2. A deviation from the diploid(2n=2x)state represents a numerical chromosomal
aberration.This is also referred to as heteroploidy.
Heteroploidy may be grouped into two categories :
1.Aneuploidy 2.Euploidy
ANEUPLOIDY:- A loss or gain of one or few chromosome as compared to
somatic chromosome number of a species is known as aneuploidy.
Aneuploidy can be following types :-
1.MONOSOMY- The diploid organism which lacks one chromosome of a
single homologous pair is called monosomic. The genomic formula 2n-1
2.NULLISOMY-Diploids organisms which have lost a pair of homologous
chromosomes are called nullisomics with genomic formula 2n-2
3.TRISOMY- When somatic cells of an organism contain three copies of one
chromosome or organisms which have extra chromosome. The genomic
formula 2n+1
4.TETRASOMY-Tetrasomics have a perticular chromosome represented four
times.i.e four homologous. The genomic formula is 2n+2
3. TRISOMY
A trisomy is a type of polysomy in which there are
three copies of a perticular chromosome, instead of
normal two.
A trisomy is atype of aneuploidy (an abnormal
number of chromosome)
The no of possible trisomics in an organisms will be
equal to its haploid chromosome number.
In plants , the first case of trisomy was investigated in
jimson weed(Datura stramonium 2n=24 ) by
A.F.Blakeslee and J.Belling in 1924.
4. HOW DOES TRISOMY OCCUR?
Trisomy often occurs because of errors during
meiosis, which is the process by which gametes , or
eggs and sprem,are formed.
In meiosis ,the replicated chromosomes are sorted
into daughter cells in two steps,called meiosis I and
meiosis II .
As you can see ,at the end of the meiosis II ,each
gamete should be haploid,meaning that it has only
one copy of each of its chromosomes.
5.
6. However , just like any other cellular process ,
meiosis is error-prone, and sometimes the
chromosomes don’t separate normally. This is
called chromosome nondisjunction, and it can
happen either in meiosis I or meiosis II .
when chromosome nondisjunction occurs,the
chromosomes don’t separate normally. This
results in some gametes that have an extra
chromosome and others gametes are completely
missing a chromosome. when these gametes fuse
with normal gametes during fertilization, the
result is an aneuploid embryo.
7. Figure showing trisomy condition,when a normal gamete fuses
with a gamete that has an extra chromosome
8. For example-
In Barley (2n=14). So,n=7
• Here 7 trisomics are possible.
• one of the pair of chromosomes has an extra member
and forms a trivalents during Anaphase I of meiosis.
• Two chromosomes will go to one pole and one
chromosome will go to other pole.
• As a result, two types of gametes are formed i.e n and
n+1 . This is very common in plants and has variable
effects on phenotype.
9. Depending on the nature of extra chromosome ,
trisomics are grouped into the following categories:
1.Primary trisomics
2.Secondary trisomics
3.Tertiary trisomics
PRIMARY TRISOMICS
The extra chromosome is completely homologous to
one the chromosome pairs in the complement. Also
called ‘full trisomy’ that means an entire extra
chromose has been copied.
10. SECONDARY TRISOMICS
The extra chromosome has two identical arm ,
that means one chromosome arm is represented
4 times in secondary trisomics. The two arm of
an iso chromosome are identical in gene content.
The iso-chromosome is produced by misdivision
of centromere.
11. TERTIARY TRISOMIC
• The cell or individual carrying a translocated extra chromosome is
called tertiary trisomic.
• The end of the extra chromosome are homologous to the ends of
two different chromosome that are non homologous.
• In a reciprocal translation ; there are two possible types of
tertiary trisomy-
• The extra chromosome carries the centromere of one
chromosome & trans-located segment of other chromosome.
• The extra chromosome carries the centromere of other non-
homologous chromosome carries & the translocated segment of
the first.
12. Fig: the chromosome constitutions of primary,secondary
and tertiary trisomics
13. Chromosome pairing
• Chromosome configuration may be a trivalent or
bivalent + univalent based on chromosome pairing
and chiasma formation
1.Both the normal homologous pair to form a
bivalent while both arms of the iso-chromosome pair
together to form a U-shaped univalent.
2.Each arm of the iso-chromosome pair with a
normal homologous chromosome to form a trivalent
3.Trivalent are of different shapes depending upon
the number & position of chiasmata.
