Isochromosome for long arm of Y chromosome in patient with Turner's syndrome and sex chromosome mosaicism (45,X-46,XYqi).

In patients with structural aberrations of the sex chromosomes a direct relation has been found between the phenotype and the extent and site of sex chromosomal deletion (Ferguson-Smith, 1965,1969). Women with deletion of the short arm of the X chromosome invariably have short stature and streak gonads, and usually have several of the other malformations of Turner's syndrome, while those whoseX chromosome deletion is confined to the long arm have shown only the gonadal defect. Deletions of the Y chromosome have also been found in Turner's syndrome, usually in association with 45,X mosaicism. In several cases, including some with presumptive short arm isochromosome of the Y, there have been ambiguous external genitalia and signs of incomplete testicular differentiation. The possible genetic implications of these findings are that the Y chromosome may bear X homologous loci, and that the short arm of the Y chromosome may bear the determinants of male sex differentiation. This hypothesis is strengthened by the findings in two patients with isochromosomes for the long arm of the Y chromosome (Jacobs and Ross, 1966) who show female sex differentiation. A third example of this apparently rare type of Y chromosome aberration is described here in a patient found at necropsy to have minor degrees of masculinization ofthe internal and external genitalia.