Download CHAPTER 21 Chromosomal Mutations

Peter J. Russell
CHAPTER 21
Chromosomal Mutations
edited by Yue-Wen Wang Ph. D.
Dept. of Agronomy,台大農藝系
NTU
遺傳學 601 20000
Chapter 21 slide 1
Types of Chromosomal Mutations
1. Variations in chromosome structure or number can arise
spontaneously or be induced by chemicals or radiation.
Chromosomal mutation can be detected by:
a. Genetic analysis (observing changes in linkage).
b. Microscopic examination of eukaryotic chromosomes at mitosis
and meiosis (karyotype analysis).
2. Chromosomal aberrations contribute significantly to human
miscarriages, stillbirths and genetic disorders.
a. About 1⁄2 of spontaneous abortions result from major
chromosomal mutations.
b. Visible chromosomal mutations occur in about 6/1,000 live
births.
c. About 11% of men with fertility problems, and 6% of those
institutionalized with mental deficiencies have chromosomal
mutations.
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Chapter 21 slide 2
Variations in Chromosome Structure
1. Mutations involving changes in chromosome structure occur in four
common types:
a. Deletions.
b. Duplications.
c. Inversions (changing orientation of a DNA segment).
d. Translocations (moving a DNA segment).
2. All chromosome structure mutations begin with a break in the DNA,
leaving ends that are not protected by telomeres, but are “sticky” and
may adhere to other broken ends.
3. Polytene chromosomes (bundles of chromatids produced by DNA
synthesis without mitosis or meiosis) are useful for studying
chromosome structure mutations.
a. Polytene chromosomes are easily detectable microscopically.
b. Homologs are tightly paired, joined at the centromeres by a proteinaceous
chromocenter.
c. Detailed banding patterns are characterized for the four polytene
chromosomes, with each band averaging 30 kb of DNA, enough to encode
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Chapter 21 slide 3
several genes.
Deletion
1. In a deletion, part of a chromosome is missing (Figure 21.1).
a. Deletions start with chromosomal breaks induced by:
i. Heat or radiation (especially ionizing).
ii. Viruses.
iii. Chemicals.
iv.Transposable elements.
v. Errors in recombination.
b. Deletions do not revert, because the DNA is missing.
2. The effect of a deletion depends on what was deleted.
a. A deletion in one allele of a homozygous wild-type organism may give a normal
phenotype, while the same deletion in the wild-type allele of a heterozygote would
produce a mutant phenotype.
b. Deletion of the centromere results in an acentric chromosome that is lost, usually
with serious or lethal consequences. (No known living human has an entire
autosome deleted from the genome.)
c. Large deletions can be detected by unpaired loops seen in karyotype analysis
(Figure 21.2).
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Chapter 21 slide 4
Fig. 21.1 A deletion of a chromosome segment
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Chapter 21 slide 5
Fig. 21.2 Cytological effects of meiosis of heterozygosity for a deletion
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Chapter 21 slide 6
3. Deletion mapping can indicate the physical location of a
gene on the chromosome, because deletion of the
dominant allele in a heterozygote results in the recessive
phenotype.
a. Expression of the recessive trait caused by the absence of a
dominant allele is called pseudodominance.
b. Demerec and Hoover (1936) studied a fly strain heterozygous for
the X-linked recessive mutations y, ac and sc (Figure 21.3).
i. Genetic analysis shows the 3 loci linked at the left end of the
X chromosome.
ii. Deletion experiments correlate the deleted DNA with loss of
dominant alleles and the appearance of pseudodominance.
iii. This technique was used to produce the detailed physical
map of Drosophila polytene chromosomes.
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Chapter 21 slide 7
Fig. 21.3 Use of deletions to determine the physical locations of genes on Drosophila
polytene chromosomes
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Chapter 21 slide 8
4. Human disorders caused by large chromosomal deletions
are generally seen in heterozygotes, since homozygotes
usually die. Examples include:
a. Cri-du-chat (“cry of the cat”) syndrome, resulting from deletion
of part of the short arm of chromosome 5 (Figure 21.4).
b. Prader-Willi syndrome, occurring in heterozygotes with part of
the long arm of one chromosome 15 homolog deleted. The
deletion results in feeding difficulties, poor male sexual
development, behavioral problems and mental retardation.
