Download X - My Teacher Site

Chapter 15
The Chromosomal Basis of
Inheritance
AP Biology
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
Concept 15.1:
Mendelian inheritance has its
physical basis in the behavior of
chromosomes
The Chromosome Theory of Inheritance
•
Mitosis and meiosis were first described in the late 1800s
–
Made possible by improved techniques in microscopy
–
Brought to light parallels between chromosome behavior and the
behavior of Mendel’s proposed hereditary factors
•
The chromosome theory of inheritance states:
–
Mendelian genes have specific loci (positions) on chromosomes
–
It is chromosomes that undergo segregation and independent
assortment
Behavior of Chromosomes and Mendel’s Laws
•
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
–
Behavior of homologous
Y
R
r

R
Green-wrinkled
seeds ( yyrr)
y
y
r
Meiosis
chromosomes during meiosis
Fertilization
can account for segregation
R
of alleles at each genetic locus
R
y
r
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
Y
r
R
R
y
Y
y
1
R
r
Y
y
r
R
Y
y
Anaphase I
for independent assortment
R
r
Y
y
Metaphase II
r
R
Y
y
2
2
Y
Y
Gametes
R
located on different
1/
4 YR
F2 Generation
chromosomes
r
Metaphase I
1
chromosomes can account
of alleles for 2 or more genes
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
Y
Behavior of nonhomologous
y
r
Y
–
r
All F 1 plants produce
yellow-round seeds (YyRr)
F1 Generation
to different gametes
y
R Y
Gametes
R
y
r
r
1/
4
Y
Y
y
r
r
1/
4 Yr
yr
y
y
R
R
1/
4 yR
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
–
Several characteristics make fruit flies a convenient organism for genetic
studies:
•
They breed at a high rate
–
A single mating produces 100s of offspring
•
A generation can be bred every two weeks
•
They have only four pairs of chromosomes
–
Include 3 pairs of autosomes and one pair of sex chromosomes
•
Females: XX
•
Males: XY
Wild Type vs. Mutant Varieties
•
Morgan faced the tedious task of carrying out many matings and then
microscopically searching for naturally occurring variant offspring
–
Morgan noted wild type, or normal, phenotypes that were common in the fly
populations
•
–
Traits alternative to the wild type are called mutant phenotypes
•
–
Ex) Red eyes in Drosophila
Due to alleles assumed to have originated as mutations in the wild-type
allele
Morgan invented a notation for
symbolizing alleles in Drosophila
•
Gene takes its symbol
first mutant discovered
–
•
w = allele for white eyes
A superscript + identifies the allele
for the wild-type trait
–
w+ = allele for red eyes
from the
Morgan’s Experiments
•
In one experiment, Morgan mated male flies with white eyes (mutant) with female
Fig. 15-4
flies with red eyes (wild type)
EXPERIMENT
–
The F1 generation all had red eyes
•
–
–
Suggested that the wild-type allele
dominant
The F2 generation showed the 3:1
red:white eye ratio
•
All females had red eyes
•
Half of the males had red eyes and
half had white eyes
Morgan determined that the white-eyed
mutant allele is located on X chromosome
•
•
A single copy of the mutant allele conferred
white eyes in males since there was no
wild-type allele to offset the recessive allele
A female could have white eyes only
both of her X chromosomes carried the
reccessive mutant allele
P
Generation

isAll offspring
F1
Generation
had red eyes
RESULTS
F2
Generation
CONCLUSION
P
Generation
w+
X
X

w+
X
Y
w
Eggs
F1
Generation
w+
Sperm
w+
w+
w
w+
Eggs
F2
Generation
w
w+
w
Sperm
w+
w+
w+
w
w
w+
if
Correlating Behavior of a Gene’s Alleles with Behavior
of a Chromosome Pair
• Morgan’s finding supported the chromosome theory of
inheritance
– Proved that a specific gene is carried on a specific
chromosome (eye color gene is located on the X
chromosome)
– Also indicated that genes located on sex
chromosomes exhibit unique inheritance patterns
Concept Check 15.1
• 1) Which one of Mendel’s laws relates to the
inheritance of alleles for a single character? Which law
relates to the inheritance of alleles for two characters
in a dihybrid cross?
• 2) What is the physical basis of Mendel’s laws?
• 3) Propose a possible reason that the first naturally
occurring mutant fruit fly Morgan saw involved a gene
on a sex chromosome.
