The Fragile X Chromosome

Abstract

The fragile X chromosome is associated with a common form of X-linked mental retardation in males and possibly females. The fragile X is so called because it has a fragile site at the distal end of the long arm. Fragile sites are morphological features of chromosomes defined as specific points liable to show some specific features. The chapter discusses tissue culture conditions. Some of the morphological appearances of the fragile X are illustrated in the chapter. In the illustration, the appearance is more distinctive in the unbanded preparations than in the G-banded ones. When G-banding methods that employ trypsin are used, the segment of chromosome distal to the fragile site becomes merely a fuzziness that is not as readily detected as the striking appearance of the fragile X on unbanded chromosomes. The autosomal fragile sites are considered to be without phenotypic effect. Homozygotes for the folate-sensitive autosomal fragile sites have not been identified yet, but it is speculated that such homozygosity can be deleterious. The chapter also discusses the karyotype–phenotype relationships.