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Chromosomal
Abnormalites

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Karyotype = stained and photographed preparation of
metaphase chromosomes arranged according to their
size and position of centromeres

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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
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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Abnormal Chromosome
Numbers
• 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
• Most chromosome
abnormalities lethal,
frequently in spontaneous
abortions.
• Exceptions are trisomy 13,
trisomy 18, and trisomy 21
(Down syndrome), and the
Sex chromosomes

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EXAMPLES:
• Trisomy-X = 47, XXX
(female)
• Double-Y = 47, XYY
(male)
• Klinefelter Syndrome =
47, XXY (male, sterile)
• Turner Syndrome = 45, X
(female, sterile)
An extra X or Y chromosome
usually has a relatively mild effect.
Why?
1) X chromosme
inactivation/Dosage Compensation
2) Not much (essential) on the Y

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Chromosome Abnormalities
•Deletions/Duplications
•Inversions
•Translocations

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Deletions
Duplications

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A B C
C B A
B
B
A
A
C
C
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

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Paracentric inversion
• Does not include
centromere
• Crossing-over
produces one acentric
(no centromere) and
one dicentric (two
centromeres)
chromosome
Pericentric inversion
• Includes centromere
• Crossing-over
results in
duplications and
deletions of genetic
information

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Reciprocal
Translocations
• 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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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.
• Polyploids can arise from genome duplications
occurring before or after fertilization
Through the formation of unreduced gametes that
have double the normal complement of
chromosomes or
Through abortive mitotic division, called
endoreduplication.

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Polyploids can generate new
species

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A seedless watermelon is a sterile hybrid which is
created by crossing male pollen for a watermelon,
containing 22 chromosomes per cell, with a female
watermelon flower with 44 chromosomes per cell.

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The karyotype of the Chinook salmon has been characterized as 2N = 68, with 16
pairs of metacentric chromosomes and 18 pairs of acrocentric chromosomes (Simon
1963)

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