ABSTRACT
FSHD Facioscapulohumeral Muscular Dystrophy: Clinical Medicine and Molecular Cell Biology, edited by Meena Upadhyaya and David N.Cooper. © 2004 Garland/BIOS Scientific Publishers Limited, Abingdon.
12.1 Introduction
The term mosaic was used in ancient times to refer to floors or windows with numerous little colourful pieces of stone or glass representing mostly religious or historical events. In a biological context, mosaicism may refer to two distinct phenomena. Mosaicism can refer to a viral disease of plants, resulting in light and dark areas in the leaves, which then often become shrivelled. An alternative definition, and one more appropriate for this chapter, is a condition in which an individual has two or more genetically distinct cell lines derived from a single zygote, but differing by virtue of the presence of a mutation or non-disjunction (Gelehrter et al., 1998). The first paper on the existence of chromosomal mosaicism was published in 1959. Here, the phenotype of a case of Klinefelter syndrome was described, in a man with two distinct cell populations in his cultured bone marrow. One cell population contained 46 chromosomes with XX, whereas the other cell population contained 47 chromosomes with XXY (Ford et al., 1959). Nowadays, it has become clear that mosaicism is an important factor that contributes to variability in phenotype, severity of disease and inheritance. Mosaicism has already been described for more than 30 different diseases, including both Mendelian and non-Mendelian disorders, e.g. haemophilia, Duchenne muscular dystrophy, Friedreich ataxia and FSHD (Gottlieb et al., 2001).
