With a large semen sample to work from, Dr Hurley set about her experiment. She had long considered the rigidity of human sexual parts to be a major fault of biology. Humans could be killed in an instant, and to replace them took over 9 months of development. Not to mention they could only be born one at a time!
Using the donated sperm, she eventually managed to convert some of the specimens into a modified chromosomal makeup. She named it Z, and it had two dramatic differences to normal human sperm: firstly, it could invert between masculine and feminine at will. Secondly, its reproductive processes were drastically faster than normal humans- instead of a 9 month gestation period, its offspring would need only 9 days to develop.
Some days later, Dr Hurley donated one of the adjusted sperm to a couple trying to get a child through artificial insemination. The mother was successfully impregnated and the foetus developed normally- with one aberration. The genetic signal that causes the masculine reproductive system to form was delayed until the feminine reproductive system finished forming. As is normal, when it received this signal the labia fused into a scrotum, the ovaries formed testicles and the clitoris grew into a penis. But the body retained a genetic record of the feminine reproductive system, keeping it dormant.
A few months later, the mother gave birth to a perfectly healthy baby boy. It seemed totally normal until a while after birth, at which point its body seemed to produce odd stimuli, and its genitalia transformed. Its penis receded to a clitoris, its testicles moved inside the body, its scrotum separated into labia and between them formed a vagina. The couple were shocked by this, but tests showed they had a perfectly healthy child, just one which could spontaneously change sex.
They named the child Cam- everyone assumed it was short for Cameron, but actually it was sort of for 'chameleon'. The name proved fitting, as Cam developed a habit of hanging out with other boys as a boy and other girls as a girl. They tended to be put down as twins on forms and the like depending on whether he or she was the one showing up.
Anyway, let's fast forwards to their first day at college, aged 18... (or not if you have ideas for other stories with them!)