Concept 15.2:
Sex-linked genes exhibit unique
patterns of inheritance
The Sex Chromosomes
•
In humans and some other animals, there is a chromosomal basis of sex
determination
–
Fig. 15-5
There are two varieties of sex chromosomes
• A larger X chromosome (XX = female)
• A smaller Y chromosome (XY = male)
–
X
Only the ends of the Y chromosome have
Y
regions that are homologous with the
X chromosome
• These regions allow X and Y
chromosomes in males to pair and
behave like homologous chromosomes during meiosis
Segregation of Sex Chromosomes
•
The 2 sex chromosomes segregate during meiosis in both testes and ovaries
–
Each gamete contains one sex chromosome
•
•
In females, each egg contains one X chromosome
In males, half the sperm contain an X
Fig. 15-6a
44 +
XY
chromosome and half contain a
44 +
XX
Parents
Y chromosome
–
Sex is determined based on which
of these sperm fertilizes the egg cell
– There is a 50/50 chance of either
22 +
22 +
or Y
X
Sperm
+
44 +
XX
or
22 +
X
Egg
44 +
XY
Zygotes (offspring)
(a) The X-Y system
sperm fertilizing an egg
•
In humans, the anatomical signs of sex begin to emerge in 2 month old embryos
–
The SRY gene on the Y chromosome codes for the development of testes
–
In the absence of SRY, the gonads develop into ovaries
Sex Determination in Other Animals
•
Other animals have different methods of sex determination
–
X-O system: there is only one type of sex chromosome (X)
•
•
–
22 +
22 +
or Y
X
Sperm
Egg
or
44 +
XY
Zygotes
(offspring)
Characteristic of grasshoppers, cockroaches, and some other
insects
(a) The X-Y system
Sex is determined by presence or absence of a second X
chromosome in sperm
22 +
–
Females: XX
–
Males: XO
Characteristic of birds and
some fish and insects
XX
22 +
X
76 +
ZW
76 +
ZZ
32
(Diploid)
16
(Haploid)
(b) The X-0 system
Z-W system: sex chromosome
present in egg determines gender
•
44 +
XX
22 +
X
+
(c) The Z-W system
– Females: ZW
–
–
Males: ZZ
Haplo-diploid system: no sex chromosomes
•
Characteristic of most bees and ants
(d) The haplo-diploid system
–
Females (diploid): develop from
fertilized egg
–
Males (haploid): develop from unfertilized egg
Inheritance of Sex-Linked Genes
•
The sex chromosomes also 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 (homozygous) of the allele
• A male needs only one copy (hemizygous) of the allele
Inheritance of Sex-Linked Genes: Males vs. Females
•
Sex-linked recessive disorders are much more common in males than in
females
Fig. 15-7
–
Ex) The chance of a female inheriting two recessive alleles is much
less than the probability of a male inheriting a single recessive allele
XNXN
Sperm Xn

XnY
(a)
Sperm XN
Y
Eggs XN XNXn XNY
XN
XNXn

XNY
Xn
(b)
Sperm Xn
Y
Eggs XN XNXN XNY
XNXn XNY
XNXn

XnY
Y
Eggs XN XNXn XNY
XnXN XnY
Xn
(c)
XnXn XnY
X-Linked Recessive Disorders
•
Some disorders caused by recessive alleles on the X chromosome in humans:
–
Color blindness
–
Duchenne muscular dystrophy: disease characterized by progressive
weakening of muscles and loss of coordination
•
Results from absence of a key muscle protein called dystrophin
•
Affects 1/3500 males born in the US
–
–
Affected individuals rarely live past their early 20s
Hemophilia: caused by absence of one or more proteins required for blood
clotting
•
Bleeding is therefore prolonged since a firm clot is slow to form
•
Affected individuals can be treated with intravenous injections of the
missing protein
X Inactivation in Female Mammals
• In mammalian females, one of the two X chromosomes in each cell is
randomly inactivated during embryonic development
–
Cells of males and females thus have the same effective dose
(one copy) of each gene
–
The inactive X condenses into a compact object called a Barr
body
• Most of the genes on this chromosome are not expressed
• Barr body chromosomes in the ovaries are reactivated in
cells that give rise to eggs so that each female gamete has
an active X
X Inactivation in Female Mammals: Tortoiseshell Cats
•
Females consist of a “mosaic” of two cell types as a result of random and
independent X inactivation of each embryonic cell present
•
–
About half contain cells with an active X derived from father
–
The remaining cells contain an active X derived from mother
A female heterozygous for a particular gene on the X chromosome will therefore be
Fig. 15-8
X chromosomes
a mosaic for that character
Early embryo:
–
Allele for
black fur
Ex) Results in mottled coloration of
tortoiseshell cats
•
Orange patches formed in regions
where orange alleles are active
Two cell
populations
in adult cat:
Active X
Cell division and
X chromosome
inactivation
Black patches are found in areas
where black allele is active
•
Only (heterozygous) females can be tortoiseshell
Active X
Inactive X
Black fur
•
Allele for
orange fur
Orange fur
Concept Check 15.2
•
1) A white-eyed female Drosophila is mated with a red-eyed (wild-type)
male, the reciprocal cross of the one shown in Figure 15.4 (pp. 289). What
phenotypes and genotypes do you predict for the offspring?
