2. Structural Aberrations
• The chromosomal aberrations in which alternation of the
structure of chromosome(sequence of genes or kind of
genes in chromosome or no. of genes)occur—Structural
Aberration.
• Types of Structural aberrations:-
a) Changes in number of genes
i. Deletion (Terminal, Intercalary)
ii. Duplication (Intrachromosomal,
Interchromosomal)
b) Changes in the location of genes
i. Inversions (Paracentric, Pericentric)
ii. Translocations (Intrachromosomal,
Interchromosomal)
3. Deletion
• It is a loss of a (generally small) segment of
chromosome during replication process. Any
number of nucleotides can be deleted, from a
single base to an entire piece of chromosome.
• It can be Spontaneous or may be induced
(radiation, UV, chemicals, viruses).
• Detection is based on unpaired region of normal
chromosome that produces a loop during
pachytene stage.
• Firstly, observed by Bridges in 1917 in Drosophila.
a) Terminal deletion: It is a loss of either terminal
segment of a chromosome.
• In 1938, Muller postulated that loss of telomere
makes chromosome unstable, so it is not
commonly observed. Ex:- maize.
b) Intercalary deletion: It is a loss of segment in
between centromere and telomere.
4. Duplication
• The occurrence of a segment twice in the same chromosomes is known
as duplication. It occurs due to unequal crossing over or crossing overin
inversion or translocated heterozygotes. It is detected by presence of
loop in duplicatedregion during pachytene stage.
• Intrachromosomal Duplication:
o Tandem – In this case sequence of genes in the duplicated segment is
similar to the sequence of genes in the original segment of a
chromosome.
o Reverse tandem – The sequence of genes in the duplicated segment is
reverse to the sequence of genes in the original segment of a
chromosome.
• Interchromosomal Duplication:
o Displaced - Duplicating segment is incorporated away from
corresponding segment on the same chromosome.
o Translocated - Duplicated chromosomal segment is
incorporated on different chromosome.
5. Effects Of Deletion & Duplication
• Deletion:-
a) Crossing over not occur.
b) Harmful effect on diploid
organisms.
c) Morphological effects.
• Duplication:-
a) Origin of new genes mainly due to
duplication results in evolution.
b) Activity of certain enzymes may be
doubled.
