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5
Human Chromosomes and
Chromosome Behavior
Human Chromosomes
• Humans contain 46 chromosomes, including 22
pairs of homologous autosomes and two sex
chromosomes
• Karyotype = stained and photographed preparation
of metaphase chromosomes arranged according to
their size and position of centromeres
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Fig. 5.3 1-10
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Fig. 5.3 11-y
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Human Chromosomes
• Each chromosome in karyotype is divided into
two regions (arms) separated by the centromere
• p = short arm (petit); q = long arm
• p and q arms are divided into numbered bands
and interband regions based on pattern of
staining
• Within each arm the regions are numbered.
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Centromeres
• Chromosomes are classified according to the relative
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position of their centromeres
In metacentric it is located in middle of chromosome
In submetacentric—closer to one end of chromosome
In acrocentric—near one end of chromosome
Chromosomes with no centromere, or with two
centromeres, are genetically unstable
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Fig. 5.3
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Human X Chromosome
• Females have two copies of X chromosome
• One copy of X is randomly inactivated in all
somatic cells
• Females are genetic mosaics for genes on the X
chromosome; only one X allele is active in each
cell
• Barr body = inactive X chromosome in the nucleus
of interphase cells
• Dosage Compensation = dosage equalization for
active genes
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Fig. 5.7
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Human Y Chromosome
• Y chromosome is largely heterochromatic
• Heterochromatin is condensed inactive chromatin
• Important regions of Y chromosome:
pseudoautosomal region = region of shared X-Y
homology
SRY=master sex controller gene which encodes
testis determining factor (TDF) for male development
The pseudoautosomal region of the X and Y
chromosomes has gotten progressively shorter in
evolutionary time.
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Fig. 5.10
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Human Y Chromosome
• Y chromosome does not undergo recombination
along most of its length, genetic markers in the Y
are completely linked and remain together as the
chromosome is transmitted from generation to
generation
• The set of alleles at two or more loci present in a
particular chromosome is called a haplotype
• The history of human populations can be traced
through studies of the Y chromosome
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Abnormal Chromosome Number
• Euploid = balanced chromosome abnormality = the
same relative gene dosage as in diploids (example:
trisomics)
• Aneuploid = unbalanced set of chromosomes =
relative gene dosage is upset (example: trisomy of
chromosome 21)
• Monosomic = loss of a single chromosome copy
Polysomic = extra copies of single chromosomes
• Chromosome abnormalities are frequent in
spontaneous abortions.
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Abnormal Chromosome Number
• Monosomy or trisomy of most human autosomes
unviable. There are three exceptions: trisomies of
13, 18 and 21
• Down Syndrome is a genetic disorder due to
trisomy 21, the most common autosomal
aneuploidy in humans
• Frequency of Down Syndrome increases with
mother’s age
• Amniocentesis = fetal cells are analyzed for
abnormalities of chromosome number and
structure
• Chorionic villus sampling (CVS) = cells from a
zygote-derived embryonic membrane (the chorion)
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analyzed
Abnormal Chromosome Number
• Trisomic chromosomes undergo abnormal
segregation
• Trivalent = abnormal pairing of trisomic
chromosomes in cell division
• Univalent = extra chromosome in trisomy is unpaired
in cell division
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Fig. 5.13
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Sex Chromosome Aneuploidies
• An extra X or Y chromosome usually has a relatively
mild effect
• Trisomy-X = 47, XXX (female)
• Double-Y = 47, XYY (male)
• Klinefelter Syndrome = 47, XXY (male, sterile)
• Turner Syndrome = 45, X (female, sterile)
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Chromosome Deletions
• Deletions = missing chromosome segment
• Polytene chromosomes of Drosophila can be used to
map physically the locations of deletions
• Any recessive allele that is uncovered by a deletion
must be located inside the boundaries of the deletion
= deletion mapping
• Large deletions are often lethal
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Fig. 5.16
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Gene Duplications
• Duplication = chromosome segment present in multiple copies
• Tandem duplications = repeated segments are adjacent
• Tandem duplications often result from unequal crossing-over
due to mispairing of homologous chromosomes during meiotic
recombination
Fig. 5.17
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Red-Green Color Vision Genes
• Genes for red and green pigments are close on Xchromosome
• Green-pigment genes may be present in multiple
copies on the chromosome due to mispairing and
unequal crossing-over
• Unequal crossing-over between these genes during
meiotic recombination can also result in gene
deletion and color-blindness
• Crossing-over between red- and green-pigment
genes results in chimeric (composite) gene
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Chromosome Inversions
• Inversions = genetic
rearrangements in which the
order of genes in a
chromosome segment is
reversed
• Inversions do not alter the
genetic content but change the
linear sequence of genetic
information
• In an inversion heterozygote,
chromosomes twist into a loop