•
2) Neither Tim nor Rhoda has Duchenne muscular dystrophy, but their
firstborn son does have it. What is the probability that a second child of this
couple will have the disease? What is the probability if the second child is a
boy? A girl?
•
3) During early embryonic development of female carriers for color
blindness, the normal allele is inactivated by chance in about half of the
cells. Why, then, aren’t 50% of female carriers color-blind?
Concept 15.3:
Linked genes tend to be inherited
together because they are located
near each other on the same
chromosome
Linked Genes
• Each chromosome has hundreds or thousands of
genes
– Genes located on the same chromosome that tend
to be inherited together are called linked genes
– These linked genes do not assort independently
and therefore deviate from the results of typical
breeding experiments
How Linkage Affects Inheritance
•
Morgan also performed 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
•
Wild-type flies have gray bodies and normal-sized wings
•
Mutant flies have black bodies and much smaller wings called vestigial
wings
–
Mutant alleles are recessive to
the wild-type alleles
–
Fig. 15-UN1
Parents
in testcross
b vg
b+ vg+

b vg
b vg
b+ vg+
b vg
Neither gene is located on
a sex chromosome
Most
offspring
or
b vg
b vg
Morgan’s Observations
•
Morgan observed a much higher proportion of parental phenotypes that would be
expected due to independent assortment
–
He therefore concluded that body color and wing size are usually inherited
together in specific combinations
•
These combination were generally those combinations seen in the parental
generation
–
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
Genetic Recombination
•
However, nonparental phenotypes were also produced
–
This suggested that body color and wing size genes are only partially linked
genetically
•
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
–
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
Fig. 15-UN2
called recombinant types, or recombinants
–
When 50% of all offspring are recombinants,
Gametes from yellow-round
heterozygous parent (YyRr)
there is said to be a 50% frequency of
YR
recombination
•
A 50% frequency
recombination
Gametes from greenwrinkled homozygous
recessive parent ( yyrr)
yr
Yr
yR
of
yr
YyRr
yyrr
Yyrr
yyRr
is observed for any two genes on
different chromosomes
Parentaltype
offspring
Recombinant
offspring
Recombination of Linked Genes: Crossing Over
•
In Morgan’s Drosophila testcross, most of the offspring had parental phenotypes for
body color and wing size
–
–
This suggested that the 2 genes were on the same chromosome
•
The occurrence of parental genotypes at a greater frequency than 50%
indicates that genes are linked
•
In this case, about 17 % of offspring
were recombinants
•
This suggested that, although genes
can be linked, the linkage can be
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
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
–
Assumed that crossing over is a random event, with the chance of
crossing over approximately equal at all points along a chromosomes
–
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
• The greater the distance between 2 genes, the more points there
are between them where crossing over can occur
Linkage Maps
•
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
Drosophila data:
–
Involved a map of 3 genes: eye
color (cn), body color (b),
and wing size (vg)
•
9% recombination frequency
for cn and b
•
9.5% between cn and vg
•
17% between b and vg
•
b-vg recombination frequency (17%) is slightly less than the sum of b-cn
and cn-vg frequencies (9+9.5=18.5%)
–
This occurs because of the few times that a crossover occurs between
both b-cn and cn-vg
–
These crossovers cancel each other out
Physically Linked Genes That Are Genetically Unlinked
•
Some genes on a chromosome are so far apart that a crossover between them is
virtually certain
–
Observed frequency of recombination in crosses such as these can have a
maximum value of 50%
•
This result is indistinguishable from that for genes on different
chromosomes
•
In such a case, the physical connection between genes on the same
chromosome is not reflected
– Such genes are physically linked, but genetically unlinked, and
behave (assort independently) as if found on different chromosomes
–
These genes are mapped by adding the recombination frequencies
from crosses involving a set of closer pairs of genes lying between the
two distant genes
Linkage Maps of Fruit Fly Genes
•
Sturtevant used recombination frequencies to make linkage maps of fruit fly genes
Fig. 15-12
Mutant phenotypes
–
Found that these genes
Short
aristae
Black
body
Cinnabar Vestigial
eyes
wings
Brown
eyes
clustered into 4 groups
of linked genes
–
0
48.5
57.5
67.0
104.5
Correlated with the
observation that there are 4 pairs
of chromosomes in Drosophila
•
Other methods allow geneticists to
Long aristae
(appendages
on head)
Gray
body
Red
eyes
Normal
wings
Red
eyes
Wild-type phenotypes
construct cytogenetic maps
–
These maps locate genes with respect to chromosomal features (ex: stained
bands)
Concept Check 15.3
•
1) When two genes are located on the same chromosome, what is the
physical basis for the production of recombinant offspring in a testcross
between a dihybrid parent and a double-mutant (recessive) parent?
