotypically female but are sterile because
their sex organs do not mature.
When given estrogen replacement therapy, girls with Turner
syndrome develop secondary sex characteristics.
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Chromosomal Translocation and CML
Figure 15.16
• Chromosomal translocations have been implicated in certain
cancers, including chronic myelogenous leukemia (CML).
• CML occurs when a large fragment of chromosome 22 switches
places with a small fragment from the tip of chromosome 9.
• The resulting short, easily recognized chromosome 22 is called the
Philadelphia chromosome.
© 2014 Pearson Education, Inc.
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
• In both cases, the sex of the parent contributing
the allele is a factor in the pattern of inheritance
– The genes involved are not necessarily sex-linked and
may or may not lie on the X chromosome
© 2014 Pearson Education, Inc.
Genomic Imprinting
• Variation in phenotype depending on whether an allele is inherited
from the male or female parent is called genomic imprinting.
• Genomic imprinting occurs during the formation of gametes and
results in the silencing of the imprinted genes.
– Because different genes are imprinted in sperm and ova, some genes in
a zygote are maternally imprinted and others are paternally imprinted.
– For a maternally imprinted gene, only the paternal allele is expressed.
– For a paternally imprinted gene, only the maternal allele is expressed.
• The maternal and paternal imprints are transmitted to all body cells
during development.
• Although only a few genes are imprinted most of these genes are
critical for embryonic development.
• The gene for insulin-like growth factor 2 (Igf2) was one of the first
imprinted genes to be identified.
– Although the growth factor is required for normal prenatal growth, only
the paternal allele is expressed.
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Only the paternal Igf2 allele is expressed
• Evidence that the Igf2 allele is
imprinted initially came from
crosses between wild-type
mice and dwarf mice
homozygous for a recessive
mutation in the Igf2 gene.
• The phenotypes of
heterozygous offspring differ,
depending on whether the
mutant allele comes from the
mother or the father.
• The Igf2 allele is imprinted in
eggs, turning off expression of
the imprinted allele.
• In sperm, the Igf2 allele is not
imprinted and functions
normally.
© 2014 Pearson Education, Inc.
Figure 15.17
• It appears that imprinting is the result of the
methylation (addition of –CH3) of cysteine
nucleotides
• Genomic imprinting is thought to affect only
a small fraction of mammalian genes
• Most imprinted genes are critical for
embryonic development
© 2014 Pearson Education, Inc.
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
– These organelles reproduce themselves
– Extranuclear genes are inherited maternally because the zygote’s
cytoplasm comes from the egg
• Because a zygote inherits all its mitochondria from the
ovum, all mitochondrial genes in most animals and plants
demonstrate maternal inheritance.
– The products of mitochondrial genes make up the protein
complexes of the electron transport chain and ATP synthase.
– 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
© 2014 Pearson Education, Inc.
Some rare human disorders are
produced by mutations to mitochondrial
DNA
• Tissues that require large energy supplies (the nervous
system and muscles) may suffer energy deprivation from
these defects.
– For example, a person with mitochondrial myopathy
suffers weakness, intolerance of exercise, and
muscle deterioration.
– Another mitochondrial disorder is Leber’s hereditary
optic neuropathy, which can produce sudden
blindness in young adults.
• Other mitochondrial mutations may contribute to
diabetes, heart disease, and other diseases of aging,
such as Alzheimer’s disease.
• Over a lifetime, new mutations gradually accumulate
in mitochondrial DNA and contribute to the aging
process.
© 2014 Pearson Education, Inc.
Figure 15.18
The first evidence of extranuclear genes came from
studies on the inheritance of yellow or white patches
on leaves of an otherwise green plant