Chapter 6 - The X chromosome

Abstract

This chapter describes the X chromosome. One of the X chromosomes in the normal female behaves differently from the other chromosomes. It becomes very condensed during interphase, forming a darkly-staining mass called the sex chromatin body. There is no sex chromatin body in the normal male and so this property of the X chromosome may be used to indicate the nuclear sex in an individual. Abnormal X chromosomes of large size are found to give rise to large bodies and deleted X chromosomes produce small bodies. Another way in which the X chromosome behaves differently is in its replicating pattern, labeled with titrated thymidine. In the normal female one of the X chromosomes replicates later than the rest of the chromosome complement. Therefore, in cells supplied with the isotope at the end of the period of DNA synthesis, the X is apparent as a heavily labeled sex chromatin body at interphase and a heavily labeled chromosome at metaphase.