•
2) For each type of offspring of the testcross in Figure 15.9 (pp. 293),
explain the relationship between its phenotype and the alleles contributed by
the female parent.
•
3) Genes A,B, and C are located on the same chromosome. Testcrosses
show that the recombination frequency between A and B is 28% and
between A and C is 12%. Can you determine the linear order of these
genes? Explain.
Concept 15.4:
Alterations of chromosome number
or structure cause some genetic
disorders
Large-Scale Chromosomal Alterations
• Large-scale chromosomal alterations can affect an
organism’s phenotype
– Can occur as a result of physical and chemical
disturbances, as well as due to errors in meiosis
– 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
–
Can also occur if sister chromatids fail to separate during meiosis II
•
As a result, one gamete receives two of the same type of
chromosome, and another gamete receives no copy
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
•
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
–
Results when fertilization involves a gamete that has no copy of a
particular chromosome, leading to a missing (2n-1) chromosome in
the zygote
•
A trisomic zygote has three copies of a particular chromosome
–
Cell has 2n+1 chromosomes
– These abnormal doses of genes can result in conditions like Down
syndrome (trisomy 21)
Polyploidy
•
Polyploidy is a condition in which an organism has more than two complete sets of
chromosomes
–
Triploidy (3n) is three sets of chromosomes
•
–
Can occur by fertilization of an abnormal diploid egg produced by
nondisjunction of all its chromosomes
Tetraploidy (4n) is four sets of chromosomes
•
Can result from failure of a 2n zygote to divide after replicating its
chromosomes in preparation for mitosis
–
•
•
Subsequent normal mitotic divisions would then produce a 4n embryo
Polyploidy is common in plants, but not animals
–
Bananas are triploid and wheat in
hexaploid (6n)
–
Known to occur among fishes, amphibians,
and a mammalian species of rodent
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
Deletion and duplications are especially
likely to occur during meiosis
–
Nonsister chromatids sometimes
exchange unequal-sized segments of
DNA during crossing over
•
The products of such
nonreciprocal crossovers are one chromosome with a deletion and the
other chromosome with a duplication
Human Disorders Due to Chromosomal Alterations
• Alterations of chromosome number and structure are associated with
some serious disorders
–
The effects of aneuploidy are generally so disastrous to
development that embryos are spontaneously aborted before
birth
–
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 (trisomy 21)
–
Causes characteristic facial features, short stature, heart defects, susceptibility
to respiratory infection, and mental retardation
•
Affected individuals tend to have a shorter average life span, though some
live to middle age and beyond
•
Most are also sexually underdeveloped and sterile
Fig. 15-16
–
Affects about one out of every 700 children born in
the US
–
The frequency of Down syndrome increases with
the age of the mother, a correlation that has not
been explained
•
0.04% for children born to mothers under 30
•
More than 0.92% for children born to mothers
over 40
Aneuploidy of Sex Chromosomes
•
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
–
Appear to cause less upset to genetic balance than aneuploid conditions
involving autosomes
•
•
Y chromosome carries relatively few genes
•
Extra copies of X chromosome become inactivated as Barr bodies
Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY
individuals
–
Affected individuals have male sex organs but abnormally small testes,
resulting in sterility
•
Monosomy X, called Turner syndrome, produces X0 females
–
Affected females are phenotypically normal but sterile
–
This is the only known viable monosomy in humans
Disorders Caused by Structurally Altered Chromosomes
•
Many deletions in human chromosomes cause severe problems
–
The syndrome cri du chat (“cry of the cat”), results from a specific deletion in
chromosome 5
–
Affected children are mentally retarded, with a small head and unusual facial
features, and have a catlike cry
•
•
Individuals usually die in infancy or early childhood
Certain cancers, including chronic myelogenous leukemia (CML), are caused by
translocations of chromosomes
–
OccursFig.when
a reciprocal translocation happens during mitosis of cells
15-17
destined to become WBCs
•
Exchange of a large portion of chromosome 22 with a small fragment from
the tip of chromosome 9 produces a short, easily recognizable
chromosome 22, called the Philadelphia chromosome
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Concept Check 15.4
•
1) More common than completely polyploid animals are mosaic polyploids,
animals that are diploid except for patches of polyploid cells. How might a
mosaic tetraploid – an animal with some cells containing four sets of
chromosomes – arise?
