Chapter 24: Patterns of
Chromosome Inheritance
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Viewing the Chromosomes
A karyotype is a display of chromosomes
paired according to their size, location
of the centromere, and staining
patterns.
A karyotype reveals abnormalities in
chromosome number or structure.
Humans have 23 pairs of chromosomes;
22 pairs of autosomes and one pair of
sex chromosomes.
Females are XX and males are XY.
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Amniocentesis
Amniocentesis uses a needle to extract
amniotic fluid from the uterus of a
pregnant woman from the 14th to 17th
week of pregnancy.
Up to 400 chromosome and biochemical
problems can be detected by culturing
fetal cells that are in the amniotic fluid.
There is a slight risk of spontaneous
abortion with this procedure.
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Amniocentesis
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Chorionic Villi Sampling
Chorionic villi sampling (CVS) uses a thin
suction tube to sample chorionic cells
from the placenta as early as the fifth
week of pregnancy.
The cells do not have to be cultured, and
karyotyping can be done immediately.
CVS carries a slightly greater risk of
spontaneous abortion but can be
performed earlier than amniocentesis.
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Chorionic villi sampling
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Karyotyping
Sampled fetal cells are stimulated to
divide in culture medium and another
chemical stops division during
metaphase when chromosomes are
highly condensed.
The stained cells are photographed and
can be paired based on stained crossbands, and size and shape.
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Human karyotype preparation
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Normal male karyotype
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Down syndrome karyotype
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Changes in Chromosome
Number
Nondisjunction occurs during meiosis I
when the members of a homologous
pair both 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.
The result is a trisomy or a monosomy.
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Nondisjunction in meiosis I
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Nondisjunction in meiosis II
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Normal development depends on the
presence of two of each kind of
chromosome, but an extra
chromosome is tolerated more than a
missing chromosome.
The Barr body is an inactive X
chromosome and is seen whenever
more than one X chromosome is
present (i.e., XX, XXY, XXX).
Cells of females function with a single
chromosome just as those of males do.
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Down Syndrome
Down syndrome is caused by trisomy 21,
three copies of chromosome 21 as a
result of nondisjunction.
Symptoms include mental retardation,
short stature, eyelid fold, flatter face, a
palm creases, and stubby fingers,
among others.
Nondisjunction usually occurred in
producing the mother’s egg and risk
increases at age 40.
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Abnormal autosomal
chromosome number
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The genes causing Down syndrome are
located on the bottom third of
chromosome 21.
In particular, the Gart gene leads to a
high level of purines, which contribute
to mental impairment but may allow
future preventive treatment.
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Changes in Sex Chromosome
Number
The presence of a Y chromosome
determines maleness.
The SRY gene on the short arm of the Y
produces a testis-determining factor that
begins the development of a male;
otherwise an embryo develops as a
female.
An abnormal number of sex chromosomes
is the result of inheriting to many or too
few X or Y chromosomes.
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Turner Syndrome
Individuals with Turner syndrome are
females that have only one X
chromosome; therefore they are XO.
They are short, with a broad chest, and
webbed neck.
They do not undergo puberty or
menstruate, and there is a lack of
breast development.
Intelligence is normal and individuals can
lead normal lives.
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Klinefelter Syndrome
Individuals with Klinefelter syndrome are
males that have two or more X
chromosomes in addition to a Y
chromosome.
The Y chromosome drives development
as a male but gonads are
underdeveloped and breasts develop.
Klinefelter males are usually slow to
learn but are not mentally retarded.
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Abnormal sex chromosome
number
24-21
Poly-X Females
A poly-X female has more than two X
chromosomes and extra Barr bodies in
the nucleus.
An XXX female has a normal phenotype
except there may be menstrual
difficulties, but she is fertile; her children
usually have normal karyotypes.
Females with XXXX are usually tall and
severely retarded; they may menstruate
normally.
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Jacobs Syndrome
Jacobs syndrome males are XYY which
can only result from nondisjuction
during spermatogenesis.
They tend to be tall, have persistent acne,
and have speech and reading
problems.
At one time it was suggested that XYY
males were unusually aggressive, but
this was found not to be true.
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Changes in Chromosome
Structure
A mutation is a permanent genetic
change.
A change in chromosome structure is a
chromosome mutation.
Radiation, organic chemicals, or even
viruses may cause chromosomes to
break, leading to mutations.
Chromosomal mutations include
inversion, translocation, deletion, and
duplication.
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Deletions and Duplications
Deletions occur when a single break
causes a lost end piece, or two breaks
result in a loss in the interior.
An individual who inherits a normal
chromosome from one parent and a
chromosome with a deletion from the
other parent no longer has a pair of
alleles for each trait, and a syndrome
can result.
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In Williams syndrome, chromosome 7
loses an end piece and children have a
pixie look and the skin ages
prematurely from lack of the gene that
governs elastin production.
An end piece of chromosome 5 produces
cri du chat syndrome where larynx is
abnormal and the infant’s cry is like
that of a cat, the head is small, and
there are facial abnormalities.