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Chapter 21 slide 9
Duplication
1. Duplications result from doubling of chromosomal
segments, and occur in a range of sizes and locations
(Figure 21.5).
a. Tandem duplications are adjacent to each other.
b. Reverse tandem duplications result in genes arranged in the
opposite order of the original.
c. Tandem duplication at the end of a chromosome is a terminal
tandem duplication (Figure 21.6).
d. Heterozygous duplications result in unpaired loops, and may be
detected cytologically.
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Chapter 21 slide 10
Fig. 21.5 Duplication, with a chromosome segment repeated
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Chapter 21 slide 11
Fig. 21.6 Forms of chromosome duplications are tandem, reverse tandem, and
terminal tandem duplications
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Chapter 21 slide 12
2. An example is the Drosophila eye shape allele, Bar, that
reduces the number of eye facets, giving the eye a slit-like
rather than oval appearance (Figure 21.7).
a. The Bar allele resembles an incompletely dominant mutation:
i. Females heterozygous for Bar have a kidney-shaped eye that
is larger and more faceted than in a female homozygous for
Bar.
ii. Males hemizygous for Bar have slit-like eyes like those of a
Bar/Bar female.
b. Cytological examination of polytene chromosomes showed that
the Bar allele results from duplication of a small segment (16A)
of the X chromosome.
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Chapter 21 slide 13
Fig. 21.7 Chromosome constitutions of Drosophila strains
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Chapter 21 slide 14
3. Duplications like Bar probably result from unequal crossing-over,
perhaps due to similar DNA sequences in neighboring chromosome
regions (Figure 21.8).
4. Bar homozygote parents (Bar/Bar X Bar/Y) sometimes produce
offspring that do not show the Bar phenotype:
a. About 1 in 1,600 progeny flies will have wild-type eyes.
b. About 1 in 1,600 will have double-Bar eyes, with 3 copies of the 16A
sequence and eyes even more reduced than Bar.
c. Unequal crossing-over would account for these results.
5. Multigene families result from duplications. Hemoglobin (Hb) is an
example:
a. Each Hb contains two copies of two subunits (e.g., 2 α-globins and 2 βglobins), and the identity of the subunits changes with the organism’s
developmental stage.
b. Genes for the α-type polypeptides are clustered together on 1
chromosome, and those for β-type polypeptides are clustered on another.
c. α-type genes have similar sequences, as do β-type. They probably result
from duplication and subsequent sequence divergence.
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Chapter 21 slide 15
Fig. 21.8 Unequal crossing-over and the Bar mutant of Drosophila
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Chapter 21 slide 16
Inversion
Animation: Crossing-over in an Inversion Heterozygote
1. Inversion results when a chromosome segment excises and reintegrates
oriented 1800 from the original orientation. There are two types
(Figure 21.9):
a. Pericentric inversions include the centromere.
b. Paracentric inversions do not include the centromere.
2. Inversions generally do not result in lost DNA, but phenotypes can arise
if the breakpoints are in genes or regulatory regions.
3. Linked genes are often inverted together. For example:
a. If a normal chromosome has the gene order ABCDEFGH, inversion of
the BCE) segment would give the gene order ADCBEFGH.
i. A fly homozygous for the inversion (ADCBEFGH/ADCBEFGH)
will have normal crossing over and meiosis, and no problems with
gene duplications or deletions.
ii. A heterozygote (ABCDEFGH/ADCBEFGH) suffers serious genetic
consequences due to unequal crossover.
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Chapter 21 slide 17
Fig. 21.9 Inversions
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 18
4. Different recombinant chromosomes are produced by crossover in a
heterozygote, depending on centromere involvement:
a. Paracentric inversions (no centromere) result in visible inversion loops between
homologous chromosomes (Figures 21.10 and 21.11).
i. Crossover in the inversion region results in unbalanced sets of genes, and
gametes or zygotes derived from recombined chromatids may be inviable
due to abnormal gene dose.
ii. Without crossover in the looped region, gametes receive complete sets of
genes (two gametes with normal and two with inversions) and are viable.
iii. Effects of a single crossover within an inverted segment in a heterozygote
include:
(1) Joining of homologous regions of two chromatids to produce a
dicentric bridge, and corresponding loss of an acentric fragment.
