For people with Turner Syndrome, there is only one and it is X. Babies are born with external female genitalia but have no ovaries and therefore do not grow breasts or menstruate. For people with Klinefelter Syndrome, there are 3 sex chromosomes, XXY. These babies are born appearing to be male but do not develop normally. Manifestations of Klinefelter Syndrome include undersized testicles, faint or non-existent body and facial hair, abnormally soft skin, and breast development. Five alpha-reductase 2 deficiency occurs when an XY fetus does not respond to testosterone in the womb and therefore appears to be female at birth, but upon puberty does respond to testosterone and develops as any other male. So you have a baby girl and then 13 years later she grows chest hair, her voice drops, and her clitoris grows into as much of a penis as it can at that point. Chimerism is when 2 eggs are fertilized but one embryo is absorbed by the other, giving the resulting baby 2 separate sets of DNA which can manifest as a child of ambiguous sex. In this case some of the body's cells will be XX and some will be XY. If all of the genitals have the same sex, this could go unnoticed forever. You may end up with one ovary that appears to be an undescended testicle, or one undescended testicle that is presumed to be an ovary. Since people rarely get DNA tested, especially against other parts of themselves, no one knows how common chimerism is.
Then we have XY embryos who are simply immune to testosterone altogether. They are born appearing female and develop as such. They don't have a uterus and their gonads aren't really testicles or ovaries, but that's rarely noticed until they fail to menstruate and even then they're often just considered women born without a uterus. They are very feminine in appearance because while most women have some level of testosterone, these don't, or at least they don't respond to it.
So that's what sex is, and some of the ways it's not as cut and dried as basic (incomplete) biology classes would have had us believe. But what about gender? Gender is the masculine and feminine, it is the manifestation of sex for most of us but even that is a spectrum. From "sensitive" boys to tomboys, and from macho men to girly girls, we all express gender in our own ways. Because we are who we are, and what we consider to be manly or womanly changes with religion, culture, environment, tradition, and all sorts of other influences. Hair length, clothing, names, all of it is societal. At one point not too long ago voting, owning property, wearing pants, and going to war were all considered masculine, but women do these things every day now and no one bats an eye. A hundred and fifty years ago a doctor named Leslie or Robin with short hair and pants would have OBVIOUSLY been a man. But gender changes with time, because it's not a biological constant.
