ch 15: the chromosomal basis of inheritance
TRANSCRIPT
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LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures byErin Barley
Kathleen Fitzpatrick
The Chromosomal Basis of Inheritance
Chapter 15
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Overview: Locating Genes Along Chromosomes
• Mendel’s “hereditary factors” were genes• 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
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Figure 15.1
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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 can account for Mendel’s laws of segregation and independent assortment
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Figure 15.2P Generation
F1 Generation
Yellow-roundseeds (YYRR)
Green-wrinkledseeds (yyrr)
×
Meiosis
Fertilization
Gametes
Y
YR R
YR
y
yr
y r
All F1 plants produceyellow-round seeds (YyRr).
Meiosis
Metaphase I
Anaphase I
Metaphase II
R R
R R
R R
R R
R R R R
r r
r r
r r
r r
r r r r
Y Y
Y Y
Y Y
Y Y
Y Y Y Y
y y
y y
y y
y y
yy y y
Gametes
LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.
LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologous chromosomesassort independently duringgamete formation.
1
2 2
1
1/41/4
1/41/4YR yr Yr yR
F2 Generation
3 3Fertilization recombinesthe R and r alleles at random.
Fertilization results in the 9:3:3:1 phenotypic ratioin the F2 generation.
An F1 × F1 cross-fertilization
9 : 3 : 3 : 1
r
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Figure 15.2a
P Generation Yellow-roundseeds (YYRR)
Green-wrinkledseeds (yyrr)
×
Meiosis
Fertilization
Gametes
Y
YR R
YR
y
yr
y r
r
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Figure 15.2b
F1 Generation
All F1 plants produceyellow-round seeds (YyRr).
Meiosis
Metaphase I
Anaphase I
Metaphase II
R R
R R
R R
R R
R R R R
r r
r r
r r
r r
r r r r
Y Y
Y Y
Y Y
Y Y
Y Y Y Y
y y
y y
y y
y y
yy y y
Gametes
LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.
LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
1
2 2
1
1/41/4
1/41/4YR yr Yr yR
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Figure 15.2c
F2 Generation
3Fertilization recombines the R and r alleles at random.
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.
An F1 × F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF SEGREGATION LAW OF INDEPENDENTASSORTMENT
3
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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 produce many offspring
– A generation can be bred every two weeks
– They have only four pairs of chromosomes
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• Morgan noted wild type, or normal, phenotypes that were common in the fly populations
• Traits alternative to the wild type are called mutant phenotypes
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Figure 15.3
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Figure 15.3a
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Figure 15.3b
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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
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Figure 15.4
All offspringhad red eyes.
PGeneration
F1
Generation
F2
Generation
F2
Generation
F1
Generation
PGeneration
Eggs
Eggs
Sperm
Sperm
XX
XY
w+
w+
w+w+ w+
w+
w+w+ w+
w+
w+
w
w
w
ww w
RESULTS
EXPERIMENT
CONCLUSION
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Figure 15.4a
All offspringhad red eyes.
PGeneration
F1
Generation
F2
Generation
RESULTS
EXPERIMENT
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Figure 15.4b
F2
Generation
PGeneration
Eggs
Eggs
Sperm
Sperm
Xw+
CONCLUSION
XX Y
w+
w+w+ w+
w+
w+w+ w+
w+
w+
w
w
w
ww w
F1
Generation
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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 X chromosome and a smaller Y chromosome
• Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome
• The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features
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Figure 15.5
X
Y
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• 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
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Figure 15.6
Parents
orSperm
or
Egg
Zygotes (offspring)
44 +XY
44 +XX
22 +X
22 +Y
22 +X
44 +XX
44 +XY
22 +XX
22 +X
76 +ZW
76 +ZZ
32 (Diploid)
16 (Haploid)
(a) The X-Y system
(b) The X-0 system
(c) The Z-W system
(d) The haplo-diploid system
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Figure 15.6a
Parents
or
Sperm
or
Egg
Zygotes (offspring)
44 +XY
44 +XX
22 +X
22 +Y
22 +X
44 +XX
44 +XY
22 +XX
22 +X
(a) The X-Y system
(b) The X-0 system
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Figure 15.6b
76 +ZW
76 +ZZ
32 (Diploid)
16 (Haploid)
(c) The Z-W system
(d) The haplo-diploid system
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• A gene that is 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
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Inheritance of X-Linked Genes
• X chromosomes have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination
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• X-linked genes follow specific patterns of inheritance
• For a recessive X-linked trait to be expressed– A female needs two copies of the allele
(homozygous)– A male needs only one copy of the allele
(hemizygous)
• X-linked recessive disorders are much more common in males than in females
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Figure 15.7
Eggs Eggs Eggs
Sperm Sperm Sperm
(a) (b) (c)
XNXN XnY XNXn XNY XNXn XnY
Xn Y XN Y YXn
Xn Xn
XN
XN
XN XNXNXn XNY
XNY
XNY XNY
XnY XnYXNXn XNXn
XNXnXNXN
XnXn
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• Some disorders caused by recessive alleles on the X chromosome in humans
– Color blindness (mostly X-linked)
– Duchenne muscular dystrophy
– Hemophilia
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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
