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Chapter 24 Chromosomal Basis
of Inheritance
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24.1 Sex-Linked Inheritance
• Normally, both males and females have 23
pairs of chromosomes
– 22 pairs are called autosomes
– One pair is the sex chromosomes
• Males are XY
• Females are XX
24.1 Sex-Linked Inheritance
• Sex-linked traits are controlled by genes
on the sex chromosomes
– X-linked are found on the X chromosome
• Most sex-linked traits
• No matching gene on the Y
– Y- linked are found on the Y chromosome
24.1 Sex-Linked Inheritance
• Most X-linked traits are recessive
– Female must receive two alleles
• One from each parent
– Male inherits X from his mother
• Y from his father does not carry an allele for the
trait
24.1 Sex-Linked Inheritance
• Sex-linked Alleles
– Example for red-green colorblindness
• Well known X-linked recessive disorder
• Carrier – female capable of passing recessive allele
• Color-blind males are more common than color-blind females
Genotypes Phenotypes
XBXB Female who has normal color vision
XBXb Carrier female who has normal color vision
XbXb Female who is color blind
XBY Male who has normal vision
XbY Male who is color blind
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All
1 1
Phenotypic Ratio
Normal vision
Color blind
Key
oocytes
sp
erm
Parents
×
Y
Offspring
Females
Males
XB= Normal vision
Xb= Color blind
XBY XBXb
XB Xb
XB XBXB XBXb
XBY XbY
♀
♀
24.1 Sex-Linked Inheritance
• Pedigree for X-linked Disorders
– Most sex-linked disorders are carried on the X
chromosome
– X-linked recessive disorder
• More males than females will have the disorder
– Recessive allele on X always expressed in males
– X-linked dominant disorder
• Only a few known
• Affected males pass the trait only to daughters
• Females can pass the trait to both sons and
daughters
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XbY
K e y
grandfather
daughter
XBXB
XBXb XBY
XBY XBXB XBXb XbY
XbY
XBY XbXb
grandson
XBXB = Unaffected female
XBXb = Carrier female
XbXb = Color-blind female
XBY = Unaffected male
XbY = Color-blind male X-linked Recessive Disorders
• More males than females are affected.
• An affected son can have parents who have the
normal phenotype .
• For a female to have the characteristic, her father
must also have it. Her mother must have it or be a
carrier. • The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons
will have it.
24.1 Sex-Linked Inheritance
• X-linked Recessive Disorders of Interest
– Red-green colorblindness
– Duchenne muscular dystrophy
• Characterized by a wasting away of the muscles
• Absence of protein dystrophin
– Fragile X syndrome
• Most common cause of inherited mental impairment
• Most common known cause of autism
– Hemophilia
• Absence or minimal presence of a clotting factor
fibrous
tissue
abnormal muscle normal tissue
Abnormal muscle in muscular dystrophy
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(left, right): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center;
(center): Courtesy Muscular Dystrophy;
Hemophiliac
Alice
Alice
12 children of 26
are shown
Key
Unaffected male
Unaffected female
Carrier Queen Victoria Prince Albert
4 children of 9
are shown
Prince Henry of
Battenberg
Beatrice Victoria Frederick III
(Germany)
Louis IV
(Hesse)
Princess
Helena of
Waldeck
Leopold
(died at 31)
Leopold
(died at 32)
Victoria Henry Irene Frederick
(died at 3)
Alexandra Nicholas II
(Russia)
Alfonso XII
(Spain)
Alexander
(Earl of
Athlone)
Waldemar
(died at 56)
Henry
(died at 4)
Alexei
(murdered)
Rupert
(died at 21)
Gonzalo
(died at 20)
Alfonso
(died at 31)
6 children of 34
are shown
(queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images;
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
24.2 Gene Linkage
• Each chromosome contains many aleles
in a definite fixed order
• Linkage group – all the alleles on one
chromosome that are inherited together
– Sex-linkage refers to sex chromosomes
• Two trait crosses assume the alleles are
on nonhomologous chromosomes
• Alleles that are linked do not show
independent assortment
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A a
B
A
B
a
b
b
50% 50%
no crossing- over
during meiosis
2 types of gametes in
equal proportions a. Complete linkage
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97% 3%
A
A
B
a
b
a
B
A
b
B b
a
crossing-over
during meiosis
no crossing- over
during meiosis
recombinant gametes
4 types of gametes in unequal proportions
b. Incomplete linkage
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97% 3%
A A a
B
A
B
A
B
a
b
a
b
a
B
A
b
b B b
a
50% 50%
no crossing- over
during meiosis
crossing-over
during meiosis
no crossing- over
during meiosis
recombinant gametes
4 types of gametes in unequal proportions
b. Incomplete linkage a. Complete linkage
2 types of gametes in
equal proportions
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Z S R G
z s r g
pair of homologous chromosomes
24.2 Gene Linkage • During meiosis, crossing-over sometimes occurs
between nonsister chromatids in a tetrad
– Chromatids exchange genetic material
• If crossing-over occurs, dihybrid produces four
types of gametes instead of two
• Occurrence of crossing-over can help tell the
sequence of genes on a chromosome
– Crossing-over occurs more often between distant
genes than between closer genes
24.3 Changes in Chromosome Number
• Nondisjunction
– Occurs during meiosis I, when both members
of a homologous pair go into the same
daughter cell
– Or during meiosis II, when the sister
chromatids fail to separate and both daughter
chromosomes go into the same gamete
– Results in trisomy or monosomy when
fertilized
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Meiosis I
Meiosis II nondisjunction
2n 2n
pair of
homologous
chromosomes
normal
normal
2n + 1 2n - 1
Zygote
Fertilization
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Meiosis I
Meiosis II
nondisjunction
pair of
homologous
chromosomes
Zygote
2n + 1 2n + 1 2n - 1 2n - 1
Fertilization
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Meiosis I
Meiosis II
nondisjunction
nondisjunction
2n
a.