•
2) About 5% of individuals with Down syndrome have a chromosomal
translocation in which a third copy of chromosome 21 is attached to
chromosome 14. If this translocation occurred in a parent’s gonad, how
could it lead to Down syndrome in a child?
•
3) The ABO blood type locus has been mapped on chromosome 9. A father
who has type AB blood and a mother who has type O blood have a child
with trisomy 9 and type A blood. Using this information, can you tell in which
parent the nondisjunction occurred? Explain your answer.
Concept 15.5:
Some inheritance patterns are
exceptions to the standard
chromosome theory
Exceptions to the Standard Chromosome Theory
• There are two normal exceptions to Mendelian genetics
– One exception involves genes located in the nucleus
– The other exception involves genes located outside the
nucleus
• In both of these cases, the sex of the parent
contributing an allele is a factor in the pattern of
inheritance
Genomic Imprinting
•
For a few mammalian traits, the phenotype depends on which parent passed along
the alleles for those traits
Fig. 15-18
–
Such variation in phenotype is called
genomic imprinting
–
Paternal
chromosome
Maternal
chromosome
Involves the silencing of
Normal Igf2 allele
is not expressed
certain genes that are stamped” with
an imprint during gamete production
•
As a result, only either the male or
Normal Igf2 allele
is expressed
Wild-type mouse
(normal size)
(a) Homozygote
Mutant Igf2 allele
inherited from mother
Mutant Igf2 allele
inherited from father
female allele is expressed in every
cell of an organism
–
Ex) In the mouse gene for
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
insulin-like growth factor 2
(Igf2), only the paternal
allele is expressed
(b) Heterozygotes
Methylation and Genomic Imprinting
•
It appears that imprinting is the result of the methylation (addition of –CH3) of
DNA
–
May directly silence genes
• Ex) Heavily methylated genes are usually inactive
–
May activate the expression of an allele
• Ex) Methylation results in paternal expression of Igf2 allele
•
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
•
Not every eukaryotic gene is located on nuclear chromosomes
–
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 can reproduce independently and thus transmit their
genes to daughter organelles
–
Distribution of these genes to offspring does not follow the same rules
or Mendelian inheritance patterns
•
Extranuclear genes are inherited maternally because the zygote’s
cytoplasm comes from the egg
Extranuclear Genes in Plants
•
The first evidence of extranuclear genes came from studies on the
inheritance of yellow or white patches on leaves of an otherwise green plant
–
These coloration patterns are due to mutations in plastid genes that
control pigmentation
• Coloration of offspring is determined by maternal parent
– Pattern depends on ratio of wild-type
to mutant plastids inherited from
the mother
Mitochondrial Genes of Plants and Animals
•
Similar maternal inheritance is also the rule for mitochondrial genes in most animals
and plants
–
Products of most mitochondrial genes help make up protein complexes of ETC
and ATP synthase
•
Defects in one or more of these proteins reduce the amount of ATP the cell
can make
•
Results in diseases that affect the muscular and nervous systems
–
Ex) Mitochondrial myopathy causes weakness, intolerance of
exercise, and muscle deterioration
– Ex) Leber’s hereditary optic neuropathy can produce sudden
blindness
Concept Check 15.5
•
1) Gene dosage, the number of active copies of a gene, is important to
proper development. Identify and describe two processes that establish the
proper dosage of certain genes.
•
2) Reciprocal crosses between two primrose varieties, A and B, produced
the following results:
A female X B male = offspring with all green (nonvariegated) leaves
B female X A male = offspring with spotted (variegated) leaves.
Explain these results.
•
3) Mitochondrial genes are critical to the energy metabolism of cells, but
mitochondrial disorders caused by mutations in these genes are generally
not lethal. Why not?
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings