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It’s simple. Having an X and Y chromosome is male. Having an X and X chromosome is female. That is biological sex. The end. Not really. That is chromosomal or genetic sex, which is determined by the last set of chromosomes in your sequence of chromosomes (set number 23 to be exact, so chromosomes 45 and 46, known as the sex chromosomes). The thing is, it is not really all that clear, nor is it non-varying.

First, let's start with how genetic sex works. The X chromosome contains hundreds of distinct stretches of DNA called genes. The smaller Y chromosome has many fewer genes but includes one important gene called SRY that causes most Y- containing embryos to develop as biologically male. During human embryonic development the SRY protein turns on male-associated genes although it has genes involved in many other processes as well.

Second, let's talk a bit about variation. A good number of people have X and Y chromosome variations. Sometimes this is called sex chromosome anomaly, sex chromosome abnormality, X and Y chromosomal variations, or sex chromosome aneuploidy. One example is Klinefelter syndrome which entails having a 47th chromosome added to an XY sex chromosome (male) so it is actually XXY. According to the academic journal Ultrasound in Obstetrics and Gynecology, this is understood to occur in about 1 in 380 live male births. According to the Focus Foundation, It is believed the diagnosis rate is likely less than 25%. Let's give the run down from the of the frequencies from the Focus Foundation which has conservative numbers compared to some other research like the aforementioned article:

X&Y chromosome variations in people assigned male at birth:

• 47,XXY (Klinefelter syndrome) 1 out of 650 live births

• 47,XYY (Jacob’s syndrome) 1 out of 1,000 live births

• 48,XXYY 1 out of 18,000 live births

• 48,XXXY 1 out of ~29,000 live births

• 49,XXXXY 1 out of ~92,500 live births

• The total frequency of these variations per 1,000 live births is approximately 2.64. 

X chromosome variations in people assigned female at birth:

• 45,X (Turner syndrome) 1 in ~2,250 live births [National Library of Medicine]

• 47,XXX (Trisomy X or Triple X) 1 in ~950 live births [Cleveland Clinic]

• 48,XXXX (Tetrasomy X) rare

• 49,XXXXX (Pentasomy X) rare

• The total frequency of these variations per 1,000 live births is approximately .749. 

In some cases, a small number of people do not have a sex of male or female recorded at birth or infancy.

Third, let's talk about why it happens.  X and Y chromosome variations are typically not inherited, as they occur due to problems during the formation of a parent’s egg or sperm. These conditions are caused by a problem in the division of the X and Y chromosomes during the formation of a parent's egg or the sperm, which are then involved in conception. The exact cause is unknown, and these conditions are typically not inherited.

Fourth, let's talk about what this means. Above just discusses chromosomal variation. There are assuredly many more physical sex characteristic variations (such as reproductive or sexual anatomy). And that is without throwing in hormonal variation debates such as natural testosterone levels. Consider Caster Semenya, a South African middle-distance runner who competed in the Olympics and won gold, but has hyperandrogenism which is a medical condition characterized by high levels of androgens (steroid hormones that play a crucial role in the development and maintenance of male characteristics; testosterone is the primary androgen hormone). Based on her case, in 2011 the World Athletics (formerly IAAF) introduced regulations requiring female athletes with high natural testosterone levels to undergo hormone-reducing treatment or surgery to lower their testosterone levels to compete in some events. 

Smash them all together as a topic and you will be discussing the meaning and frequency of intersex. As Intersex: MedlinePlus Medical Encyclopedia notes, the term intersex refers to people who are born with genetic, hormonal, or physical sex characteristics that are not typically 'male' or 'female'. Intersex people have a diversity of bodies and gender identities, and may identify as male or female or neither. The reality is that defining male or female sex is actually anything but clear. Add to that gender (gender is about social and cultural differences in identity, expression, and experience) and sexual preferences which are impacted by biological, psychological, social, and cultural factors and it gets messier still. So, be kind, things are not binary. You can't have a binary classification system with more than two configurations even if some configurations are more common than others.

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