2n
pair of
homologous
chromosomes
pair of
homologous
chromosomes
normal
normal
2n + 1 2n - 1
Zygote
b. 2n + 1 2n + 1 2n - 1 2n - 1
Fertilization
24.3 Changes in Chromosome Number
• Normal development depends on exactly
two of each kind of chromosome
– Trisomies are tolerated better than
monosomies
• Only trisomy 21 (Down syndrome) has a
reasonable chance of survival after birth
• Chromosome 21 is one of the smallest
chromosomes
– Chances of survival are greater when trisomy
or monosomy involves the sex chromosomes
24.3 Changes in Chromosome Number
• Abnormal numbers of sex chromosomes
– Normal XX females have one X inactive
• Barr body
• Cells of females function with 1 X just like males
• Zygote with one X chromosome (Turner syndrome) can
survive
– All extra X chromosomes become deactivated
• Poly-X females and XXY males are seen fairly frequently
– Extra Y chromosomes are also tolerated
• XYY (Jacobs syndrome) is due to nondisjunction during
meiosis II of spermatogenesis
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24.3 Changes in Chromosome Number
• Down Syndrome – trisomy 21
– Most common autosomal trisomy seen among
humans
– Easily recognized physical features
– Mild to severe mental impairment
– Chances of a woman having a Down syndrome child
increase rapidly with age, starting at about age 40
– Karyotyping can be used with amniocentesis or
chorionic villus sampling to diagnose a fetus
– Symptoms may be due to expression of Gart gene
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© Jose Carrilo/PhotoEdit
21
extra chromosome 21
Gart
gene
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© CNRI/SPL/Photo Researchers, Inc.
21
a. b .
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a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers, Inc.
extrachr omosome 21
Gart
gene
• Changes in sex chromosome number
– Presence of Y chromosome, not the number of X,
determines maleness
• SRY gene produces testis-determining factor
24.3 Changes in Chromosome Number
24.3 Changes in Chromosome
Structure
• Chromosomal mutations occur when
chromosomes break
– Environmental agents or viruses can cause
break
– Ordinarily, break reunites with same
sequence of genes
– Failure to reunite correctly can result in:
• Deletion, duplication, translocation, or inversion
24.3 Changes in Chromosome
Structure
• Deletion
– Occurs when a single break causes a
chromosome to lose an end piece or when
two simultaneous breaks lead to the loss of
an internal chromosomal segment
– Inheriting one normal chromosome and one
with a deletion can result in a syndrome due
to not having a pair of alleles
– William’s syndrome – loss of small piece of 7
– Cri du chat – 5 missing an end piece
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deletion lost
a.
+
a
b
c
d
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f
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a
b
c
d
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f
g
h
b. Courtesy The Williams Syndrome Association
Deletion
a
b
c
d
e
d
e
f
a
b
c
d
e
f
g
g
duplication
inversion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Courtesy Kathy Wise
a b Duplication
• Duplication
– Chromosomal segment
is repeated in the same
chromosome or in a
nonhomologous
chromosome
– Individual has more than
two alleles for certain
traits
– Inv dup 15 syndrome
• Inverted duplication on
chromosome 15
24.3 Changes in Chromosome
Structure
24.3 Changes in Chromosome
Structure
• Translocation
– Exchange of chromosomal segments between two
nonhomologous chromosomes
– A person who has both of the involved chromosomes
has the normal amount of genetic material and is
healthy
• Unless the chromosome exchange breaks an allele into two
pieces
– The person who inherits only one of the translocated
chromosomes will have only one copy of certain
alleles and three copies of certain other alleles
• One type of Down syndrome is caused by a
translocation between chromosomes 21 and 14
• Alagille syndrome
– Translocation between chromosomes 2 and 20
– Father did not realize he had the syndrome until he
had a child with the syndrome
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a
b
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translocation
Translocation
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A
A
A
B
B
B
C
C C
D
D
D
E
E
E
F
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inverted
segment
region of
crossing-over
duplication and
deletion in both
homologous
chromosomes Inversion
• Inversion
– Segment of a chromosome
is turned 180 degrees
– Reverse sequence of
alleles can lead to altered
gene activity
– Crossing-over between an
inverted chromosome and
the noninverted homologue
can lead to recombinant
chromosomes that have
both duplicated and deleted
segments
24.3 Changes in Chromosome
Structure
A
A
A
B
B
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C
C C
D
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E
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F
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G
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inverted
segment
region of
crossing-over
duplication and
deletion in both
homologous
chromosomes
deletion lost
+
a
b
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d
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f
g
h
a
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a Deletion
a
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c
d
e
d
e
f
a
b
c
d
e
f g
g
duplication
inversion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a Duplication
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translocation
Translocation Inversion