Sex - Biological
Sex is biological; it underlies our reproductive mechanisms. It is chromosomal; we tend to be divided into male (X-Y) and female (X-X), but such differentiation is not absolute, because a significant number of persons are born with other configurations. Correct chromosomes are not necessarily essential for individual survival. Biological sex is fixed at birth and does not change with time.
The deterministic importance of sex is debatable. Does our sex make us who we are, or is it just a feature that is given importance based on gender, not biology?