(2) During anaphase the two centromeres of the dicentric chromosome
migrate towards opposite poles, causing the bridge to break, and
producing two chromatids with deletions.
(3) The second meiotic division distributes one chromatid to each
gamete:
(a) Two gametes carry normal sets of genes (one in the normal order and the other in inverted
order).
(b) Two gametes are missing many genes, and are inviable.
(4) Female mammals often shunt dicentric
chromosomes or acentric
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Chapter 21 slide 19
fragments to the polar bodies, so fertility may not be so reduced.
Fig. 21.10 Consequences of crossing-over in a paracentric inversion
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 20
Fig. 21.11 Meiotic products resulting from a single crossover within a heterozygous,
pericentric inversion loop
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 21 slide 21
b. Pericentric inversions that undergo a single crossover will result
in:
i. Two viable gametes, one with genes in normal order, the
other with the inversion.
ii. Two inviable gametes, each with some genes deleted and
others duplicated.
c. Some crossover events within the inversion loop do not affect
gamete viabiity Examples:
i. A double crossover close together involving the same two
chromatids (a 2-strand double crossover).
ii. Changes where duplicated and deleted segments do not
affect gene expression (e.g., very small segments).
iii. In mammals, inverted segments may remain unpaired, and
so avoid crossing-over.
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Chapter 21 slide 22
Translocation
Animation: Meiosis in a Translocation Heterozygote
1. A change in location of a chromosome segment is a translocation. No
DNA is lost or gained. Simple translocations are of two types (Figure
21.12):
a. Intrachromosomal, with a change of position within the same
chromosome.
b. Interchromosomal, with transfer of the segment to a nonhomologous
chromosome.
i. If a segment is transferred from one chromosome to another, it is
nonreciprocal.
ii. If segments are exchanged, it is reciprocal.
2. Gamete formation is affected by translocations.
a. In homozygotes with the same translocation on both chromosomes,
altered gene linkage is seen.
b. Gametes produced with chromosomal translocations often have
unbalanced duplications and/or deletions and are inviable, or produce
disorders like familial Down syndrome.
c. Strains homozygous for a reciprocal translocation form normal gametes.
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Chapter 21 slide 23
Fig. 21.12 Translocations
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 24
d. Strains heterozygous for a reciprocal translocation must pair a set of
normal chromosomes (N) with a set of translocated ones (T).
i. Result is a cross-like configuration in meiotic prophase I of four
associated chromosomes, each partially homologous to two others in
the group (Figure 21.13).
ii. Anaphase I segregation may occur in three different ways (crossover
will not be considered).
(1) Alternate segregation moves alternate centromeres to the same
poles (e.g., N1 and N2 one direction, T1 and T2 the other). Gametes
are viable, with either normal or translocated chromosomes.
(2) Adjacent 1 segregation moves adjacent nonhomologous
centromeres to the same pole (e.g., N1 and T2 one direction, N2
and T1 the other). Gametes are inviable due to gene duplications
and deletions.
(3) Adjacent 2 segregation is rare, moving different pairs of
adjacent homologous centromeres to the same pole (N1 and T1 one
direction, N2 and T2 the other). These gametes are usually
inviable.
iii. Thus, heterozygotes for a reciprocal translocation are considered
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Chapter 21 slide 25
semi-sterile.
Fig. 21.13 Meiosis in a translocation heterozygote in which no crossover occurs
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Chapter 21 slide 26
3. Animal gametes with large duplications or deletions may function, but the
zygote generally dies. Small duplications or deletions may be viable. Plant
pollen with duplications or deletions is usually nonfunctional.
4. Some tumors have chromosomal abnormalities (either in number or structure)
early in inception, and develop more mutations over time that often correlate
with progression to uncontrolled growth. Examples:
a. Chronic myelogenous leukemia (CML) involves a reciprocal translocation of
chromosomes 9 and 22.
i. Myeloblasts (stem cells of white blood cells) replicate
uncontrollably.
ii. 90% of CML patients have the Philadelphia chromosome (Ph1) reciprocal
translocation.
iii. The reciprocal translocation causes transition from a differentiated cell to a
tumor cell, by translocating a proto-oncogene from chromosome 9 to
chromosome 22, and probably converting it to the c-abl oncogene.
iv. The hybrid gene arrangement causes expression of a leukemia-producing gene
product.
b. Burkitt lymphoma (BL) involves a reciprocal translocation of chromosomes 8 and
14.
