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IN THIS CHAPTER we deal with three kinds of classical chromosomal variation, having the quality in common that typically they carry no implications for abnormality in the person. First, there is the matter of normal variation. Normal chromosomes do not necessarily look exactly alike in different individuals, and some chromosomes show a remarkable degree of variation in their morphology. Obviously enough, it is crucial that the cytogeneticist distinguishes normal variation from abnormality. Generally, there is no point in reporting a particular variant to the referring practitioner or to the patient. But it is sometimes necessary to pursue the matter, with family studies, when it is not clear whether a particular finding is a normal variant or an abnormality, or when a previous report has sown a seed of doubt in the patient's mind. The study of normal variants is also a research activity in its own right.
Second, we bring together in this chapter a group somewhat flowing on from the first, comprising structural rearrangements due to the translocation of harmless material, such as heterochromatin and nucleolar organizing regions, from one chromosome to another. Third, we treat a group categorized as the “euchromatic abnormality without phenotypic effect.” These are chromosomal differences that do not too comfortably accommodate the expressions “normal” and “variant,” and yet “abnormality” may convey too strong a sense. Anomaly may be a better word, or if “abnormality” is to be used, it should retain, in the reader's mind (if not in the text hereafter), its quotation marks. These are deletions and duplications that might at first sight have been thought to be abnormalities that would be associated with some clinical defect—deletions up to 16 Mb and duplications as large as 30 Mb in size—but which are in fact observed in normal persons. Finally, in the new era of microarrays, copy number variants loom large as a new form of variation, and this demands its own chapter (which follows this one).
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