The chromosomal basis for sex determination:
It has been known for decades that human male and female cells have different sex chromosomes (Painter, 1921) and that the difference is visible in interphase as well as in mitosis (Barr and Bertram, 1949). Although Painter's discovery of the human sex chromosomes could not be exploited clinically at the time because cytogenetic techniques were inadequate, the discovery of sex chromatin masses (Barr Bodies) in interphase cells of females but not of males was soon followed by the development of a simple technique that allowed Barr bodies to be studied in buccal smears. As a result, it was quickly recognized that although most females were "chromatin positive" and most males were "chromatin negative", there were exceptions. It was especially noteworthy that many short, infertile females with a condition known as Turner syndrome had no Barr bodies, whereas tall, infertile males with a condition known as Klinefelter syndrome did have Barr bodies.
Soon after cytogenetic analysis became feasible, the chromosomal basis for these discrepancies became apparent. Because the anomalous sex chromotin findings had suggested that the Turner and Klinefelter syndromes were characterized by unusual sex chromosome constitutions, they were two of the first conditions for which chromosome studies were performed. Patients with Klinefelter syndrome were found to have 47 chromosomes with two X Chromosomes as well as a Y Chromosome (karyotype 47, XXX ; Jacobs and Strong, 1959), whereas most Turner syndrome patients were found to have only 45 chromosomes with a single X Chromosome (Karyotype 45, X; Ford et al.,1959). These findings promptly established the crucial role of the Y chromosome in normal male development.
The next step in the understanding of the human sex chromosomes was an explanation of sex chromatin in terms of X inactivation. As additional sex chromosome abnormalities were identified, the number of Barr bodies seen in interphase cells was observed to be always one less than the total number of X chromosomes per cell:
| Sexual Phenotype | Karyotype | Barr Bodies |
|---|---|---|
| Male | 46,XY;47,XYY 47,XXY;48,XXYY 48,XXXY,49,XXXYY 49,XXXXY | 0 1 2 3 |
| Female | 45,X 46,XX 47,XXX 48,XXXX 49,XXXXX | 0 1 2 3 4 |
The theory of X inactivation (the Lyon hypothesis) is that in somatic cells in normal females (but not in normal males), one X chromosome is inactivated, thus equalizing the expression of X-linked genes in the two sexes. The Barr body represents the late-replicating, inactive X chromosome. The replication asynchrony between active (early-replicating) and inactive (late-replicating) X chromosomes can be recognized cytogenetically by specialized banding procedures called "replicating banding". In patients with extra X chromosomes, any X chromosome in excess of one is inactivated and forms a Barr body (see table above). Thus all diploid somatic cells in both males and females have a single active X chromosome, regardless of the total number of Xs or Ys present.
Even though many abnormalities of the sex chromosomes have been defined and their clinical consequences reported in detail, there still remain mysteries about the precise role of the sex chromosomes in sexual determination. There are exceptions, not yet fully understood, to the rule that females are always XX and males always XY. These exceptions, which include XX males, XY females, and XX true hermaphrodites, suggest that the entire Y chromosome is not the sole determinant of phenotyoic sex. Molecular analysis is currently being used to find an explanation for these unusual