What is sex?

The English word sex can be traced to the late 14th century and comes, via French, from the Latin sexus which means (only) the state of being either male or female, possibly deriving from the root sec- meaning ‘cut’ or division. Since then, ‘sex’ has meant

either of the two main categories (male and female) into which humans and many other living things are divided on the basis of their reproductive functions (OED). 

It is a biological category that divides people according to their genetics as well as sex characteristics such as reproductive organs. Nobody has any choice in what sex they are born as. They pop out into the world as either one or the other [1], and must deal with that as they will.

There are processes that exist in the material world, and then there are the concepts (or universals) that we humans abstract from the material particulars in order to think and talk about them: ‘sex’, ‘male’, ‘female’, etc. These concepts are culturally mediated, but changing the words, or changing the way we think about and categorise sex, will make no difference whatever to the material facts. Those exist regardless, independently of human thought. 

As a concept sex seems simple enough, until you look at it closely. Some of what I’ll cover below is a bit technical, but as with genetics, it’s important to ground ourselves in facts before we get onto the social and philosophical questions.

Methods of reproduction

No living organism is immortal, so for a species to keep existing, it has to reproduce, i.e. to create new generations that will live on when the older generations die. Natural selection has produced, over billions of years, a rich diversity of methods, but all can be categorised as either asexual or sexual. All bacteria reproduce asexually, most animals reproduce sexually, and some species can adopt either method, depending on the circumstances (e.g. wasps, starfish, aphids, komodo dragons).

In essence, sex is an evolved mechanism for the reproduction of species. 

Asexual reproduction

In asexual reproduction, one organism makes a clone of itself. Bacteria, some plants and even some animals (e.g. some sponges and sea anenomes) reproduce this way.

There are several methods of asexual reproduction. We needn’t look at them in detail, as my concern here is with human beings, but very briefly, the four main types are:

• Binary fission:  meaning ‘division in half’, this usually occurs in bacteria. A single parent cell splits into two, copying its DNA into the new cell.
• Budding: the parent organism grows a bud or outgrowth which develops into a new organism, which may or may not separate. Used by some plants and some animals. 
• Parthenogenesis: meaning ‘virgin birth’, this is reproduction from an egg without fertilisation by sperm. Used by some plants and some animals.
• Fragmentation: part of the parent organism breaks off and then develops into a new individual. This is different to budding because the new organism begins as a separate piece to the parent, not just an outgrowth.

What the methods have in common is that they allow one individual to churn out offspring all by itself, with relatively little expenditure of energy.

Sexual reproduction

In sexual reproduction, two parents of the same species – in anisogamous species (see below), one male, one female – combine their genetic material to create offspring that inherits a mixture of its parents’ genes and will therefore be slightly different to both.

Sexual reproduction requires the fusion of the two parents’ genetic material in the form of gametes, or sex cells. In animals, the male gamete is the sperm and the female is the egg, and fertilisation happens when the two come together. Fertilisation doesn’t necessarily involve sexual intercourse – and thanks to the technology of artificial insemination, this can now be true even in human beings.

The two parents don’t always have to be separate individuals: hermaphroditic organisms (e.g. earthworms) can supply both male and female gametes, and self-fertilise. Since two gametes are required, however, this is still considered sexual reproduction.

In summary, there are two models for organisms to reproduce themselves:

Asexual	Sexual
No sexes, one parent	Two sexes, two parents
Individual makes a copy or clone of itself	Two gametes whose fusion leads to fertilisation
Offspring genetically identical to parents	Offspring genetically variant to parents
Quick and easy, but no genetic variation	More resource-costly, but gives genetic variation

Of course there are exceptions. Some plants and fungi can do either asexual or sexual reproduction depending on the conditions. Even some animals can do this, via parthenogenesis. There are plants and a nematode worm that are trioecious, i.e. the organism has three forms – male, female, and bisexual or hermaphrodite. However the latter possesses both sex parts at once, so even this ‘third sex’ is built upon the male-female binary.

Evolution is conservative. Sexual reproduction involves two parents, and two gametes. 

Isogamy and anisogamy

The vast majority of sexually reproducing species are divided into ‘male’ and ‘female’, defined by the size of the gametes they produce:

Male: small gametes

Female: large gametes

This is the textbook, scientific definition of male and female. 

This dichotomy in gamete size, which is found in almost all complex multicellular eukaryotes [2] (a group which humans belong to), is known as anisogamy. When the differential in size is accompanied by a differential in motion where the small gamete is mobile and the large gamete is immobile, this more extreme form of anisogamy is known as oogamy, which is found in almost all animals. This dichotomy leads to varying degrees of sexual dimorphism, i.e. when males and females have different body types, as in our own species. 

