“In one out of every 3,000 births, the physician cannot tell the new parents what some have waited to hearnamely, whether they have had a boy or a girl. But the results of a study described in the May issue of the American Journal of Human Genetics may offer new hope to parents whose infants are born sexually ambiguous. According to the report, researchers have determined that a second copy of a sex determination gene known as WNT-4 can change an embryo from male to female, which often results in ambiguous genitalia. “
https://www.scientificamerican.com/article/gene-duplicate-causes-sex/
b/s
It should first be noted that the numbers of “intersexual” children are often greatly exaggerated:
“Anne Fausto-Sterling’s suggestion that the prevalence of intersex might be as high as 1.7% has attracted wide attention in both the scholarly press and the popular media. Many reviewers are not aware that this figure includes conditions which most clinicians do not recognize as intersex, such as Klinefelter syndrome, Turner syndrome, and late-onset adrenal hyperplasia. If the term intersex is to retain any meaning, the term should be restricted to those conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female. Applying this more precise definition, the true prevalence of intersex is seen to be about 0.018%, almost 100 times lower than Fausto-Sterling s estimate of 1.7%. https://www.ncbi.nlm.nih.gov/pubmed/12476264
The incidence of “intersexuality” should be viewed as a result of negative mutations, defined as “a change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke. ... Often cells can recognise any potentially mutation-causing damage and repair it before it becomes a fixed mutation.”
Babies who are born with sexual deformities no more consitute a “third (or 4th or 5th “ad infinitum”) sex” than babies born with club feet or cleft lips constitute separate human sub-species.
There is no denying that the rare cases of babies born with sexual deformities constitute a difficult problem that requires wise medical judgment, but they in no wise justify the patent fiction that sexuality exists on a “continuum.”