Earlier in this article I mentioned that humans with single X chromosomes are females with associated abnormalities (Turner syndrome). This poses an apparent paradox: males with single X-chromosome are normal and females are also physiologically haploid for X-chromosome because of the inactivation of one of the two X-chromosomes; why then are females with an XO constitution abnormal? The reason seems to lie in the fact that not all the genes on the X-chromosome are inactivated. There are some genes that escape inactivation, for example steroid sulphatase gene (STS). Does this mean that females have two