In many single-celled organisms, sexual reproduction occurs without a dichotomy in gamete size (and thus also no dichotomy of sexual dimorphism). This is known as isogamy, and the outcome is the same as in anisogamy, i.e. the fusion of the two gametes leads to fertilisation. However, because the gametes in such species are the same size, we don’t classify them as male and female. Instead, isogamous organisms use a system of mating types. Most commonly there are two, called + and - (plus and minus), though there can be more. 

Isogamy is thought to be the ancestral form from which anisogamy evolved – the origin of male and female lies in this transition. In isogamy the parental investment is equal, whereas in anisogamy the female invests more resources in each gamete than the male, and the potential appears for natural and sexual selection to produce morphological differences between the sexes. 

Overwhelmingly, organisms have either no sex or two sexes. 

Why have two sexes?

Sexual reproduction is more costly in time and energy. It requires organisms to find and court a mate, involves lengthy gestation, and demands parental care, and therefore constrains reproduction. Asexual reproduction is quicker and easier: one aphid for example can have 600 billion offspring in one season.

So why have two sexes? The most likely explanation relates to genetic diversity.

In asexual reproduction, because there is no other parent, i.e. no other source of genetic material, every new generation will be genetically identical to the previous one. (Genetic variation does arise, but through genetic mutation.) This works fine when conditions are stable, but it can potentially put the species at risk.

• Each generation has the same vulnerabilities. If a disease appears that can kill one individual, it can kill the entire population. 
• Each generation depends upon the same habitat, or at least there must be no change in habitat that will harm the species’ survival. If the selection pressures change, the population will adapt much more slowly. 

Sexual reproduction by contrast relies upon the combination of genetic material from different parents, which over several generations can result in a lot of variation.

• Each generation has genetic variation within the population. If a disease appears, there is more chance that some individuals will have resistance and survive. 
• If habitat changes, there is more chance that some individuals will have traits that allow them to adapt.

These are two strategies developed by evolution to ensure the survival of organisms. The fact that organisms of both kinds still exist tells us that both strategies have (so far) been successful in perpetuating species. No one method of reproduction is ‘superior’ to another: there are simply different strategies that have proved successful for certain species under certain conditions.

The answer, then, to the question ‘what is sex?’ is – from a narrow biological perspective – that sex is a means for certain species to reproduce themselves by fusing male and female gametes to ensure genetic diversity. In the next article I’ll look at how this works in our species, Homo sapiens.

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Footnotes

[1] Even people with DSDs are basically either male or female, unless they’re one of the tiny percentage whose sex is ambiguous. I discuss intersex/DSDs separately here. 

[2] Jussi Lehtonen, Hanna Kokko, and Geoff A. Parker, ‘What do isogamous organisms teach us about sex and the two sexes?’ (2016).

A short introduction to genetics

This article may seem like a digression from our investigation of sex and gender, but it isn’t possible to sustain an authoritative discussion without at least a basic understanding of the relevant aspects of genetics, such as what chromosomes are. This is just a short, simple introduction.

Of course, science does not claim to fully understand this staggering achievement of evolution any more than I do.

DNA

All genetic material is made from a chemical called deoxyribonucleic acid, or DNA for short.[1] This is the code of life. The information is stored as a code made from four basic building blocks, or chemical bases (known as nucleotides) which we render as letters: adenine (A), guanine (G), cytosine (C), and thymine (T). These pair up with each other to form units called base pairs. The order, or sequence, of these bases provides the instructions for how an organism should be made, like a recipe of the traits we inherit from our parents. In humans there are roughly 3 billion bases – the number is different for other species.

Credit: US National Library of Medicine

Structurally the DNA molecule is made of two strands that wind together to form the famous ‘double helix’ or double-stranded spiral. This remarkable, elegant structure is shared by all living organisms.[2] The structure resembles a twisted ladder: the base pairs are the rungs, and the two long strands are the uprights.

The advantage of this ladder structure is that it’s highly stable. This is important because when cells divide, the DNA copies itself to the new cell. An exact copy of our DNA is thus found in the core or nucleus of every cell in our body, i.e. each cell contains our entire genetic code. This copying ability allows all known living organisms to grow and reproduce. Cells are the basic structural unit of such organisms (the human body is estimated to have over 30 trillion), and the nucleus controls the processes of the cell.

