• Chromosomal mutations are studied under two broad
categories:
(A) Changes in chromosomal morphology/structure or
chromosomal aberrations.
(B) Changes in number of chromosomes or ploidy.
Chromosomal aberrations
• In a chromosomal aberration, there is an alteration in
the structural organization of the chromosome.
• A break in the chromatid may lead to a loss of material,
called a deletion, and in cases, in which there are two
breaks, a translocation, an inversion, or an exchange
between sister chromatids may occur.
• In all these cases, the chromosome change can be
observed microscopically, especially at the stages of
chromosome condensation.
• Chromosomal aberrations should be differentiated from
gene mutations (also called point mutations), in
which the changes are produced at the molecular
level.
• In these, the alteration resides in the genetic code (i.e.,
in the sequence of DNA bases), and can be detected
only by its genetic or molecular expression.
intra and interchromosomal aberrations
• Aberrations are of two main types : intrachromosomal
and interchromosomal aberrations.
• Aberrations may be intrachromosomal, when
the alteration is within a single chromosome, or
interchromosomal, when it involves the intervention
of two or more chromosomes.
• The main types of chromosomal aberrations are briefly
mentioned below:
intrachromosomal aberrations
- Deficiency or deletion
• This aberration involves the loss of chromosomal
material, and maybe either terminal (at the end of a
chromosome) or intercalary (within the chromosome);the
former aberration originates from a single break at
the G 1 phase, and the latter from two breaks.
• In heterozygous deficiency, one chromosome is
normal, but its homologue is deficient.
• Animals with a homozygous deficiency usually do
not survive to an adult stage, because a complete
set of genes is lacking.
• Notched wing margin in Drosophila is due to
deletion of a segment in X-chromosome.
• Cri-du-chat syndrome in humans is caused by deletion
of a small part of short arm of chromosome 5.- inversion
• An inversion is a chromosomal aberration in which a
chromosome segment is inverted by 180º.
• Inversions are called pericentric, when the segment
includes the kinetochore, and paracentric, if the
kinetochore is located outside the segment.
• In these aberrations, there is a typical configuration
at pachynema consisting of a loop, that allows the
pairing of the inverted segment.
• In the paracentric inversion, acentric and dicentric
chromatids are formed.
interchromosomal aberrations
- Duplication
• Duplication occurs when a segment of the chromosome
is represented two or more times.
• The duplicated fragment may be free, with a kineto-
chore of its own, or it may be incorporated into a
chromosomal segment of the normal complement.
• Duplication increases the number of genes in the
genotype. It increases genetic redundancy, protects
the organism against harmful mutations, and allows
development of new traits.
• Sometimes, duplication of a gene has deleterious
effects. For example, in Drosophila, when the B gene