Structural abnormalities
It results from chromosome breakage followed by reunion of the
broken ends in different configurations.
The factors responsible for structural abnormalities are mainly:
 Ionizing radiation
 Chemical agents
 Viral infections
Structural abnormalities-cont’d
Structural Abnormalities involved two main types:
 Balanced rearrangement : the exchange or rearrangement of
genetic material does not cause any missing or extra genetic
material:
 Balanced rearrangement are generally harmless with the
exception of some cases in which one of the breakpoints damages
an important functional genes.
 Balanced rearrangement carriers are often at risk of producing
children with an unbalanced chromosomal complement.
Structural abnormalities-cont’d
 Unbalanced rearrangement : In which the chromosomal
complement contains an incorrect amount of chromosomal
material and usually associated with very severe clinical effects.
Structural abnormalities-cont’d
Types of sstructural abnormalities:
1) Deletion-loss of genetic material results in monosomy for this
segment of chromosome and usually large deletion will be
incompatible with survive to term.
 Terminal deletion: involves a single break of the terminal part
of chromosome and broken part is subsequently lost
 Interstitial deletion: involves two breaks and the intervening
portion of the chromosome is lost.
Terminal
deletion
Interstitial
deletion
Structural abnormalities-cont’d
Terminal deletion: Cri-du Chat syndrome (46,XX,5p- or
46,XY,5p-)
 It results from deletion of a short arm of chromosome number 5.
 The incidence 1per 50 000 birth.
 The clinical features are:
 mewing cry of affected babies which disappear after the first few
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months of infancy.
Microcephaly.
Wide set eyes.
Epicanthus
Low – set or malformed ears .
Sever mental retardation .
Structural abnormalities-cont’d
Deletion
Some of deletion occur as micrdeletion which can not be detected by
microscope. It is needed for high resolution cytogenetics
/molecular Cytogenetics (FISH) or Molecular genetics
analysis ( PCR): e.g. Prader-Willi & Angelman syndromes
Structural abnormalities-cont’d
2) Duplication: some genetic material doubled
3) Inversion: It involves two breaks along the
chromosome in which a given segment is reversed in
position
 Pericentric inversion: inverted segment involved
both p & q arms with centromere.
 Paracentric inversion: inverted segment involved
only one arm.
Pericentric
inversion
Paracentric
inversion
duplication
Structural abnormalities-cont’d
4) Ring chromosome: loss of telomeres or ends of both arms of
chromosome and formation of a ring
structure
5) Isochromosome: creation of two non- identical chromosomes,
one is a combination of the two short arms, the other a
combination of the two long arms. New chromosome in which
the two arms genetically similar.
Structural abnormalities-cont’d
6) Insertion -no loss of genetic material, but a rearrangement of
genetic material to a non-homologous chromosome.
It results from two breaks of one chromosome
release a
fragment and one break in another chromosome to insert this
fragment
Structural abnormalities-cont’d
7) Translocation: It involves exchange of genetic material between
two chromosomes, involving two types:
 Reciprocal translocation: no loss of genetic material, but an
exchange of genetic material between two non-homologous
chromosomes to form two derivative chromosomes.
Structural abnormalities-cont’d
 Robertsonian translocation (centric fusion)
It results from break at or near the centromere in two acrocentric
chromosomes (D/D, D/G, G/G) and subsequent fusion of their long
arms. The fragment formed by fusion of their short arms is lost.
• It is a functionally balanced
translocation though chromosome
number is reduced 45.
• carriers are normal because of two
short arms that lost do not carry
important genetic material.