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Chapter 21 slide 27
Fig. 21.14 Origin of the Philadelphia chromosome in chronic myelogenous leukemia
(CML) by a reciprocal translocation involving chromosomes 9 and 22
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 28
Position Effect
1. Sometimes inversions or translocations change phenotypic expression of genes by the
position effect, for example by moving a gene from euchromatin to heterochromatin
(transcription generally occurs in euchromatin but not heterochromatin).
2. An example is the white-eye (w) locus in Drosophila:
a. An inversion moves the w gene from a euchromatin region of the X chromosome to a position in
heterochromatin.
b. In a w+ male, or a w+/w female, where w is involved in the inversion, the eyes will have white spots
resulting from the cells where the w allele was moved and inactivated.
3. The Bar eye in Drosophila shows a different kind of position effect.
a. Recall that different alleles are produced by duplication of the 16A region of the X chromosome
(Figure 21.7).
i. Wild-type has one copy of the 16A segment.
ii. Bar has two copies of 16A.
iii. double-Bar has three copies of 16A.
b. Different combinations of locations may be tested for this allele. For example:
i. Flies that are Bar/Bar have four copies of the 16A segment (two on each chromosome).
ii. Flies that are double-Bar/+ also have four copies of the 16A segment (three on one
chromosome and one on the other).
iii. Sturtevant (1925) showed that these arrangements do not produce the same phenotype:
(1) Bar/Bar flies have an average of 68 eye facets, while double-Bar/ + has only 45.
(2) In some way, the three adjacent 16A segments exert a greater effect on eye
development than the same number of segments on different chromosomes, indicating
that position can affect gene expression.
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Chapter 21 slide 29
Fig. 21.15 Position effect and Bar eye in Drosophila
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Chapter 21 slide 30
Fragile Sites and Fragile X Syndrome
1. Chromosomes in cultured human cells develop narrowings or unstained
areas (gaps) called fragile sites; over 40 human fragile sites are known.
2. A well-known example is fragile X syndrome, in which a region at
position Xq27.3 is prone to breakage and mental retardation may result.
a. Fragile X syndrome has an incidence in the U.S. of about 1/1,250 in males,
and 1/2,500 in females (heterozygotes).
b. Inheritance follows Mendelian patterns, but only 80% of males with a fragile
X chromosome are mentally retarded. The 20% with fragile X chromosome
but a normal phenotype are called normal transmitting males.
i. A normal transmitting male can pass the chromosome to his daughter(s).
ii. Sons of those daughters frequently show mental retardation.
c. About 33% of carrier (heterozygous) females show mild mental retardation.
i. Sons of carrier females have a 50% chance of inheriting the fragile X.
ii. Daughters of carrier females have a 50% chance of being carriers.
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Chapter 21 slide 31
Fig. 21.16 Diagram of a human X chromosome showing the location of the fragile site
responsible for fragile X syndrome
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Chapter 21 slide 32
d. Molecular analysis shows a repeated 3-bp sequence, CGG, in the FMR-1
(fragile X mental retardation-1) gene, at the fragile X site.
i. Normal individuals have 6–54 CGG repeats, with an average of 29.
ii. Normal transmitting carrier males, their daughters and some other
carrier females have 55–200 copies, but do not show symptoms.
iii. Individuals with fragile X syndrome have 200–1,300 copies,
indicating that tandem amplification of this sequence is tolerated until a
threshold number of copies is reached.
iv. Amplification of CGG repeats occurs only in females, perhaps during
a slipped mispairing process during DNA replication.
v. The function of the FMR-1 gene is unknown. It encodes a protein of
unidentified function.
3. There are other examples of triplet repeat amplifications that cause
disease above a threshold number of copies. In these examples,
amplification can occur in both sexes.
a. Myotonic dystrophy.
b. Spinobulbar muscular atrophy (Kennedy disease).
c. Huntington disease.
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Chapter 21 slide 33
Variations in Chromosome Number
1. An organism or cell is euploid when it has one complete set of
chromosomes, or exact multiples of complete sets. Eukaryotes that
are normally haploid or diploid are euploid, as are organisms with
variable numbers of chromosome sets.
