THE HUMAN Y CHROMOSOME AND THE ETIOLOGY OF TRUE HERMAPHRODITISM

Natural human chimeras: A review

The term chimera has been borrowed from Greek mythology and has a long history of use in biology and genetics. A chimera is an organism whose cells are derived from two or more zygotes. Recipients of tissue and organ transplants are artificial chimeras. This review concerns natural human chimeras. The first human chimera was reported in 1953. Natural chimeras can arise in various ways. Fetal and maternal cells can cross the placental barrier so that both mother and child may become microchimeras. Two zygotes can fuse together during an early embryonic stage to form a fusion chimera. Most chimeras remain undetected, especially if both zygotes are of the same genetic sex. Many are discovered accidently, for example, during a routine blood group test. Even sex-discordant chimeras can have a normal male or female phenotype. Only 28 of the 50 individuals with a 46,XX/46,XY karyotype were either true hermaphrodites or had ambiguous genitalia. Blood chimeras are formed by blood transfusion between dizygotic twins via the shared placenta and are more common than was once assumed. In marmoset monkey twins the exchange via the placenta is not limited to blood but can involve other tissues, including germ cells. To date there are no examples in humans of twin chimeras involving germ cells. If human chimeras are more common than hitherto thought there could be many medical, social, forensic, and legal implications. More multidisciplinary research is required for a better understanding of this fascinating subject.

Epithelial ovarian tumor in a phenotypic male

Laparotomy in a 41-year-old married man with non-treated left cryptorchidism revealed female internal genitals on the left side, and an epithelial ovarian tumor of intermediate malignancy. Germinal malignancies are frequent in intersexes, but non-germinal gonadal neoplasms are rare. This is the second reported case of epithelial ovarian tumor in intersexes, and the first case of epithelial ovarian tumor in an intersex registered as male.

A true hermaphrodite dispermic chimera with 46,XX and 46,XY karyotypes

A 16-year-old male with hypospadias and gynaecomastia had a rudimentary uterus with a right Fallopian tube and ovary; the left gonad was a functioning testis. Cytogenetic studies showed cells with 46,XX and 46,XY sex chromosomes in cultured blood, skin and gonadal tissues. Cells with the 46,XX constitution predominated in all tissues. Extensive investigations failed to demonstrate blood cell and serum chimerism, but there was little genetic variation of these characters between family members. Cytogenetic studies demonstrated that the father had contributed different marker chromosomes to the 46,XY and 46,XX cell lines of the propositus, whereas the mother had contributed the same two informative markers to both cell lines. The patient was a chimera with two diploid cell lines of different sex that had developed from the products of two separate acts of syngamy. Dispermy was demonstrated, and, whereas there was no evidence of different maternal contributions to the chimeric cell lines, uncertainty remains that these were identical.

Serologic detection of a y-linked gene in xx males and xx true hermaphrodites

To test the hypothesis that H-Y antigen (present on both somatic and germ cells in normal males but not normal females) is essential for testicular differentiation, we studied four XX males and three XX true hermaphrodites. Blood cells from six subjects and cultured gonadal fibroblasts from a seventh expressed H-Y antigen. Since expression of this antigen requires the presence of a gene normally carried by the Y chromosome, this gene, and perhaps additional Y chromosomal material, should have been present in the genome of these subjects. In one patient this presence is accounted for by a Y-to-X translocation, detectable by chromosome banding. In another a normal Y chromosome was present in a minor population of cells. In the remaining five, no karyotypic abnormality was detectable. Immunologic detection of H-Y antigen is a sensitive test for the presence of the Y chromosome or of its male-determining segment.

Evidence of XX-XY sex chromosome mosaicism in a child with true hermaphroditism

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Cytogenetics and sex determination in man and mammals

Sex in man and probably throughout the class mammalia is normally determined by the presence of a Y chromosome (male) or its absence (female). The presence of genetic loci on both the long and the short arm of the X chromosome in double dose appears to be essential for the development of mature functional ovaries in the human female though a single X suffices in the female mouse.

The development of masculine genital anatomy and phenotype is a consequence of prior formation of testes. In the absence of gonads of either kind, female internal and external genitalia are formed but secondary sex development fails. In rare human families a mutant gene suppresses the development of male external genitalia in 46, XY embryos but permits the development of testes and male internal genitalia. The external phenotype is normal female (syndrome of testicular feminization). A sex-linked mutant gene in the mouse has a similar effect.

The locus or loci directly concerned with male development might lie wholly on the Y chromosome or might be located on another chromosome or chromosomes. In the latter case it (or they) must be repressed in the female and normally activated by a locus or loci on the Y chromosome in the male. Present evidence does not permit the exclusion of either possibility.