Numerical Chromosomal Aberration

1. A
somewhat
brief
REVIEW

2.  A state or condition markedly different from the norm.
 Chromosomal aberration:
Possession of sets of chromosomes or genomes, gives a specific
chromosome number to each species. But sometimes, some irregularities may occur
during mitosis, meiosis or fertilization and may produce cells with variant chromosome
number.
 A deviation from the ploidy states represents a Numerical Chromosomal Aberration .

4. It is a condition of having fewer or extra chromosomes than the normal genome number of the species.
Organism exhibiting the condition is called Aneuploids or Heteroploids.
Aneuploidy commonly arise due to non disjunction of 2 chromosomes of homologous pairs so that one gamete
comes to have an extra chromosome (N+1)while the other becomes deficient in one chromosome(N-1).
Another method of obtaining aneuploids is through loss of chromosomes from a normal or polyploidy karyotype
due to faulty mitosis.
Now,fusion with similar or normal gamestes give rise to 4 types of aneuploids.
2N-1(monosomy)
2N-2(nullisomy)
2N+1(trisomy)
2N+2(tetrasomy)

5.  Has one chromosome in triplicate format of 2N+1.
Trisomy shows a number of changes of which some are lethal.
In a trisomic, one of the pairs of chromosomes has an extra member and forms a
trivalent 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.
In plants, the first case of trisomy was investigated in Jimson weed i.e. Datura
stramonium by A.F. Blakeslee and J.Belling in 1924. The size, shape and spine
characteristics of seed capsule of this trisomic plant were different from seed capsule of
wild type species.
Human trisomic syndrome i.e Down’s syndrome ,triplicate form of the chromosome
number 21 leading to developmental delays and physical disabilities.
Patau’s syndrome -13th chr. Trisomy has high MR.

6.  A particular chromosome represented four times.
 2N+2
 The four homologues tend to form a quadrivalent at meiosis and disjunction often
proceeds fairly regularly, two by two.
 All the 21 possible tetrasomics in wheat are viable.

7.  The diploid organism which lacks one chromosome of a single homologous pair is
called monosomic with genomic formula 2n-1.
 A monosomic produces two types of gametes n and n-1 because single chromosome
without a pairing partner may go to either of poles during meiosis. i.e. formula 2n-
1.
 Monosomics have been used extensively in wheat breeding for the purpose of
chromosome substitution.
 Humans suffer from Turner’s syndrome(44+X).

8.  Diploid organisms which have lost a pair of homologous chromosomes are called
nullisomics with genomic formula 2n-2.
 Nullisomics are not usually found in natural populations, but have to be obtained
by inter crossing or selfing of monosomics (2n-1). These can occur by fusion of two
gametes that are lacking in the same chromosome.

9.  The organism or karyotype with more than two genomes are called polyploids.
 Generally ploidy levels higher than tetraploid are not commonly encountered in the
natural population. However there are some exceptions. Eg: hexaploid (6x) wheat,
octaploids (8x) straw berries, many commercial fruits and ornamental plants, liver
cells of man etc.
 These occur in nature due to failure of chromosome to separate at the time of
anaphase either due to non disjunction or due to non formation of spindle fibres.
 Can also be artificially induced by application of Colchicine & Granosan.
 Polyploids with odd number of genomes are sexually sterile because the odd
chromosomes cannot form synapsis i.e. the pairing of 2 non sister homologous
chromosomes and being odd in number the recombination does not happen.
 So they are propagated vegetatively e.g. Banana ,pineapple(more common in plants)
 Polyploids also do not cross breed freely with diploids.They generally have a gigas
effect at both morphological and biochemical level due to increase in frequency of
dominant alleles.

10. Now,the polyploidy is of 2 types,

11.  It is a type of polyploidy in which there is a numerical increase of same genome.
e.g. Autotriploid (AAA)
Autotetraploid(AAAA)
Some of the crop & garden plants are autopolyploids .
e.g. Maize, Rice,Gram
Autopolyploidy induces the Gigas effecf.
In general, expression is exaggerated either in positive / negative direction.

12.  a) Auto triploids: Auto triploids have three complete sets of genomes of
the same species in somatic cell.
 Generally, in nature they originate by the fusion of a haploid gamete with a
diploid gamete (unreduced gamete).
 Triploids can be produced artificially by crossing between autotetraploid and
diploid species.
 Triploids are useful only in those plant species which propagate asexually like
banana, sugarcane, apple, sugarbeet, watermelon, tomato, Cynodon dactylon
(doob grass).
 b) Auto tetraploids: In autotetraploids, four copies of the genome of same
species (AAAA or BBBB) are present.
 They may arise spontaneously or can be induced artificially by doubling the
chromosomes of a diploid species with colchicine treatment.
 Eg: Rye, alfalfa, grapes, groundnut, potato, coffee, Oenothera lamarckiana.

13.  These has been developed through hybridization between 2 species followed by
doubling of the chromosomes.
e.g. AABB
Here allotetraploid is the common type and for fact Allopolyploids functions as a
complete new species.

14.  Inter-specific crossing followed by chromosome doubling in nature have resulted
in origin of some natural allopolyploid crops like cotton, tobacco, mustard, wheat,
etc.
 (i) Cotton: The new world cotton (Gossypium hirsutum)
J.O. Beasley crossed old world cotton (Gossypium herbaceum) with
American cotton (Gossypium raimondii) and doubled the chromosome number in F1
hybrids. The allopolyploid thus produced resembled the cultivated new world cotton
(Gossypium hirsutum) and when crossed, with it gave fertile F1 hybrids.
 (ii)Tobacco: There are two cultivated species of tobacco. i. e. Nicotiana tabacum
and Nicotiana rustica.
Nicotiana tabacum is an allotetraploid and available evidence suggests that it is
derived from a cross between Nicotiana sylvestris x Nicotiana tomentosa

15.  Artificial allopolyploids have been synthesized in some crops either to study the
origin of naturally available allopolyploids or to explore the possibilities of creating
new species.
 i) Triticale: Triticale, a man made cereal, is first produced by Muntzing. Triticale is a
new crop species synthesized by crossing wheat and rye (Secale cereale).
a) Some triticales are hexaploids and are developed from a cross between tetraploid
wheat (Triticum turgidum ) and rye
b) Octaploid triticales are produced from a cross between hexaploid wheat (Triticum
aestivum) and rye.
 (iii) Raphanobrassica : Russian geneticist G.D. Karpechenko in 1927 synthesized
Raphanobrassica, which is an allopolyploid resulting from a cross between
Radish and cabbage.

16. For giving me your valuable time
and a much needed opportunity.