Biological sex is the physical sex we are born with. It differs from gender in that it is unaffected by culture, social norms or mores, or an individual's mind or brain. You are typically the biological sex you are born as due to whether you inherited an X or Y chromosome from your biological father's sperm. You inherit the X chromosome you carry from your biological mother's egg.
People are typically born with the XY or XX chromosome pairs created when a sperm implants in an egg. We in the US learn about this process in our early education. A person with XY chromosomes is biologically male. A person with XX chromosomes is biologically female. This is the simple split of our sexes, male and female, that we are familiar with because it is the typical way we as people develop. However, these are not the only combinations of the X and Y chromosomes that can occur in humans. In fact, there are many conditions that involve different combinations or an X or Y that does not copy over as is typical. These combinations result in intersex individuals.
We will return to gender identity after a brief look at intersex conditions.
There are many different syndromes that can occur when one X and one Y or two X chromosomes are not combined completely as is typical. These conditions are referred to as "intersex". People who have intersex characteristics may present physically as typically masculine or typically feminine or some combination or degree of both.
Here are several, but not all, possible intersex conditions a person may possess. There are others not described in this article.
Klinefelter syndrome, also referred to as karyotype (complete set of chromosomes in an individual) 47,XXY, it is a condition that results from two or more X chromosomes in males. Occurs in one or two subjects per 1000 males. This condition can result in breast growth, weaker muscles, less than typical body hair, and several other symptoms. This condition is often not noticed until puberty. The symptoms can be more severe if three or more X chromosomes are present. This is a random occurrence and is not inherited.
Turner syndrome also referred to as 45,X in which a female is missing partially or completely an X chromosome. This has several physical characteristics that can be present from birth but usually also includes infertility. This, too, is typically not inherited but results from a genetic defect in early cell division.
De la Chapelle syndrome, also known as XX male syndrome. Rare congenital intersex condition where an individual with 46,XX karyotype (typical female karyotype) has male phenotypical (outward physical traits) characteristics. Occurs 1 in 20,000 approximately. Can result in internal and external genitalia ambiguities.
Swyer syndrome also referred to as XY gonadal dysgenesis. A type of hypogonadism (diminished functional activity of the gonads) in a person whose karyotype is 46,XX and typically has female external genitalia. Internally they have gonads that develop into neither testes or ovaries. If left untreated a person with this condition will not experience puberty.
As we can see while the typical person has either an XX or XY karyotype, resulting in a genetic female or male person, there are other combinations that can result in possibly ambiguous or androgynous physical characteristics. This can include ambiguities in the person's phenotype, possibly even with physically ambiguous reproductive organs or genitals.
In the past, many such individuals had surgery performed in infancy or while young to create more typical appearing male or female genitalia. However, there is growing concern that this may not be the best for an individual without allowing them to choose whether or not to receive such surgery. This can be complicated by whether the individual grows to identify with the gender that matches the sex chosen for such a surgery.
Which brings us back to gender identity.
While biological sex is determined by a person's genetic makeup, gender identity is more complicated. Gender is, in part, a social construct and at least in part biological. That is, part of what defines male and female gender is created socially, rather than being purely biologically based. A person whose gender and biological sex are aligned and the same is referred to as cisgendered. This is sometimes called just cis in cultural slang.
A person with an XX, or female biological karyotype, may however identify as a male gendered person. This may be a desire to present themselves in a more typically masculine manner, including minimizing or hiding female phenotypical characteristics (such as wearing clothing that constricts or hides breasts) or possibly even surgery to remove or reduce female characteristics and induce male characteristics. It may also include a desire to be considered male by other people.
This type of individual is referred to as a transgender person. In this case, a Female-to-Male transgender person or FtM transgender person. In the case of a biological male who identifies as a female gendered person they would be a Male-to-Female or MtF transgender person.
In addition, a person may not identify as either male or female. They may identify as an agendered, non-gendered, or non-binary person. While this may seem relatively new in US culture, other cultures have a long history of what is sometimes referred to as 'third gender', or people whose gender does not match their biological sex (or consider themselves non-binary).
It may be helpful to understanding to consider gender a spectrum, with fully male at one end and fully female at the other with a person possibly identifying at different points along that spectrum. Also, keep in mind that gender is a label. It is useful in describing, but is not definitive.
While typically the gender a person identifies with is the same as their biological sex, there are these cases of individuals who do not. Like intersex conditions there is almost certainly a biological component to this. We have already discovered differences in the brains of people who identify as transgender.
Just as your DNA contains the 'blue prints' to build organs such as your heart and lungs, it contains the instructions on the formation of brain structures and systems. There is likely a type of condition where a person's biological sex is mismatched with their brain or mind's gender. Whether this is similar to the conditions that cause such as De la Chapelle syndrome to arise or an entirely different mechanism is not well understood at this time. However, we are starting to learn more.
What is clear at this time is that a transgendered or non-binary person is naturally occurring and is a normal difference from the typical cisgendered person. As our society and culture catches up to the latest scientific understanding, however, transgendered and non-binary people can face significant social and cultural obstacles, discrimination, and trials.