As both a genetics-loving biology major and a supporter of LGBT rights, something that irks me to no end is when people try to discredit the LGBT movement, particularly for trans individuals, by claiming that it is only genetically possible to have two sexes, meaning that there can only be two genders that match the sexes.

To give them a little credit, I can see how having only a basic understanding of genetics would lead to those conclusions. Most people know that females have the sex chromosomes XX and males XY, but that’s about the extent of their knowledge. However, when you really dive into the field of genetics and learn more about it, you discover that determination of sex is not nearly so cut and dry.

Both male and female sex cells, called gametes, are haploid, meaning they only have one set of each chromosome. (Regular body cells have two sets of each chromosome, one from each parent.) In order to get cells with only one chromosome set, the cells have to undergo the process of meiosis, in which two rounds of cell division take place. The first round separates the chromosome pairs, and the second separates the duplicated chromosomes.

Usually, this process works just fine, and any future offspring will end up with a set of perfectly functional chromosomes, including sex chromosomes. You will get either XX of XY sex chromosomes, and will subsequently have female or male body parts. However, this is not always the case, making it difficult to determine what someone’s sex is in the first place.

Sometimes, the sex chromosomes won’t split correctly during meiosis, and so the subsequent cells will contain too many Xs or Ys or none at all. This can result in offspring that can have any assortment of sex chromosomes, including X, XXX, XXY, XYY, and many more. All of these combinations have various effects on the way bodies grow and develop, including the development of reproductive organs. Some of these individuals can even have physical traits associated with both sexes.

Other times, a person will have additions, deletions, or mutations to their sex chromosomes that influence the person’s development. For example, a person could have two X chromosomes, but could still develop male traits if one of the Xs had the information for male traits added on by accident.

An example that is perhaps easier to understand is androgen insensitivity. This occurs when an individual has XY chromosomes, but the body’s cells can’t bind the androgen hormones necessary for the development of male traits. That individual will then develop female traits by default. People with androgen insensitivity will be female in appearance but will have male chromosomes; they will often times have a blind uterus as well as undescended testes that are surgically removed.

So how does this tie in with society’s views of sex and gender? Well genetically speaking, there are so many X and Y combinations that assigning a sex at all can sometimes be a major ethical dilemma.

Because of this, we can’t just say that one person has to be one sex or another, because there are actually people out there who are a mix of both. And if we can’t tell everyone what sex they are, we definitely can’t tell people what gender they have to identify as either.