OK, are we sitting comfortably?
Once upon a time, possibly around a billion years ago, some very simple life-forms hit upon the idea of exchanging little bits of genetic material, which gave them the advantage of variation in changing environments. Amazingly, this is still how the majority of life-forms have “sex” - you may have heard of the worrying phenomenon of antibiotic resistance spreading through populations of bacteria.
Eventually though, some slightly less simple life-forms partially formalised the genetic-stuff-swapping arrangements by requiring different “mating types” - in some species of fungi, there can be a hundred or so mating types. These, therefore, are the only group of living things known to have anything remotely resembling a “spectrum” of sexes.
However, in pretty much all other known life-forms, the number of mating types settled to just two. In higher eukaryotes (this includes humans) there are no additional gametes. (There is extensive debate as to why, because on the face of it mathematically it means that only 50% of other members of your species are suitable mates; I have read various mathematical-modelling scenarios involving distribution of mitochondria etc.)
The reproductive cells, known as gametes, can be isogamous (similar in morphology/size) or anisogamous (different in morphology/size). In anisogamous species, the difference boils down to:
- Male gametes: Small, motile
- Female gametes: Large, immotile
(Sometimes with isogamous species, we don’t call them male and female, but + and -.)
Some species produce both male and female gametes in the same individual, these are monoecious species (or in animals can be called hermaphrodites). Some species produce male and female gametes on separate individuals, these are dioecious species (in animals called gonochoric). The primary sexual characteristics of the individual refer to the systems for production of the gametes.
Typically, gonochoric species are sexually dimorphic (they have differences in their bodies as well as the gametes they make) - this is because of the unavoidable differences in necessary adaptions required for successful reproduction; the secondary sexual characteristics are connected with these adaptations.
The manner in which the sexual role of an individual of a gonochoric species is established is variable. Some animals have sex determined by temperature during development (e.g. crocodilians), some can indeed change sex in the truest sense by producing the other gamete (e.g. some species of fish), some have gone for a hardwired chromosomal method (e.g. birds and mammals).
Humans, like other mammals, are a gonochoric anisogamous species whose sex is determined chromosomally, or specifically by the presence of the SRY gene on the Y-chromosome. Your complement of chromosomes is determined the moment your male parent’s small motile gamete fertilised your female parent’s large immotile gamete, and is passed into every cell of your body (except red blood cells, but they are descended from nucleated precursors), where this chromosomal make-up determines how your body develops all those secondary sexual characteristics that evolution has determined are needed for the continuation of the species.
Errors in chromosome segregation, problems with hormone production or receptor function, variations in anatomical growth during embryo development - these are not a spectrum of multiple rainbow sexes, but are DSDs.
There is no known way for human sex, nor any of the true underlying architecture associated with our sexed bodies, to be re-written. We can make superficial modifications to appearance and circulating levels of sex hormones, but that’s it.
The TLDR take home message is that in Homo sapiens sex is binary, is determined at conception and is immutable. We are not Mr/Mrs Potato Head.
No other species has “gender” and most humans do not either.