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Figure 15.8
Early embryo:
X chromosomesAllele fororange fur
Allele forblack fur
Two cellpopulationsin adult cat:
Cell division andX chromosomeinactivation
Active XInactive X
Active X
Black fur Orange fur
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Figure 15.8a
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• Each chromosome has hundreds or thousands of genes (except the Y chromosome)
• 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
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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
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Figure 15.9-1P Generation (homozygous)
Wild type(gray body, normal wings)
b+ b+ vg+ vg+ b b vg vg
Double mutant(black body,vestigial wings)
EXPERIMENT
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Figure 15.9-2P Generation (homozygous)
Wild type(gray body, normal wings)
F1 dihybrid(wild type)
TESTCROSS
b+ b+ vg+ vg+
b+ b vg+ vg
b b vg vg
b b vg vg
Double mutant(black body,vestigial wings)
Double mutant
EXPERIMENT
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Figure 15.9-3P Generation (homozygous)
Wild type(gray body, normal wings)
F1 dihybrid(wild type)
Testcrossoffspring
TESTCROSS
b+ b+ vg+ vg+
b+ b vg+ vg
b b vg vg
b b vg vg
Double mutant(black body,vestigial wings)
Double mutant
Eggs
Sperm
EXPERIMENT
Wild type(gray-normal)
Black-vestigial
Gray-vestigial
Black-normal
b+ vg+ b vg b+ vg b vg+
b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg
b vg
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Figure 15.9-4P Generation (homozygous)
Wild type(gray body, normal wings)
F1 dihybrid(wild type)
Testcrossoffspring
TESTCROSS
b+ b+ vg+ vg+
b+ b vg+ vg
b b vg vg
b b vg vg
Double mutant(black body,vestigial wings)
Double mutant
Eggs
Sperm
EXPERIMENT
RESULTS
PREDICTED RATIOS
Wild type(gray-normal)
Black-vestigial
Gray-vestigial
Black-normal
b+ vg+ b vg b+ vg b vg+
b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg
965 944 206 185
1
1
1
1
1
0
1
0
If genes are located on different chromosomes:
If genes are located on the same chromosome andparental alleles are always inherited together:
:
:
:
:
:
:
:
:
:
b vg
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• 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
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Figure 15.UN01
Most offspring
F1 dihybrid femaleand homozygousrecessive malein testcross
or
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
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• 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
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Genetic Recombination and Linkage
• 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
• A 50% frequency of recombination is observed for any two genes on different chromosomes
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Figure 15.UN02
Gametes from green-wrinkled homozygousrecessive parent (yyrr)
Gametes from yellow-rounddihybrid parent (YyRr)
Recombinant offspring
Parental-type
offspring
YR yr Yr yR
yr
YyRr yyrr Yyrr yyRr
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Recombination of Linked Genes: Crossing Over
• Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed
• He proposed that some process must occasionally break the physical connection between genes on the same chromosome
• That mechanism was the crossing over of homologous chromosomes
© 2011 Pearson Education, Inc.
Animation: Crossing Over
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Figure 15.10Testcrossparents
Replicationof chromosomes
Gray body, normal wings(F1 dihybrid)
Black body, vestigial wings(double mutant)
Replicationof chromosomes
Meiosis I
Meiosis II
Meiosis I and II
Recombinantchromosomes
Eggs
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 vgb vg
b vg
b+vg+ b+ vgb vg b vg+
Testcrossoffspring
965Wild type
(gray-normal)
944Black-
vestigial
206Gray-
vestigial
185Black-normal
Sperm
Parental-type offspring Recombinant offspring
Recombinationfrequency
391 recombinants2,300 total offspring
× 100 = 17%=
b+ vg+ b vg+b+ vgb vg
b vg b vg b vg b vg
b vg
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Figure 15.10a
Testcrossparents
Replicationof chromosomes
Gray body, normal wings(F1 dihybrid)
Black body, vestigial wings(double mutant)
Replicationof chromosomes
Meiosis I
Meiosis II
Meiosis I and II
Recombinantchromosomes
Eggs
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 vgb vg
b vg
b+vg+ b+ vgb vg
b vg+
Spermb vg
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Figure 15.10b
Testcrossoffspring
965Wild type
(gray-normal)
944Black-
vestigial
206Gray-
vestigial
185Black-normal
Sperm
Parental-type offspring Recombinant offspring
Recombinationfrequency
391 recombinants2,300 total offspring
× 100 = 17%=
b+ vg+ b vg+b+ vgb vg
b vg b vg b vg b vg
b vg
Eggs
Recombinantchromosomes
b+vg+ b vg b+ vg b vg+
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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
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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
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• 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
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Figure 15.11
Chromosome
Recombinationfrequencies
9% 9.5%
17%
b cn vg
RESULTS
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• 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
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• 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
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Figure 15.12
Mutant phenotypes
Shortaristae
Blackbody
Cinnabareyes
Vestigialwings
Browneyes
Long aristae(appendageson head)
Gray body
Red eyes
Normalwings
Redeyes
Wild-type phenotypes
104.567.057.548.50
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Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders
• Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