A small amount of DNA also appears in the mitochrondria (structures within cells that convert energy for the cell to use): this mitochondrial DNA is useful for tracing our (maternal) ancestors but needn’t detain us here.

Genomes

A genome is an organism’s full set of genetic instructions, which tell it how to grow and develop – the total of all its DNA. It is the complete set of the billions of ATCG letters that make you what you are. The complete human genome was mapped in 2000, helping us to explore how genetics works, for example researching the causes of genetic diseases.

Different species vary because each has its own distinct genome, but every individual within a species also has (smaller-scale) variations that give it its own particular configuration. Apart from identical twins, everyone’s genome is unique; we inherit mutations from our parents, but we also have our own mutations, combinations which may never have existed before.

These mutations or glitches – a bit like a spelling mistake – can result in our cells receiving the wrong instructions. This might work to our advantage, or it might confuse the instructions and cause something not to work properly, possibly even with fatal consequences.

Chromosomes

There is an immense amount of information contained within the spiral, and the DNA in each cell would stretch to two metres long if unravelled. That’s a lot to pack into a tiny cell. So evolution has devised a solution: it divides the DNA into (in humans) 46 sections and coils each one into a structure we call a chromosome (along with some proteins that help to stabilise and package it).

Chromosomes are usually pictured in a characteristic X form, but a chromosome only really looks like this during cell division, when it condenses to avoid getting tangled, and has a copy of itself attached. For most of a cell’s life chromosomes look more like string.

The number of chromosomes in the nucleus of a cell is called ploidy; the condition of chromosomes existing in pairs is diploidy; the condition of having an atypical number of chromosomes, i.e. some are missing or extra, is called aneuploidy. The number varies by species (and is not, by the way, a measure of complexity) – fruit flies for example have only 8, dogs have 78 and goldfish have 94. 

Humans are diploid organisms. Each human cell contains 46 chromosomes [3], organised into pairs: we humans have 23 types of chromosome, and two of each type. One of each pair is inherited from our father, the other from our mother. In other words, we inherit half our DNA from either parent: 23 chromosomes from each. This is why we can share some characteristics from our father and others from our mother.

All chromosomes do not look the same: there are several types based on variations in their structure.

22 of these pairs are called autosomes and are the same in both males and females. The 23rd pair, the one of particular interest to us here, is different: the so-called ‘sex chromosomes’. There are two types of sex chromosome, either X or Y.

• Females have two X chromosomes
• Males have one X chromosome and one Y chromosome
• These are the two normal patterns. However, other combinations can be found

An individual’s collection of chromosomes is called a karyotype (‘carry-o-type’). This is also a lab technique that produces an image of the chromosome pairs lined up like in the example below, so they can be checked for abnormalities. Pairs 1-22 are autosomes, pair 23 are the sex chromosomes.

Although female embryos inherit one X chromosome from each parent, in every cell one of the two is ‘switched off’ and is not expressed. This could be either the maternal or the paternal X; in males, the X chromosome is always from the mother.

The X and Y chromosomes are often described as ‘sex chromosomes’ (I’ve done it here myself) but we shouldn’t think of X as female and Y as male, because men have an X too, and thanks to rare DSDs like CAIS it’s possible for females to have Y chromosomes. Also, note that sex is produced by a number of genes interacting in different ways at different stages of development: it is not the product of the X and Y chromosomes alone. 

Chromosomes, then, determine sex but are not synonymous with sex. Karyotypes are not sexes. Having variations in your karyotype – such as XXX (Triple X syndrome) found in about 1 in 1000 females, or XXY (Klinefelter syndrome) found in 1 in 1000 males – does not mean you are a ‘new’ sex. You are just a male or female with an unusual karyotype. 

I shall say more about chromosomes when we discuss human sexual reproduction.

Genes

A gene is a small segment of DNA – a section of a chromosome – that is the basic unit of heredity. Humans are estimated to have about 20,000-100,000 genes, depending on who you ask (they are hard to count and we don’t know for sure). Each of us has two copies of each gene, one inherited from each parent, and while most are the same, a small number (less than 1 per cent) are different. These are known as alleles (‘a-LEELs’), i.e. variant forms of the same gene that allow for the differences in people’s physical characteristics. We’ll talk about those in a moment.

If DNA is a recipe for making an organism, genes are sub-units that contribute the specific ingredients. They convey the information for setting up particular physical traits: whether we have light or dark skin, blue or brown eyes, whether we are short or tall, and so on. Such characteristics may be determined either by a single gene or by several genes in interaction. (Bear in mind however that our traits can also be determined by our environment – genes do not act in isolation from the world.)

