Normal Chromosomal Variation

Abstract

Abstract Knowing what is normal and what is not is becoming a particular challenge in this era of molecular karyotyping. This chapter reviews the normal chromosome variation from classical times, now very well understood. This is followed by a discussion of the complexity and uncertainty that the molecular approach has, in this century, challenged researchers with. In particular, the chapter discusses the concept of the copy number variant (CNV) and how the harmlessness, or not, of a CNV may be assessed. Mention is made of CNVs potentially acting as “second hits,” such that, while nonpathogenic in one setting, they may contribute to an abnormal phenotype in the context of a..