in the region in which the gene
order is inverted
Fig. 5.22
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Chromosome Inversions
• Paracentric inversion = does not include centromere;
• Crossing-over within a paracentric inversion loop
during recombination produces one acentric
(no centromere) and one dicentric (two centromeres)
chromosome
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Fig. 5.23
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Chromosome Inversions
• Pericentric inversion = includes centromere
• Crossing-over within a pericentric inversion loop during
homologous recombination results in duplications and deletions
of genetic information
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Reciprocal Translocations
• A chromosomal aberration resulting from the
interchange of parts between nonhomologous
chromosomes is called a translocation
• There is no loss of genetic information but the
functions of specific genes may be altered
• Translocations may produce position effects =
changes in gene function due to repositioning of
gene
• Gene expression may be elevated or decreased in
translocated gene
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Reciprocal Translocation
• Heterozygous translocation = one pair interchanged,
one pair normal
• Homozygous translocation = both pairs interchanged
Fig. 5.25
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Reciprocal Translocations
• Synapsis involving heterozygous reciprocal
translocation results in pairing of four pairs of sister
chromatids = quadrivalent
• Chromosome pairs may segregate in several ways
during meiosis, with three genetic outcomes:
• Adjacent-1 segregation: homologous centromeres
separate at anaphase I; gametes contain
duplications and deletions
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Reciprocal Translocation
• Adjacent-2 segregation: homologous centromeres
stay together at anaphase I; gametes have a
segment duplication and deletion
• Alternate segregation: half the gametes receive
both parts of the reciprocal translocation and the
other half receive both normal chromosomes; all
gametes are euploid, i.e have normal genetic
content, but half are translocation carriers
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Reciprocal Translocation
• The duplication and deficiency of gametes produced
by adjacent-1 and adjacent-2 segregation results in
the semisterility of genotypes that are heterozygous
for a reciprocal translocation
• The frequencies of each outcome is influenced by the
position of the translocation breakpoints, by the
number and distribution of chiasmata, and by whether
the quadrivalent tends to open out into a ring-shaped
structure on the metaphase plate
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Fig. 5.26
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Robertsonian Translocation
• A special case of nonreciprocal translocation is a
Robertsonian translocation = fusion of two
acrocentric chromosomes in the centromere region
• Translocation results in apparent loss of one
chromosome in karyotype analysis
• Genetic information is lost in
the tips of the translocated
acrocentric chromosomes
Fig. 5.27
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Robertsonian Translocation
Robertsonian translocations are an important risk
factor to be considered in Down syndrome. When
chromosome 21 is one of the acrocentrics in a
Robertsonian translocation, the rearrangement
leads to a familial type of Down syndrome
The heterozygous carrier is phenotypically
normal, but a high risk of Down syndrome results
from aberrant segregation in meiosis
Approximately 3 percent of children with Down
syndrome are found to have one parent with such
a translocation
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Polyploidy
• Polyploid species have multiple
complete sets of chromosomes
• The basic chromosome set, from
which all the other genomes are
formed, is called the monoploid set
• The haploid chromosome set is the
set of chromosomes present in a
gamete, irrespective of the
chromosome number in the species.
Fig. 529
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Polyploidy
• Polyploids can arise from genome duplications
occurring before or after fertilization
• Two mechanisms of asexual polyploidization:
 the increase in chromosome number takes place in meiosis
through the formation of unreduced gametes that have double
the normal complement of chromosomes
 the doubling of the chromosome number takes place in mitosis.
Chromosome doubling through an abortive mitotic division is
called endoreduplication
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Polyploidy
• Autopolyploids have all chromosomes in the
polyploid species derive from a single diploid
ancestral
• Allopolyploids have complete sets of
chromosomes from two or more different
ancestral species
• Chromosome painting = chromosomes
hybridized with fluorescent dye to show their
origins
• Plant cells with a single set of chromosomes can
be cultured
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Fig. 5.31
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Polyploidy
• The grass family illustrates the importance of
polyploidy and chromo-some rearrangements in
genome evolution
• The cereal grasses (rice, wheat, maize, millet, sugar
cane, sorghum, and other cereals) are our most
important crop plants
• Their genomes vary enormously in size: from 400 Mb
found in rice to 16,000 Mb found in wheat
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Polyploidy
• In spite of the large variation in chromosome
number and genome size, there are a number of
genetic and physical linkages between single-copy
genes that are remarkably conserved in all grasses
amid a background of rapidly evolving repetitive
DNA
• Each of the conserved regions (synteny groups)
can be identified in all the grasses and referred to
a similar region in the rice genome.
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