A functionally relevant deviation from the normal
structure is called a structural aberration.
This is to be differentiated from chromosomal
polymorphism. Loss (deletion) or doubling (duplication)
of a particular segment may occur. A
deletion may occur at the end of a chromosome
(terminal deletion) or within a chromosomal
segment (interstitial deletion). Prerequisite for
a terminal deletion is one break; for an interstitial
deletion, two breaks. A segment that has
been doubled is called a duplication. In
metaphase chromosomes, an aberration is seen
in both chromatids because as a rule it has occurred
before the S phase. Duplications and
deletions represent opposite, and in some respects
complementary, aberrations of chromosomal
structure
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