Mosaicism involving the Y chromosome can be associated with abnormalities of sex differentiation. Of the variety of karyotypes possible from loss of the Y by nondisjunction, 45,X/46,XY is the most common. A wide variety of phenotypes is displayed by these individuals from newborns with ambiguous genitalia to normal fertile males or normal female phenotype with bilateral streak gonads. Most have short stature, and one-third have other Turner stigmata.

In the "typical" mixed gonadal dysgenesis case presenting with abnormal sex differentiation the usual gonadal pattern is a streak gonad on one side and a dysgenetic or normal appearing testis on the other side of the abdomen. Miillerian and wolffian duct development correlates with the character of the ipsilateral gonad. All possible permutations combining streaks, and dysgenetic and apparently normal testes have been encountered. The diversity of presentation is presumed to reflect the relative proportion of 45,X and 46,XY cells in the gonadal ridge. The incidence of gonadal tumors is 25%.

The application of molecular biology methods allittles markers for DNA sequences on the Y chromosome to seek out similar sequences on patients' sex chromosomes. In one series of 40 Turner syndrome patients, one patient had identifiable Y chromosome material using polymerase chain reaction of the gene from SRY region of the Y chromosome. In 3 of 18 patients (none of whom had evidence of Y chromosomal material by cytogenetic analysis), Y chromosomal segments from the SRY region were detected by polymerase chain amplification and Southern blot analysis. The short coming of this method is that polymerase chain reaction and in situ hybridization identify only those DNA sequences that correspond to the probes used in the analysis. There may be a sequence from a Y chromosome that does not correspond to the selected probes. A totally reliable method awaits identification of the gene responsible for tumor formation or the complete mapping of the Y chromosome sequences. Until then, patients with gonadal dysgenesis having any Y chromosome fragments must be considered to be at risk for gonadal tumors and virilization at puberty.