2. Aneuploidy results from variations in the number of individual
chromosomes (not sets), so that the chromosome number is not an
exact multiple of the haploid set of chromosomes.
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Chapter 21 slide 34
Changes in One or a Few Chromosomes
1. Aneuploidy can occur due to nondisjunction during meiosis.
a. Nondisjunction during meiosis I will produce four gametes, two with a chromosome
duplicated, and two that are missing that chromosome.
i. Fusion of a normal gamete with one containing a chromosomal duplication will
produce a zygote with three copies of that chromosome, and two of all others.
ii. Fusion of a normal gamete with one missing a chromosome will result in a
zygote with only one copy of that chromosome, and two of all others.
b. Nondisjunction during meiosis II produces two normal gametes and two that are
abnormal (one with two sibling chromosomes, and one with that chromosome
missing).
i. Fusion of abnormal gametes with normal ones will produce the genotypes
discussed above.
ii. Normal gametes are also produced, and when fertilized will produce normal
zygotes.
c. More complex gametic chromosome composition can result when:
i. 1 chromosome is involved.
ii. Nondisjunction occurs in both meiotic divisions.
iii. Nondisjunction occurs in mitosis (result is somatic cells with unusual
chromosome complements).
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Chapter 21 slide 35
2. Autosomal aneuploidy is not well tolerated in animals, and in mammals
is detected mainly after spontaneous abortion. Aneuploidy is much
better tolerated in plants. There are four main types of aneuploidy
(Figure 21.17):
a. Nullisomy involves loss of 1 homologous chromosome pair (the cell is
2N - 2).
b. Monosomy involves loss of a single chromosome (2N - 1).
c. Trisomy involves one extra chromosome, so the cell has three copies of
one, and two of all the others (2N + 1).
d. Tetrasomy involves an extra chromosome pair, so the cell has four
copies of one, and two of all the others (2N + 2).
3. More than one chromosome or chromosome pair may be lost or added.
Examples:
a. A double monosomic aneuploidy has two separate chromosomes
present in only one copy each (2N - 1 - 1).
b. A double tetrasomic aneuploidy has two chromosomes present in four
copies each (2N + 2 + 2).
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Chapter 21 slide 36
Fig. 21.17 Normal (theoretical) set of metaphase chromosomes in a diploid (2N)
organism (top) and examples of aneuploidy (bottom)
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Chapter 21 slide 37
4. Some types of aneuploidy have serious meiotic consequences.
Examples:
a. A monosomic cell (2N - 1):
i. May produce gametes that are N (normal) and N - 1(monosomic).
ii. Or, the unpaired chromosome may be lost completely, producing
gametes that are all N - 1.
b. A trisomic cell (2N + 1) with the genotype +/+/a, would be an example
(assuming that this organism can tolerate trisomy, and no crossing-over
occurs) (Figure 21.18).
i. Gametes produced belong to four genotypic classes, in these
proportions:
(1) Two gametes with genotype +/a.
(2) Two gametes with genotype +.
(3) One gamete with genotype +/+.
(4) One gamete with genotype a.
ii. The cross of a +/+/a trisomic to an a/a individual will produce a
phenotypic ratio of 5 wild type : 1 mutant (a).
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Chapter 21 slide 38
Fig. 21.18 Meiotic segregation possibilities in a trisomic individual
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Chapter 21 slide 39
5. Human examples of aneuploidy in autosomes and sex chromosomes are
summarized in Table 21.1.
a. Sex chromosome aneuploidy is found more often than autosome
aneuploidy, because lyonization compensates for chromosome dosage.
b. Autosomal monosomies are rarely found in humans, presumably because
they are lost early in pregnancy.
c. Autosomal trisomies account for about half of fetal deaths, and only a
few are seen in live births. Most (trisomy-8, -13 and -18) result in early
death, with only trisomy-21 (Down syndrome) surviving to adulthood.
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Chapter 21 slide 40
d. Trisomy-21 occurs in an estimated 3,510/106 conceptions, and 1,430/106
births.
i. Down syndrome individuals are characterized by:
(1) Low IQ.
(2) Epicanthal folds over eyes.
(3) Short and broad hands.
