Two types of chromosomal aberrations can occur in cells: changes in
chromosome structure and changes in chromosome number. Structural
changes include deletions, duplications, inversions, and translocations
(Figure 6-2). Deletions are chromosomal changes in which one or
more genes or chromosomal segments are lost. Duplications occur when
one or more copies of a chromosomal segment are present on the same
or different chromosomes. Deletions and duplications can occur in the
same mutational event when two homologous DNA strands overlap,
break at the same time at two different (nonhomologous) points, and then
rejoin with the wrong strand. One of the strands will be missing one or more genes, and the reciprocal strand will have an extra copy of one or
more genes. Inversions occur when a breakage in one of the chromosomes
occurs and the segment rotates 180° before it rejoins. Translocations
take place when nonhomologous chromosomes break and exchange
|Figure 6-2 Structural aberrations of chromosomes.
In diploid (2n) organisms, there are two major types of chromosomal
aberrations that are the result of changes in chromosome number.
These are polyploidy and aneuploidy. Polyploidy results when cells acquire
one or more sets of chromosomes beyond the "normal" number of
sets. For example, triploids (3n) contain one extra set of chromosomes,
and would therefore be sterile, since they cannot produce balanced gametes
|Figure 6-3 Aneuploid gamete formation by nondisjunction
during (a) meiosis I and (b) meiosis II.
Aneuploids are the result of changes in the individual number of homologous
chromosomes in a set. This usually results from nondisjunction
during meiosis (Figure 6-3). The aneuploid condition that results
in three copies of a given chromosome is known as trisomy (2n+1).