Ron Burgundy

You should probably look into what science actually say before making a meme like this; as our understanding of human genome has increased the information many of learned in school has been disproved.

Although there is no third biologically functioning gender/sex, there are plenty of variations if the development was unable to be fully completed. According to hospital records, about one in every 1,500 to 2,000 births are born with completely ambiguous external genitals.

Your genes act as instructions for your body to follow while developing. Hormones are sent out to carry out these instructions. Sometimes non genetic factors can get in the way of developing. (For example a person who has a partially grown leg was probably not caused by genetics, but an issue that prevented the body from fully completing the genetic instructions.) There can be a lot of natural variety in biology and nature and everything basically. When you are developing into either a male or female your body has to go through a set of steps to get to that point. For example, the normal path of development for a male would be starting off with XY sex chromosomes, with a Y chromosome containing a typical male SRY gene. Testes would develop, releasing androgens (testosterone and other male hormones) would prevent the baby from becoming female and from there the primary sexual characteristics (the developmental traits you are born with, such as genitals) and the secondary sexual characteristics as well (dimorphism that develops later in life, what happens during puberty) would be primarily male.
Just like how there is variety in everything, ANYTHING can go wrong ANYWHERE along that line, and it’s actually more common than you would think. Starting from the beginning, complications in early mitosis or meiosis can cause sex chromosomes to be neither XX nor XY. Some people have XXXXY, XXXYY, XO, XXX, half of cells have an XY chromosome while the other has XX, sometimes someone has XY chromosomes with no SRY gene so it basically just functions as a XX sex chromosome, as well as many more other chromosomal possibilities. A health condition or enviromental factors can cause a hormonal imbalance that will prevent the primary sex characteristics (which includes the genitals) from developing to be fully identifiable as male or female, a hormonal imbalence can affect the secondary sex characteristics as well.

And people in these conditions are usually assigned either male or female based off of what seems the most logical, surgery is often preformed in the incidence of an ambiguous genitalia. But interestingly, many of these people socially transition to the gender opposite of rearing, whether or not they were ever informed of the incidents that occured early in life. They claim to identify that way very strongly. They become "transgender". Sexually dimorphic behaviour is a trait that has developed through evolution. It physically manifests within complex structures within the brain, the hypothalamic uncinate nucleus is an essential part in this. You can never raise a boy as a girl, or a girl as a boy, it's just how biology works. This has been done and been studied in cases such as David Reimer, who was brought up as a girl and ended up comiting suicide in the end. But what these "transgender" people... I mean the ones who everyone talkes about. People who have a normally masculinized external appearence yet claims to be female, or vice versa (who I will here refer to as "transsexuals" for simplicity). These people seem strangely similar to the other people who choose to transition genders contrary to rearing. Studies on animals suggests that the development of neurological dimorphism occurs around the third trimester, while the reproductive system developes quite early on, so theoretically, you could easily have a disconnect between the reproductive system and behavioural sexual dimorphism. Decades of research has been dedicated to finding causes of transsexuality. As with anything, it is hard to fit everyone in the same group. Of course *someone* could just choose to "transition" as a way to fit in or express themselves. Research on the topic is quite expancive, and quite interesting. People have so far been unable to find and prove a psycologically linked cause for transsexality, but of course, that doesnt mean it couldnt happen in an individual. The original reaserch on the transsexual brain was preformed in the 1990s, since then, many follow up studies have been preformed to answer lingering questions. The studies are mainly focused on structures in the hypothalamus, such as the hypothalamic uncinate nuclus. This part of the brain could only be studied throught physical examination. A collection of brains from deceased transsexuals has been gathered over time to be studied, so far, all of the brains, strongly more closely resembled the opposite sex..

I cant really say how many genders there are. It really just depends on how YOU interprete information, and the conclusions you make from it, because gender is only a word when it comes down to it.

Everyone has had a unique and individual experience from each other, so there is always something you can learn from anyone, no matter what it is. I have honestly approched this subject in an unbiased way, but I would still recommend doing your own reasearch. Try to focus a little less on your ingroups and your outgroups and more on just learning and understanding the differences people have with you.