So, with the title of this post I presumably got your attention!! Unless you have been living under a rock, you are likely aware of the controversy in women's boxing at the Paris Olympic Games. A Facebook friend has linked a couple of articles that had some factual errors, but when I tried to make some comments for clarification, I was accused of spreading misinformation, and was basically told to buzz of and stop commenting. So I did. However, I felt the need to more clearly state my understanding of the classification and determination of sex in humans. This mini-essay is basically a portion of a lecture that I give in a university-level Human Genetics class that I have taught numerous time.
The definition of sex has three basic components - chromosomal, gonadal, and phenotypic [basically secondary sex characteristics]
CHROMOSOMAL SEX
Human chromosomes come in pairs, and there are two sex chromosomes, X and Y. Humans have 46 chromosomes, 23 pairs [22 non-sex chromosome pairs and 1 sex chromosome pair. For all 23 pairs, you received one from your mother and one from your father. Females have 2 X chromosomes and males have one X chromosome and one Y chromosome. The standard nomenclature for most people would be 46XX females and 46XY for males. Thus, the number of options for sex chromosomes is two, either X or Y, which makes the choice binary. There is no third sex chromosome, nor is there some sort of spectrum of sex chromosome choices. However, that being said, there can certainly be differences in the number and structure of the two sex chromosomes. There are individuals that have only one X chromosome [interestingly there are no individuals with only a Y chromosome]. and the 45XO individuals have Turners Syndrome with significant abnormalities. There are instances of 47XXX with no apparent abnormalities. These individuals develop as females because of the absence of the Y chromosome. Another example of abnormal numbers of sex chromosomes is Kleinfelter's Syndrome with 47XXY or sometimes 48 XXXY. They develop as males, but with characteristic abnormalities. There are some males that are 47XYY but are apparently not significantly different the 46XY males. There also some uncommon sex chromosomal abnormalities such as the 46XX46XY chimeras. There are also examples of mutations on the Y chromosome that inactivates/deletes the gene most responsible for development as a male. Thus, it is possible to have a 46XY karyotype but develop as a female, albeit with abnormalities. To iterate, chromosomal sex is binary [just 2 choices] but in a small percentage of individuals, there are abnormalities of the sex chromosomes.
GONADAL SEX
Like chromosomal sex, gonadal sex is binary - there are two choices, ovaries or testes. There is no third gonad nor is there a spectrum of gonads. In over 99 percent of individuals, gonad development matches the chromosomal sex - XX embryos develop ovaries and the female reproductive structures, and XY develop testes and male reproductive structures. Once again, a very small percentage of individuals can develop ambiguous gonads, or as mentioned above, gonads that do not correlate to the chromosomal sex. To reiterate, gonadal sex is a binary choice.
PHENOTYPIC SEX
Here is where variability comes into play. Whereas chromosomal sex and gonadal sex are immutable, phenotypic sex [secondary sex characteristics] is not only variable, but it can be changed. If one examines the phenotypes of either males or females, it's obvious that there truly is a spectrum Indeed there are some phenotypes that do not fit as either male or female. Since hormones play a fundamental role in development of secondary characteristics, it makes sense that such characteristics can be manipulated by hormones, as well as by surgery, drugs, and also possibly nutrition and environment. For example, Caitlyn Jenner clearly has female secondary sex characteristics. However. her chromosomal sex remains the same and she will never develop ovaries. Similarly Elliot Page now has male secondary sex characteristics, but his sex chromosomes are unchanged, and he will never develop testes. Thus, phenotypic sex is definitely not binary. It could possibly considered bimodal with lots of variation around the mods and considerable overlap between the two distributions. For me, spectrum doesn't quite describe the variability, mainly because I either think of the light spectrum or some sort of continuum between two extremes.
Commentary - many people conflate gender with sex, when they usually mean phenotypic sex. Both are variable and can be changed or manipulated, and even abused. Another problem - some secondary sex characteristics continue to develop during adolescence and post-puberty. This is particularly relevant in sports because of the well-documented physical and physiological differences between males and females.
