Biomedical Engineering Reference
In-Depth Information
Table 7.1
Common chromosomal abnormalities
Type of abnormality
Defi nition
Monosomy
One chromosome absent
Trisomy
One chromosome extra
Deletion
Part of a chromosome missing
Duplication
Part of a chromosome present twice
Inversion
Segment of a chromosome reversed
Translocation
Two chromosomes join long arms or exchange parts
Ring chromosome
A chromosome that forms a ring due to deletions in
telomeres, which cause ends to adhere
Isochromosome
A chromosome with identical arms
Aneuploidy
An extra or missing chromosome
Polypoidy
Extra chromosome sets
the classical clinical signs associated with a particular chromosomal disor-
der. For example, mosaic Down syndrome patients can escape some of the
stigmata associated with the condition. However, each mosaic case is unique
and must be evaluated on an individual basis.
7.5.2 Structural abnormalities
The second basic mutational event one sees at the chromosomal level is
structural changes to the chromosomes. These are rearrangements to the
chromosomes that are either balanced or unbalanced. In balanced changes,
the genetic material has been displaced but the amount of genetic informa-
tion has remained intact. In unbalanced changes, there is either a gain or
loss of genetic material. Both events can have severe clinical signifi cance
since they basically disturb the amount or correct orientation of the genes
located on the chromosomes. Table 7.1 illustrates some of the more com-
mon structural changes that have been identifi e d .
While it is easy to see how deletions or duplications of the chromosomes
can cause dramatic phenotypic effects because of the loss or gain of genes,
reciprocal translocations, even when they appear balanced, can have impor-
tant consequences.
Reciprocal translocations
are exchanges between two
or more chromosomes that result in recombinant chromosomes that have
structures that are composed of the exchanged chromosomes. Sometimes
these translocation events cause the gene structure or expression of the
gene to be disturbed and can result in serious phenotypic consequences for
the carrier individual. At other times, the exchange does not directly affect
the gene structure of its expression, but carriers of the reciprocal transloca-
tion may be at a much higher than normal risk of having genetically unbal-
anced offspring with serious birth defects.
