Chromosomal aberrations arise from structural changes or alterations in chromosome number. There are two main types of chromosomal aberrations: structural aberrations which involve changes in chromosome structure, and numerical aberrations which involve changes in chromosome number. Common types of structural aberrations discussed in the document include deletions, duplications, inversions, and translocations. Deletions involve the loss of a chromosome segment, duplications the addition of an extra segment, inversions reverse the orientation of a segment, and translocations involve segments moving to new chromosomes. These structural changes can have varying genetic effects depending on the location and size of the alteration.

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2. Chromosomal Aberrations
• The somatic (2n) and gametic (n) chromosome numbers of a
species ordinarily remain constant.
• This is due to the extremely precise mitotic and meiotic cell
division.
• Somatic cells of a diploid species contain two copies of each
chromosome, which are called homologous chromosome.
• Their gametes, therefore contain only one copy of each
chromosome, that is they contain one chromosome complement or
genome.
• Each chromosome of a genome contains a definite numbers and
kinds of genes, which are arranged in a definite sequence.
• Sometime due to mutation or spontaneous (without any known
causal factors), variation in chromosomal number or structure do
arise in nature. - Chromosomal aberrations.
• Chromosomal aberration may be grouped into two broad classes:
1. Structural and 2. Numerical 2

3. INTRODUCTION:
• Chromosome structure variations result from
chromosome breakage.
• Broken chromosomes tend to re-join; if there is more
than one break, rejoining occurs at random and not
necessarily with the correct ends.
• The result is structural changes in the chromosomes.
• Chromosome breakage is caused by X-rays, various
chemicals, and can also occur spontaneously.
• There are four common type of structural aberrations:
1.Deletion or Deficiency,
2.Duplication or Repeat
3.Inversion, and
4.Translocation 3

4. DELETION OR DEFICIENCY
Loss of a chromosome segment is known as deletion
or deficiency
It can be terminal deletion or interstitial or
intercalary deletion.
A single break near the end of the chromosome
would be expected to result in terminal deficiency.
If two breaks occur, a section may be deleted and an
intercalary deficiency created.
Terminal deficiencies might seem less complicated.
But majority of deficiencies detected are intercalary
type within the chromosome.
Deletion was the first structural aberration detected
by Bridges in 1917 from his genetic studies on X
chromosome of Drosophila. 4

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Chromosome with deletion can never revert back to a normal
condition and transmitted to next generation.
In intercalary deletion, broken acentric fragment of
chromosomes appear as small chromatin bodies in cells known
as Micronuclei.
Homozygous deletion is lethal.
Heterozygous deletions can revealed a phenomenon known as
Pseudodominance /
 One copy of a gene is deleted
 So the recessive allele on the other chromosome is now
expressed
Eg., sex linked lethal and dominant notch-wing etc. in
Drosophila.
Cri-du-chat (Cat cry syndrome) of babies results from the
chromosome deficiency in the short arm of chromosome 5 .

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•Deletion can be recognized during meiotic pairing of
homologus chromosome and during somatic pairing in
specialized tissue like salivary gland of Drosophila or during
pachytene in Maize.
•Due to intercalary deletion unpaired loop is formed.

8. DUPLICATION
The presence of an additional chromosome
segment, as compared to that normally present in a
nucleus is known as Duplication.
In a diploid organism, presence of a chromosome
segment in more than two copies per nucleus is
called duplication.
Four types of duplication:
1. Tandem duplication
2. Reverse tandem duplication
3. Displaced duplication
4. Translocation duplication
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9.  The extra chromosome segment
may be located immediately after
the normal segment in precisely
the same orientation forms the
tandem .
 When the gene sequence in the
extra segment of a tandem in the
reverse order i.e, inverted , it is
known as reverse tandem
duplication.
 In some cases, the extra segment
may be located either on the
same chromosome or on a
different one but away from the
normal segment –termed as
displaced duplication.
 Later condition is also termed as
translocation duplication. 9

10. ORIGIN
 Origin of duplication involves
chromosome breakage and
reunion of chromosome segment
with its homologous chromosome.
 As a result, one of the two
homologous involved in the
production of a duplication ends
up with a deficiency, while the
other has a duplication for the
concerned segment.
 Another phenomenon, known as
unequal crossing over, also leads
to exactly the same consequences
for small chromosome segments.
 For e.g., duplication of the band
16A of X chromosome of
Drosophila produces Bar eye.
 This duplication is believed to
originate due to unequal crossing
over between the two normal X
chromosomes of female. 10

11. GENETIC EFFECTS
 Majority of small duplications have no phenotypic effect
 However, they provide raw material for evolutionary change
 Lead to the formation of gene families.
 A gene family consists of two or more genes that are
similar to each other derived from a common gene
ancestor ex- globin gene family whose Genes encode
proteins that bind oxygen
Tandem duplications play a major role in evolution, because
it is easy to generate extra copies of the duplicated genes
through the process of unequal crossing over. These extra
copies can then mutate to take on altered roles in the cell, or
they can become pseudogenes, inactive forms of the gene, by
mutation. 11