How this works is that genes tell cells how to build proteins. Cells need to multiply in order to build up into a new young creature’s body parts: each time a cell multiplies, it copies itself along with the DNA in its nucleus. Then the cells read the instructions in the DNA (this is called gene expression) to make proteins, substances that are essential to structuring and regulating the tissues and organs that make up a living body. Proteins are made from building blocks called amino acids. A gene codes a series of 20 amino acids into one of thousands of possible sequences to produce different proteins that fulfil different functions.

Chromosomes contain the genes that code for the proteins that make up our bodies

Most DNA however (called non-coding DNA) doesn’t code for proteins: i.e. all genes are DNA, but not all DNA is genes. In fact genes only make up 1-5% of your genome. Most DNA is not genes, and instead controls other things like gene expression and a process called gene regulation or gene switching. Each cell performs a different role in the body, so individual cells express (or ‘turn on’) only a fraction of their genes. Genes are switched on or off depending on the intended function of a cell: in eyeball cells, only the eyeball genes are turned on, etc.

This is how the DNA recipe builds a wide range of particular parts into an entire living organism.

Genotype vs phenotype

Sex can be divided into genotype and phenotype. These work together to give people their particular characteristics.

• Genotype is an organism’s genetic constitution: the information or code contained in its genes that determines its physical traits. 
• Phenotype is the expression of the genotype, i.e. the observable physical traits that result.

Genes contain all of your options for how a physical trait might be, passed down by your parents. For each trait, you receive information from either parent in the form of alleles – again, you can think of these as versions of the same gene, containing the different forms the trait can take, such as whether eyes are blue, brown or green. Because we have two parents, we have two copies of each gene, that is, two alleles. These alleles might be the same (both parents have alleles for brown eyes), or they might be different (one has an allele for brown eyes, one has an allele for blue eyes). Where they are different, one allele will be dominant and the other recessive, and it is the former that gets expressed. Your genotype is your entire collection of alleles, and your phenotype is the body that results.

When we talk about ‘sex’, we are talking about both genotype and phenotype.

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Footnotes

1. The discovery of DNA began in the late 1860s with the work of Swiss chemist Friedrich Miescher. The double helix structure was identified in 1953 through the work of Watson, Crick, Franklin and Wilkins.
2. The exception being those viruses which use RNA instead of DNA, assuming you regard viruses as living things, which is another debate.
3. The exception is your sex cells or gametes, which have only 23 each. I’ll discuss those in due course.

Sex and gender

In popular usage, the terms ‘sex’ and ‘gender’ are often used interchangeably, as if they meant the same thing. Sociologists, philosophers and others however mostly agree that they refer to different things. 

• Sex is biological: whether a person is male or female. It is a material reality: you have certain physical features or you don’t. A small minority of people have DSDs (sometimes called intersex), i.e. their sexual characteristics don’t fit easily into the categories of male or female.
• Gender is social: whether a person behaves in masculine or feminine ways. Gender refers to behavioural stereotypes to which people are expected to conform depending on whether they are male or female. Gender is a social construct: it has been built (and can therefore also be unbuilt). 

A whole host of other terms come piling in after. For example, neither term should be confused with sexual orientation, which is who you are sexually attracted to. The terminology and the arguments, like in any field, change over time. This can be a good or bad thing, depending on whose interests are served by the change.

I am going to delve into the topic of sex in the next few articles. Discussions of sex can get controversial and there are wildly divergent opinions. But we can only try to work out what is true, as far as we can.

For the best understanding we need to think about these categories in various ways. We must approach them scientifically, insofar as we are discussing facts about people’s actually existing bodies. We assess material reality with reference to such observable and testable features as biology, anatomy and chromosomes; and this overlaps with approaching them psychologically. We must also approach them philosophically, since we are dealing with concepts that require clarification. Finally we must consider them politically: we live in a world where resources (and therefore power) are distributed unequally, and so whether people are male or female, or masculine or feminine, has profound consequences for their lives. Politics penetrates everything. It would be naive and incorrect to suggest that the science and philosophy of sex and gender have no politics.

To begin with a caveat: describing people using labels is convenient and, to an extent, necessary. Without universal concepts, we couldn’t even hold a conversation. But concepts have their limitations: they are approximations of a world that is infinite. Every human being is particular, and like all particulars is complex and unique, living in a dynamic interaction with an infinitely complex world. Each person has their own experiences, emotions, and relationships. No actually living human being is reducible to a concept or a stereotype.