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.
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| Credit: US National Library of Medicine |
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).
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.
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.
I shall say more about chromosomes when we discuss human sexual reproduction.
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.
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.
Sex can be divided into genotype and phenotype. These work together to give people their particular characteristics.
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.
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.
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
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.
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| 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.
When we talk about ‘sex’, we are talking about both genotype and phenotype.
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.
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