The condition in which there is a change as a result of
gains or losses in the number of chromosomes but not in
the complete set is called aneuploidy.The word
aneuploid is derived from a greek word (aneu=uneven
and ploid=unit).Aneuploidy can be divided into following
two classes:
Hypoploid(due to loss of one or more chromosomes)
Hyperploidy(due to addition of one or more
chromosomes to the complete chromosome set).
The aneuploidy arise as a result of primary or secondary
nondisjunction.The loss of one chromosome produces a
monosomic(2n-1) and the condition is termed as
monosomy and the gain of one chromosome produces a
trisomic (2n+1) and the conditions is known as
trisomy.The addition of two or more chromosomes is
known as tetrasomy and pentasomy respectively, and
the individuals are known as tetrasomic and
pentasomic.In some cases a pair of homologus
chromosomes is lost (2n-2) such individuals are termed
as Nullisomics and the condition is called nullisomy.
Monosomy (2n-1)
The monosomics lack a complete chromosomes and this
create a genetic imbalance as the expression of only one
allel at each locus of the chromosomes is
inadequate.The expression of genetic information during
early development is very delicately regulated and a
sensitive equilibrium of gene product is required to
ensure normal development.
Due to loss of one chromosome in a monosomic, this
equilibrium is not achieved, therefore monosomy is not
tolerated in diploids.
Double monosomic (2n-1-1) and triple monosomic
(2n-1-1-1) could also could also be produced in polyploid
such as wheat.Double monosomic means that the
chromosome number is 2n-2 like that in nullisomic, but
the missing chromosomes are nonhomologus .The same
explanation is applied to the triple monosomics.
Trisomy(2n+1)
Trisomics are those diploid organisms which have an
extra chromosome (2n + 1).
Trisomics were obtained for the first time in jimson weed
(Datura stramonium) by A.F. Blakeslee and J. Belling
(1924).
Trisomics may be of follwoing types:
Primary trisomics
when the extra chromosome is identical to its homologs,
such a trisomic is called primary trisomic
Secondary trisomics
the secondary trisomic means that the extra
chromosome should be an isochromosome (i.e., both
chromosome arms genetically similar).
tertiary trisomic
In this case the extra chromosome is the product of
translocation.
gains or losses in the number of chromosomes but not in
the complete set is called aneuploidy.The word
aneuploid is derived from a greek word (aneu=uneven
and ploid=unit).Aneuploidy can be divided into following
two classes:
Hypoploid(due to loss of one or more chromosomes)
Hyperploidy(due to addition of one or more
chromosomes to the complete chromosome set).
The aneuploidy arise as a result of primary or secondary
nondisjunction.The loss of one chromosome produces a
monosomic(2n-1) and the condition is termed as
monosomy and the gain of one chromosome produces a
trisomic (2n+1) and the conditions is known as
trisomy.The addition of two or more chromosomes is
known as tetrasomy and pentasomy respectively, and
the individuals are known as tetrasomic and
pentasomic.In some cases a pair of homologus
chromosomes is lost (2n-2) such individuals are termed
as Nullisomics and the condition is called nullisomy.
Monosomy (2n-1)
The monosomics lack a complete chromosomes and this
create a genetic imbalance as the expression of only one
allel at each locus of the chromosomes is
inadequate.The expression of genetic information during
early development is very delicately regulated and a
sensitive equilibrium of gene product is required to
ensure normal development.
Due to loss of one chromosome in a monosomic, this
equilibrium is not achieved, therefore monosomy is not
tolerated in diploids.
Double monosomic (2n-1-1) and triple monosomic
(2n-1-1-1) could also could also be produced in polyploid
such as wheat.Double monosomic means that the
chromosome number is 2n-2 like that in nullisomic, but
the missing chromosomes are nonhomologus .The same
explanation is applied to the triple monosomics.
Trisomy(2n+1)
Trisomics are those diploid organisms which have an
extra chromosome (2n + 1).
Trisomics were obtained for the first time in jimson weed
(Datura stramonium) by A.F. Blakeslee and J. Belling
(1924).
Trisomics may be of follwoing types:
Primary trisomics
when the extra chromosome is identical to its homologs,
such a trisomic is called primary trisomic
Secondary trisomics
the secondary trisomic means that the extra
chromosome should be an isochromosome (i.e., both
chromosome arms genetically similar).
tertiary trisomic
In this case the extra chromosome is the product of
translocation.
 |
| Trisomy |
Example of Trisomy in humans
Down’s syndrome (DS) or Trisomy-21
Down’s syndrome is named after the physician
J.Langdon Down who first described this genetic defect
in 1866 and it was formally called mongolism or
mongolian idiocy. It is usually associated with a trisomic
condition for one of the smallest human autosomes (i.e.,
chromosome 21). It is the most common chromosomal
abnormality in live births (1/650 births). There are about
50 physical characteristics shown by DS infants soon
after birth. These include mild or moderate mental
retardation; eyes that slant up and out with internal epicanthal folds; a tongue that is large, swollen and
protruding ; small and under developed ears;short
stature; stubby fingers; an enlarged liver and spleen.
Nondisjunction of chromosome pair 21 during oogenesis
is the main cause of occurrence of trisomy-21.
Nullisomy
An organism which has lost a chromosome pair is a
nullosomic. The nullosomic organism has the genomic
formula (2n–2). A nullosomic diploid often does not
survive, however, a nullosomic polyploid (e.g., hexaploid
wheat, 6x–2) may survive but exhibit reduced vigour and
fertility.
 |
| Nullisomy |
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