(4) Below-average height.
ii. Table 21.2 shows the correlation of maternal age and probability of
trisomy-21.
(1) A female fetus before birth produces primary oocytes in her
ovaries that stop their development at prophase I of meiosis.
(2) After puberty, secondary oocytes develop from the primary
ones, entering the second meiotic division but again arresting, this
time at metaphase II.
(3) If fertilization occurs, the second meiotic division is
completed.
(4) The probability of nondisjunction increases with length of time
the primary oocyte is in the ovary.
(5) Amniocentesis or chorionic villus sampling can determine
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Chapter 21 slide 41
whether the fetus has a normal台大農藝系
complement
of chromosomes.
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Chapter 21 slide 42
Animation: Down Syndrome Caused by a Robertsonian
Translocation
iii. Robertsonian translocation (centric fusion) produces three
copies of the long arm of chromosome 21, resulting in
familial Down syndrome.
(1) In this nonreciprocal translocation, two
nonhomologous acrocentric (centromeres near end)
chromosomes break at centromeres.
(a) Both long arms become attached to the same centromere, creating a
chromosome with the long arm of chromosome 21 and the long arm of
chromosome 14 (or 15).
(b) The short arms also fuse, forming a reciprocal product that is usually
lost within a few cell divisions.
(c) The heterozygous carrier of this chromosome is phenotypically normal,
since the two copies of each major chromosome arm supply two copies
of all essential genes.
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Chapter 21 slide 43
Fig. 21.20 Robertsonian translocation
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 44
(2) Mating of a heterozygous carrier and a normal
individual has a high risk of Down syndrome offspring.
(a)The normal parent produces normal gametes, with one copy each of
chromosomes 14 and 21.
(b) The heterozygous carrier parent produces three reciprocal pairs of
gametes, each produced by different segregation of the three
chromosomes involved.
(c)Theoretically, the zygotes produced would be:
(i) 1⁄6 with normal chromosomes 14 and 21 (like 1 parent).
(ii) 1⁄6 heterozygous carriers with normal phenotype (like other parent).
(iii) 1⁄6 inviable due to monosomy of chromosome 14.
(iv) 1⁄6 inviable due to monosomy of chromosome 21.
(v) 1⁄6 inviable due to trisomy of chromosome 14.
(vi) 1⁄6 with trisomy of chromosome 21. These individuals have a normal
chromosome number (46) but three copies of the long arm of
chromosome 21, sufficient to cause Down syndrome.
(vii) In summary, 1⁄2 the zygotes are inviable, and 1⁄3 of the live offspring
are predicted to have Down syndrome, although the actual birth rate is
lower
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Chapter 21 slide 45
Fig. 21.21 The three segregation patterns of a heterozygous Robertsonian translocation
involving the human chromosomes 14 and 21
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台大農藝系 遺傳學 601 20000
Chapter 21 slide 46
e. Trisomy-13 (Patau syndrome) occurs in 2/104 live births, and most die
within the first 3 months. Characteristics include (Figure 21.22):
i. Cleft lip and palate.
ii. Small eyes.
iii. Polydactyly (extra fingers and toes).
iv. Mental and developmental retardation.
v. Cardiac and other abnormalities.
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Chapter 21 slide 47
f. Trisomy-18 (Edwards syndrome) occurs in 2.5/104 live births, and 90%
die within 6 months. About 80% of Edwards syndrome infants are
female. Characteristics include (Figure 21.19):
i. Small size with multiple congenital malformations throughout the
body.
ii. Clenched fists.
iii. Elongated skull.
iv. Low-set ears.
v. Mental and developmental retardation.
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Chapter 21 slide 48
Changes in Complete Sets of Chromosomes
1. Monoploidy and polyploidy involve complete sets of chromosomes,
and so both are cases of euploidy. Euploidy is lethal in most animal
species, but often tolerated in plants, where it has played a role in
speciation and diversification.
2. Monoploidy and polyploidy can result when either round of meiotic
division lacks cytokinesis, or when meiotic nondisjunction occurs for
all chromosomes.
a. Complete nondisjunction at meiosis I will produce 1⁄2 gametes with normal
chromosomes, 1⁄4 with two sets of chromosomes and 1⁄4 with no
chromosomes.
b. A gamete with two sets of chromosomes fused with a normal gamete
produces a triploid (3N) zygote.
c. Fusion of two gametes that each have two sets of chromosomes produces a
tetraploid (4N) zygote.
d. Polyploidy of somatic cells can result from mitotic nondisjunction of
complete chromosome sets.
