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Variation in chromosome structure and
number
Short arm;
For the French, petite
Long arm
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• Genetic variation refers to differences between
members of the same species or those of different
species
– Allelic variations are due to mutations in particular
genes
– Chromosomal aberrations are substantial changes
in chromosome structure or number
• These typically affect more than one gene
• They are quite common, which is surprising
Cytogenetics -The field of genetics that involves the
microscopic examination of chromosomes
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Human
chromosome 1
Human
chromosome 21
Deficiencies/ Deletion

A chromosomal deficiency occurs when a chromosome
breaks and a fragment is lost
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Duplications

A chromosomal duplication is usually caused by
abnormal events during recombination

A repetitive sequence contained multiple times can cause
alignment where the alleles don’t actually match.
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Genes derived
from a single
ancestral gene
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Inversions

A chromosomal inversion is a segment that has
been flipped to the opposite orientation
Centromere lies
within inverted
region
Centromere lies
outside
inverted region
Translocation
VARIATION IN CHROMOSOME NUMBER
• Chromosome numbers can vary in two main ways
– Euploidy
• Variation in the number of complete sets of chromosome
– Aneuploidy
• Variation in the number of particular chromosomes within a set
– Euploid variations occur occasionally in animals and
frequently in plants
– Aneuploid variations, on the other hand, are regarded
as abnormal conditions
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Polyploid organisms
have three or more
sets of chromosomes
Individual is said
to be trisomic
Individual is said
to be monosomic
In most cases, these
effects are detrimental
They produce
individuals that are less
likely to survive than a
euploid individual
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Trisomics for individual chromosomes
Datura stramonium
Genetic variation is a result of various types of
changes at the molecular level
Changes in chromosome number
•Single chromosomes may be lost or gained
•A whole set of chromosomes may be
inherited

Some human aneuploidies are influenced by the age of the parents


Older parents more likely to produce abnormal offspring
Example: Down syndrome (Trisomy 21)
 Incidence rises with the age of either parent, especially mothers
Familial down Syndrome

Complete nondisjunction can produce an individual
with one or more sets of chromosomes

This condition is termed autopolyploidy
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Interspecies Crosses

A much more common mechanism for changes in
the number of sets of chromosomes is alloploidy


It is the result of interspecies crosses
Most likely occurs between closely related species
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

An allodiploid has one set of chromosomes from two
different species
An allopolyploid contains a combination of both
autopolyploidy and alloploidy
An allotetraploid:
Contains two
complete sets of
chromosomes
from two different
species
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
Polyploids having an odd number of chromosome sets are usually
sterile
 These plants produce highly aneuploid gametes

Example: In a triploid organism there is an unequal separation of
homologous chromosomes (three each) during anaphase I
Anaphase I of meiosis
AnaaAanand two copies
of other chromosomes
During
fertilization,
these gametes
produce an
individual that
is trisomic
for the
missing
chromosome
During
fertilization,
these gametes
produce an
individual that
is monosomic
for the
missing
chromosome
All four gametes are abnormal
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50 % Abnormal
gametes
50 % Normal
gametes
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This cell will be
trisomic
This cell will be
monosomic
This cell will be
monosomic
This cell will be
normal
Will be degraded if
left outside of the
nucleus when nuclear
envelope reforms
No synapsis
between the 9
radish and 9
cabbage
chromosomes
Proper synapsis
between the 18
radish
chromosomes
and the 18
cabbage
chromosomes
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Octoploid strawberries
Hexaploid wheat
Triploid seedless fruits
Asian carp is a polyploid, a mix of four species, the bighead carp, the black
carp, the grass carp, and the silver carp.
Mule is a result of a cross between
a male donkey (Chr# 62) & a female
horse (Chr# 64)