• Plants tolerate such genetic changes better than animals do
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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
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Meiosis I
Nondisjunction
Figure 15.13-1
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Meiosis I
Meiosis II
Nondisjunction
Non-disjunction
Figure 15.13-2
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Meiosis I
Meiosis II
Nondisjunction
Non-disjunction
Gametes
Number of chromosomes
Nondisjunction of homo-logous chromosomes inmeiosis I
(a) Nondisjunction of sisterchromatids in meiosis II
(b)
n + 1 n − 1n + 1 n − 1 n − 1n + 1 n n
Figure 15.13-3
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• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
• Offspring with this condition have an abnormal number of a particular chromosome
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• A monosomic zygote has only one copy of a particular chromosome
• A trisomic zygote has three copies of a particular chromosome
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• 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
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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 orientation of a segment
within a chromosome
– Translocation moves a segment from one chromosome to another
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Figure 15.14(a) Deletion
(b) Duplication
(c) Inversion
(d) Translocation
A deletion removes a chromosomal segment.
A duplication repeats a segment.
An inversion reverses a segment within a chromosome.
A translocation moves a segment from onechromosome to a nonhomologous chromosome.
A B C D E F G H
A B C E F G H
A B C D E F G H
A B C D E F G H B C
A B C D E F G H
A D C B E F G H
A B C D E F G H M N O P Q R
GM N O C H FED A B P Q R
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Figure 15.14a
(a) Deletion
(b) Duplication
A deletion removes a chromosomal segment.
A duplication repeats a segment.
BA C D E F G H
A B C D E F G H
A B C D E F G H B C
A B C E F G H
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Figure 15.14b
(c) Inversion
(d) Translocation
An inversion reverses a segment within a chromosome.
A translocation moves a segment from onechromosome to a nonhomologous chromosome.
A B C D E F G H
A D C B E F G H
A B C D E F G H M N O P Q R
GM N O C H FED A B P Q R
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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
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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
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Figure 15.15
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Figure 15.15a
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Figure 15.15b
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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
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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
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Figure 15.16
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
Translocated chromosome 9
Translocated chromosome 22(Philadelphia chromosome)
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Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance
• 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 an allele is a factor in the pattern of inheritance
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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
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Figure 15.17
(a) Homozygote
Paternalchromosome
Maternalchromosome
Normal Igf2 alleleis expressed.
Normal Igf2 alleleis not expressed.
Normal-sized mouse(wild type)
Mutant Igf2 alleleinherited from mother
Mutant Igf2 alleleinherited from father
Normal-sized mouse (wild type) Dwarf mouse (mutant)
Normal Igf2 alleleis expressed.
Mutant Igf2 alleleis expressed.
Mutant Igf2 alleleis not expressed.
Normal Igf2 alleleis not expressed.
(b) Heterozygotes
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Figure 15.17a
(a) Homozygote
Paternalchromosome
Maternalchromosome
Normal Igf2 alleleis expressed.
Normal Igf2 alleleis not expressed.
Normal-sized mouse(wild type)
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Figure 15.17b
Mutant Igf2 alleleinherited from mother
Mutant Igf2 alleleinherited from father
Normal-sized mouse (wild type) Dwarf mouse (mutant)
Normal Igf2 alleleis expressed.
Mutant Igf2 alleleis expressed.
Mutant Igf2 alleleis not expressed.
Normal Igf2 alleleis not expressed.
(b) Heterozygotes
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• 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
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Inheritance of Organelle Genes
• Extranuclear genes (or cytoplasmic genes) are 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
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Figure 15.18
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• 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
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Figure 15.UN03
P generationgametes
Sperm Egg
This F1 cell has 2n = 6 chromo-somes and is heterozygous for allsix genes shown (AaBbCcDdEeFf).Red = maternal; blue = paternal.
Each chromosome hashundreds or thousandsof genes. Four (A, B, C, F) are shown on this one.
The alleles of unlinkedgenes are either onseparate chromosomes(such as d and e)or so far apart on thesame chromosome(c and f) that theyassort independently.
Genes on the same chromo-some whose alleles are soclose together that they donot assort independently(such as a, b, and c) are saidto be genetically linked.
D
CBA
F
E d
cba
f
e
D
CB
A
F
e
d
E
f
c ba
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Figure 15.UN04
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Figure 15.UN05
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Figure 15.UN06