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Chapter 21 slide 49
Fig. 21.24 Variations in number of complete chromosome sets
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 21 slide 50
3. Monoploidy is rare in adults of diploid species due to
recessive lethal mutations.
a. Males of some species (e.g., wasps, ants and bees) develop
from unfertilized eggs and are monoploid.
b. Plant experiments often use monoploids.
i. Haploid cells are isolated from plant anthers and grown
into monoploid cultures.
ii. Colchicine (which inhibits mitotic spindle formation)
allows chromosome number to double, producing
completely homozygous diploid breeding lines.
iii. Mutant genes are easily identified in monoploid
organisms.
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Chapter 21 slide 51
4. Polyploidy involves three or more sets of chromosomes,
and may occur naturally (e.g., by breakdown of the mitotic
spindle), or by induction (e.g., with chemicals such as
colchicine).
a. Nearly all plants and animals probably have some polyploid tissues.
Examples:
i. Plant endosperm is triploid.
ii. Liver of mammals (and perhaps other vertebrates) is polyploid.
iii. Giant abdominal neuron of Aplysia has about 75,000 copies of
the genome.
iv. Wheat is hexaploid (6N) and the strawberry is octaploid (8N).
v. North American sucker fish, salmon and some salamanders are
polyploid.
b. There are two classes of polyploids based on the number of
chromosome sets:
i. Even-number polyploids are more likely to be at least partially
fertile, because the potential exists for equal segregation of
homologs during meiosis.
ii. Odd-number polyploids will always have unpaired
chromosomes. Balanced gametes are rare and these organisms
are usually sterile or have increased zygote death.
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Chapter 21 slide 52
c. Triploids are unstable in meiosis, because random segregation
means that balanced gametes (either exactly N or exactly 2N) are
rare.
i. The probability of a triploid organism producing a haploid
gamete is (1⁄2)n, where n is the number of chromosomes.
ii. Triploidy is always lethal in humans, accounting for 15–20% of
spontaneous abortions and 1/104 live births, with most dying in
the first month.
iii. Tetraploidy in humans is also lethal, usually before birth,
accounting for 5% of spontaneous abortions.
d. Polyploidy is more common in plants, probably due to selffertilization, allowing an even-number of polyploids to produce
fertile gametes and reproduce. Plant polyploidy occurs in two types:
i. Autopolyploidy results when all sets of chromosomes are from
the same species, usually due to meiotic error. Fusion of a
diploid gamete with a haploid one produces a triploid organism.
Examples include:
(1) “Seedless” fruits like bananas, grapes and watermelons.
(2) Grasses, garden flowers, crop plants and forest trees.
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Chapter 21 slide 53
ii. Allopolyploidy results when the chromosomes are from two
different organisms, typically from the fusion of haploid
gametes followed by chromosome doubling. For example:
(1) Fusion of haploid gametes from plant 1 and plant 2
produces an N1 + N2 hybrid plant. No chromosomal pairing
occurs at meiosis, viable gametes are not produced and the
plants are sterile.
(2) Rarely, division error doubles the chromosome sets (2 N1
+ 2N2). The diploid sets function normally in meiosis, and
fertile allotetraploid plants result.
(3) An example is crosses between cabbages (Brassica
oleracea) and radishes (Raphanus sativus), which both have
a chromosome number of 18.
(a) The F1 hybrids have 9 chromosomes from each parent, and have a
morphology intermediate between cabbages and radishes. They are
mostly sterile.
(b) A few seeds, some fertile, can be produced by the F1 through meiotic
errors.
(i) Somatic cells in the resulting plants have 36 chromosomes, a full diploid
set from both cabbages and radishes.
(ii) These fully fertile plants look much like the F1 hybrids, and are named
Raphanobrassica.
(4) Polyploidy is the rule in agriculture, where polyploids include all
commercial grains (e.g., bread wheat, Triticum aestivum, an
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601 20000
allohexaploid of three plant species),
most遺傳學
crops and
common Chapter
flowers.21 slide 54