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Deletion
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Duplication results in a chromosome
segment being repeated in the same
chromosome or in a nonhomologous
chromosome, producing extra alleles
for a trait.
An inverted duplication in chromosome
15 causes inv dup 15 syndrome with
poor muscle tone, mental retardation,
and related symptoms.
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Duplication
24-29
Translocation
Translocation is exchange of
chromosomal segments between two,
nonhomologous chromosomes.
In a small percent of cases, a
translocation between chromosomes
21 and 14 causes Down syndrome.
The tendency for this particular
translocation can run in the family of
either the mother or father of affected
individuals.
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Alagille syndrome results from a deletion
of chromosome 20 or a translocation
that disrupts an allele on chromosome
20.
The symptoms for Alagille syndrome
range from mild to severe, so people
may not be aware they have the
syndrome.
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Translocation
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Inversion
Inversion involves a segment of a
chromosome being turned 180
degrees; the reverse sequence of
alleles can alter gene activity.
Crossing-over between inverted and
normal chromosomes can cause
recombinant chromosomes due to the
inverted chromosome needing to form
a loop to align.
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Inversion
24-34
Sex-Linked Traits
Traits controlled by genes on the X or Y
chromosomes are sex-linked although
most are unrelated to gender.
An allele on the X chromosome that is in
the region where the Y chromosome
has no alleles will express even if
recessive; it is termed X-linked.
A female would have to have two
recessive genes to express the trait; a
male would only need one.
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X-Linked Alleles
The key for an X-linked problem shows
the allele attached to the X as in:
XB = normal vision
Xb = color blindness.
Females with the genotype XBXb are
carriers because they appear to be
normal but each son has a 50% chance
of being color blind depending on
which allele the son receives.
XbXb and XbY are both colorblind.
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Cross involving an X-linked allele
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X-Linked Disorders
In pedigree charts that show the inheritance
pattern for X-linked recessive disorders,
more males than females have the trait
because recessive alleles on the X
chromosome are expressed in males.
A grandfather passes an X-linked recessive
disorder to a grandson through a carrier
daughter.
X-linked recessive disorders include redgreen color blindness, muscular
dystrophy, and hemophilia.
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X-linked recessive pedigree chart
24-39
Color Blindness
Three types of cones are in the retina
detecting red, green, or blue.
Genes for blue cones are autosomal;
those for red and green cones are on
the X chromosome.
Males are much more likely to have redgreen color blindness than females.
About 8% of Caucasian men have redgreen color blindness.
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Muscular Dystrophy
Muscular dystrophy is characterized by
the wasting of muscles.
The most common form is Duchenne
muscular dystrophy; this is an X-linked
disorder, occurring in 1 of 3,600 males.
Muscles weaken, frequent falls and
difficulty in rising occur early; death
occurs by age 20.
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Duchenne muscular dystrophy involves
the absence of a protein called
dystrophin that is involved in the
release of calcium from the
sarcoplasmic reticulum of muscle cells.
The lack of dystrophin causes calcium to
leak into the cell, which promotes the
action of an enzyme that dissolves
muscle fibers.
A test is now available to determine the
carriers of Duchenne muscular
dystrophy.
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Hemophilia
Hemophilia refers to the lack of one of
several clotting factors that leads to
excessive bleeding in affected
individuals.
Hemophiliacs bleed externally after
injury, but also bleed internally around
joints.
Hemorrhages can be stopped with blood
transfusions or a biotechnology
clotting factor.
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Fragile X Syndrome
Fragile X syndrome is an X-linked genetic
disorder with an unusual pattern of
inheritance.
Individuals with this syndrome (one in
1,500 males and one in 2,500 females)
have a base triplet repeat (CGG) in a
gene on the X chromosome.
Children may be autistic or hyperactive
with speech difficulties.
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Adults have large testes if male, and big
protruding ears.
They are short in stature and the face is
long with a prominent jaw.
A person with a smaller number of CGG
repeats and minor or no symptoms is
said to have a premutation and can
pass it to their children where the
number increases and the condition is
severe.
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Linked Genes
All the alleles on one chromosome form
a linkage group, which is inherited
together.
If crossing-over occurs, a dihybrid cross
gives all possible phenotypes among
the offspring, but the expected ratio is
greatly changed.
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Linkage group
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24-48
The frequency of recombinant gametes
that occurs due to the process of
crossing-over has been used to map
chromosomes.
Crossing-over data is used to map the
chromosomes of animals, such as fruit
flies, but is limited in mapping human
chromosomes because we do not
control the crosses.
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Cross involving linked genes
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Chapter Summary
Humans normally inherit 22 pairs of
autosomes and one pair of sex
chromosomes for a total of 46
chromosomes.
Abnormalities arise when humans inherit
an extra or missing autosome or
abnormal autosomes.
In humans, males are normally XY and
females are XX.
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Abnormalities arise when humans inherit
an incorrect number of sex
chromosomes.
Traits unrelated to the gender of an
individual are controlled by genes
located on the sex chromosomes.
Males express X-linked recessive disorders
because they inherit only one X
chromosome.
Genes that occur on the same
chromosome form a linkage group and
tend to be inherited together.
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