A girl and her father, both apparently normal, had been reported earlier as carriers of a satellited submetacentric chromosome in place of a long acrocentric chromosome. A morphologically identical chromosome has now been found in the proposita’s paternal uncle, but in neither a paternal cousin nor a paternal great uncle. Clinical studies of the proposita did not reveal any deviations from normality.

By application of the “centroid” method of Barton, David and Merrington to a number of H³-thymidine radioautographs, it was determined that the human late-replicatingX chromosome lies towards the periphery of the metaphase in a significant number of cases. Further, in agreement with earlier findings by other authors, the centromeres of chromosomes 1, 2 and 3 were found to lie closer together than is to be expected on a random hypothesis.

We studied the effect on female viability oftrans-heterozygous combinations of X-chromosome deficiencies andSxl^f1, a null allele ofSex-lethal. Twentyfive deficiencies, which together covered 80% of the X chromosome, were tested. Seven of thesetrans-hcterozygous combinations caused significant levels of female lethality. Two of the seven interacting deficiencies include the previously known sex determination genessans fills andsisterless-a. Four of the remaining uncover X-chromosomal regions that were not hitherto known to contain sex determination genes. These newly identified regions are defined by deficienciesDf(1)RA2 (7D10; 8A4-5),DJ(1)KA14 (7F1-2; 8C6),Df(1)C52 (8E; 9C-D) andDf(1)NI9 (17A1; 18A2). These four deficiencies were characterized further to determine whether it was the maternal or zygotic dosage that was primarily responsible for the observed lethality of female embryos,daughterless andextra macrochaetae, two known regulators ofSxl, influence the interaction of these deficiencies withSxl.

The geneSex-lethal (Sxl) plays a pivotal role inDrosophila sexual development. Once activated in response to the X: A ratio signal in XX embryos,Sxl participates in appropriate implementation of all known aspects of sexual differentiation. We have attempted to identify new X-linked genes involved in sex determination, especially those involved in the regulation ofSxl. Since misregulation ofSxl, or that of the genes that regulate it, leads to female-specific lethality, or synergistic female-lethal gene interactions, or both, we used these criteria to screen about 10,000 EMS-treated chromosomes for (i) recessive female-specific lethality or (ii) enhanced female lethality intransheterozygous combination withSxl. Four potentially useful mutations—Sxldlf, fl-35, fl-46, 1–43—were recovered and a few of their properties were characterized. Approximate map positions of these mutations were determined by meiotic mapping. To understand their probable position(s) in the hierarchy of genes regulating sex determination, we studied dose-dependent interactions between them and mutations in genes known to affect sex determination by generating double and triple heterozygotes. These studies suggest that (i)Sxldlf is not defective in the ‘early’ regulation or functions ofSxl, and (ii)fl-35, fl-46 and1–43 are unlikely to be a part of the X: A ratio signal, i.e. they are not needed for the transcriptional activation ofSxl. On the other hand, they could be affecting post-transcriptional processing ofSxl transcripts.

Drosophila melanogaster females homozygous forflex, an X-linked recessive mutation, do not survive. Hemizygous males are unaffected. Homozygous embryos appear to lack SXL, the product of theSex-lethal (Sxl) gene, apparently as a result of disruption ofSxl splicing. It is known that bothSxl and its somatic splicing regulators [snf andfl(2)d] also function in the development of the female germ line. For this reason, we investigated the role offlex in the germ line by generatingflex/flex clones inflex/+ females. Females carrying such clones in their germ lines do not lay eggs whereas females carryingflex+ eggs lay viable eggs. Additionally, DAPI staining of ovarioles showed that diploid germ cells that are homozygous mutant forflex do not complete oogenesis. These results indicate that theflex+ gene product may be required for the development of the female germ line.

We describe a novel mutation in the coding region of theSRY gene in a 46, XY female with Swyer syndrome. Analysis ofSRY was carried out by direct sequencing of a 780-bp PCR product that included theSRY open reading frame (ORF). This revealed the presence of a point mutation, ins 108A, in the coding region 5’ to the HMG box which results in a frame shift and premature termination of the encoded protein. No other mutation was found in theSRY ORF. We infer that sex reversal in this individual is a result of this insertion. In none of the 13 other 46, XY females that were studied was a mutation detected inSRY, confirming earlier findings that most cases of XY femaleness are due to causes other than mutation inSRY. These observations and those of others are discussed in relation to the aetiology of XY sex reversal.

By employing a procedure that combines ELISA and photoacoustic spectroscopy, we have examined the content of 5-methylcytosine (m⁵C) in DNA of individuals who differed from one another in the number of X chromosomes in their genomes. The results show that the human inactive X chromosome (Xi) contains very high amounts of this modified nucleotide. We estimate that in the 46, XX female there is more m⁵C in Xi (∼ 3.6 x 10⁷) than in all the remaining chromosomes put together (∼2.1 x 10⁷). Our results also suggest that nearly one-fifth of all cytosines in Xi are methylated and that, in addition to CpG methylation, there is extensive non-CpG methylation as well.