Sex Chromatin

A history of the discovery of random x chromosome inactivation in the human female and its significance

Genetic determinants of sex in placental mammals developed by the evolution of primordial autosomes into the male and female sex chromosomes. The Y chromosome determines maleness by the action of the gene SRY, which encodes a protein that initiates a sequence of events prompting the embryonic gonads to develop into testes. The X chromosome in the absence of a Y chromosome results in a female by permitting the conversion of the embryonic gonads into ovaries. We trace the historical progress that resulted in the discovery that one X chromosome in the female is randomly inactivated in early embryogenesis, accomplishing approximate equivalency of X chromosome gene dosage in both sexes. This event results in half of the somatic cells in a tissue containing proteins encoded by the genes of the maternal X chromosome and half having proteins encoded by the genes of the paternal X chromosome, on average, accounting for the phenotype of a female heterozygote with an X chromosome mutation. The hypothesis of X chromosome inactivation as a random event early in embryogenesis was first described as a result of studies of variegated coat color in female mice. Similar results were found in women using the X chromosome-linked gene, glucose-6-phosphate dehydrogenase, studied in red cells. The random inactivation of the X chromosome-bearing genes for isoenzyme types A and B of glucose-6-phosphate dehydrogenase was used to establish the clonal origin of neoplasms in informative women with leiomyomas. Behind these discoveries are the stories of the men and women scientists whose research enlightened these aspects of X chromosome function and their implication for medicine.

Anatomic changes due to interspecific grafting in cassava (Manihot esculenta)

Cassava rootstocks of varieties UnB 201 and UnB 122 grafted with scions of Manihot fortalezensis were prepared for anatomic study. The roots were cut, stained with safranin and alcian blue, and examined microscopically, comparing them with sections taken from ungrafted roots. There was a significant decrease in number of pericyclic fibers, vascular vessels and tyloses in rootstocks. They exhibited significant larger vessels. These changes in anatomic structure are a consequence of genetic effects caused by transference of genetic material from scion to rootstock. The same ungrafted species was compared. This is the first report on anatomic changes due to grafting in cassava.

'Your true and proper gender': the Barr body as a good enough science of sex

In the late 1940s, a microanatomist from London Ontario, Murray Barr, discovered a mark of sex chromosome status in bodily tissues, what came to be known as the 'Barr body'. This discovery offered an important diagnostic technology to the burgeoning clinical science community engaged with the medical interpretation and management of sexual anomalies. It seemed to offer a way to identify the true, underlying sex in those whose bodies or lives were sexually anomalous (intersexuals, homosexuals and transsexuals). The hypothesis that allowed the Barr body to stand in for 'chromosomal' or 'genetic' sex was provisional, but it supported the expectation that genetic information established one's primary identity, and the conviction that the animal world could be neatly divided into two, and only two, sexes. Ultimately, this provisional hypothesis, and its status as an unambiguous arbiter of true sex, was overturned. But during much of the 1950s, Barr's thesis about the identity of the Barr body was consistent with a coherent set of theories and evidence explaining sexual development and sexual pathology. Though provisional, the scientific status of the sex chromatin within this system of knowledge was good enough to support a flourishing research enterprise in the clinical sciences.