12. INVERSION
• When a segment of chromosome is oriented in the reverse
direction, such segment said to be inverted and the phenomenon
is termed as inversion.
• The existence of inversion was first detected by Strutevant and
Plunkett in 1926.
• Inversion occur when parts of chromosomes become detached ,
turn through 1800 and are reinserted in such a way that the genes
are in reversed order.
• For example, a certain segment may be broken in two places,
and the breaks may be in close proximity because of chance loop
in the chromosome.
• When they rejoin, the wrong ends may become connected.
• The part on one side of the loop connects with broken end
different from the one with which it was formerly connected.
• This leaves the other two broken ends to become attached.
• The part within the loop thus becomes turned around or invert
12

13.  Inversion may be classified into two types:
 PERICENTRIC - include the centromere
 PARACENTRIC - do not include the centromere
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14.  Individuals with one copy of a normal chromosome and one copy of an inverted
chromosome
 Usually phenotypically normal
 Have a high probability of producing gametes that are abnormal in genetic
content
 Abnormality due to crossing-over within the inversion interval
 During meiosis I, homologous chromosomes synapse with each other
 For the normal and inversion chromosome to synapse properly, an inversion
loop must form
 If a cross-over occurs within the inversion loop, highly abnormal
chromosomes are produced
Inversions in natural populations
 In natural populations, pericentric inversions are much less frequent than
paracentric inversions.
 In many sp, however, pericentric inversions are relatively common, e.g., in
some Drosophila. Grasshoppers etc.
 Paracentric inversions appear to be very frequent in natural populations of
Zea maize etc.
INVERSION HETEROZYGOTES
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15. When a paracentric inversion crosses
over with a normal chromosome, the
resulting chromosomes are an acentric, with
no centromeres, and a dicentric, with 2
centromeres.
The acentric chromosome isn't attached to
the spindle, so it gets lost during cell
division, and the dicentric is usually pulled
apart (broken) by the spindle pulling the two
centromeres in opposite directions. These
conditions are lethal.
Eg, Dicentric bridges formed in Maize
female tissue and pollen grains are sterile. 15

16. Crossing Over Within Inversion Interval
Generates Unequal Sets of Chromatids
PARACENTRIC INVERSION
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17. When a pericentric inversion crosses over
with a normal chromosome, the resulting
chromosomes are both duplicated for some
genes and deleted for other genes. (They do
have 1 centromere apiece though). The gametes
resulting from these are aneuploid and do not
survive. Eg, Drosophila.
Thus, either kind of inversion has lethal
results when it crosses over with a normal
chromosome. The only offspring that survive
are those that didn't have a crossover. Thus
when you count the offspring you only see the
non-crossovers, so it appears that crossing over
has been suppressed. 17

18. Crossing Over Within Inversion Interval
Generates Unequal Sets of Chromatids
PERICENTRIC INVERSION
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19. Inversions Prevent Generation of Recombinant
Offspring Genotypes
• Only parental chromosomes (non-
recombinants) will produce normal progeny
after fertilization
PARACENTRICPERICENTRIC 19

20. TRANSLOCATION
Integration of a chromosome segment into a nonhomologous
chromosome is known as translocation.
Three types:
1. simple translocation
2. shift
3. reciprocal translocation.
1. Simple translocation: In this case, terminal segment of a
chromosome is integrated at one end of a non-homologous
region. Simple translocations are rather rare.
2. Shift: In shift, an intercalary segment of a chromosome is
integrated within a non-homologous chromosome. Such
translocations are known in the populations of
Drosophila, Neurospora etc.
3. Reciprocal translocation: It is produced when two non-
homologous chromosomes exchange segments – i.e., segments
reciprocally transferred. Translocation of this type is most
common eg, Rhoeo, Oenothera, Tradescantia etc. 20

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SHIFT RECIPROCAL

22.  In reciprocal translocations two non-homologous
chromosomes exchange genetic material
 Usually generate so-called balanced translocations
 Usually without phenotypic consequences
 Although can result in position effect.
CYTOLOGY OF TRANSLOCATION HETEROZYGOTE
 In simple translocations the transfer of genetic material
occurs in only one direction
 These are also called unbalanced translocations
 Unbalanced translocations are associated with phenotypic
abnormalities or even lethality
 Example: Familial Down Syndrome
 In this condition, the majority of chromosome 21 is attached to
chromosome 14. 22

23.  Individuals carrying balanced translocations have
a greater risk of producing gametes with
unbalanced combinations of chromosomes.
 This depends on the segregation pattern during
meiosis I
 During meiosis I, homologous chromosomes
synapse with each other
 For the translocated chromosome to synapse
properly, a translocation cross must form
Balanced Translocations and Gamete Production
BALANCED LETHALS AND GAMETIC
COMPLEXES
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24.  Meiotic segregation can occur in one of three ways
 1. Alternate segregation
 Chromosomes on opposite sides of the translocation cross
segregate into the same cell
 Leads to balanced gametes
 Both contain a complete set of genes and are thus viable
 2. Adjacent-1 segregation
 Adjacent non-homologous chromosomes segregate into the
same cell
 Leads to unbalanced gametes
 Both have duplications and deletions and are thus inviable
 3. Adjacent-2 segregation
 Adjacent homologous chromosomes segregate into the same
cell
 Leads to unbalanced gametes
 Both have duplications and deletions and are thus inviable24

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