Genes and Chromosomes
A gene is a short stretch of DNA that contains the instructions for making one particular protein (the molecules that do most of the work in a cell). Your DNA contains around 20,000 genes: ones that decide your hair colour, ones that build your eyes, ones that tell your stomach how to digest food, and many more. These genes are organised into bigger packages called chromosomes. Humans have 46 chromosomes in every cell, arranged as 23 pairs.
- Human genesapprox. 20,000A surprisingly small number
- Human chromosomes46 total23 pairs
- Chromosomes from mum23One from each pair
- Chromosomes from dad23One from each pair
- Sex chromosomesXX or XYXX = typically female; XY = typically male
- Banana chromosomes22Closer to ours than you might think
What is a gene?
A gene is a section of DNA that the cell can "read" to make a particular protein. Different genes carry the instructions for different proteins. For example:
- One gene tells your cells how to make haemoglobin, the protein that carries oxygen in your blood.
- Another gene contains the recipe for insulin, the hormone that controls your blood sugar.
- Yet another gene tells your stomach how to make pepsin, the enzyme that digests proteins in your food.
You have around 20,000 genes in total. Surprisingly that is not many more than a worm (which has about 19,000) or a banana (which has about 36,000, more than us). It turns out that complex creatures like humans manage to do more with fewer genes by combining them in cleverer ways.
What is a chromosome?
The DNA inside a cell is enormously long (around 2 metres if stretched out). To fit it into the tiny nucleus, the cell winds it up tightly around proteins called histones, then coils it up further, and further, until it forms compact bundles called chromosomes.
You can think of a chromosome as one very long stretch of DNA, folded thousands of times to fit. Each of your 23 chromosome pairs is a different size, with different genes on it. Chromosome 1 (the longest) carries about 2,800 genes. The Y chromosome (the smallest) carries only about 60 genes.
Why we have 23 pairs
Humans have 23 pairs of chromosomes: 46 chromosomes in total. You inherited one of each pair from your mother (through the egg cell) and one from your father (through the sperm cell). The two chromosomes in a pair carry the same genes in the same order, but the exact versions of those genes (alleles) often differ slightly. That is why you can have your mother's eyes and your father's nose.
Different species have different chromosome numbers, with no clear pattern. A fruit fly has 8 chromosomes; a chicken has 78; a goldfish has 100; the Indian fern Ophioglossum reticulatum has the record at over 1,260. There is no link between intelligence and chromosome count.
The sex chromosomes
One pair of human chromosomes is special: the sex chromosomes. There are two types, called X and Y. Typical biological females have two X chromosomes (XX); typical biological males have one X and one Y (XY).
The Y chromosome is much smaller than the X and carries very few genes. It includes one critical gene called SRY that triggers the development of male reproductive organs. Without the Y, an embryo develops along the female path by default. (There are also more unusual chromosome arrangements like XXY or XO; biology is more varied than the simple textbook picture.)
How genes get passed on
When a sperm meets an egg, the new fertilised egg cell has 23 chromosomes from each parent, making 46 total. As the embryo grows, every one of its trillions of cells copies that same complete set. So every cell in your body carries an identical mix of half your mother's genes and half your father's.
But which particular half? When eggs and sperm are made (a process called meiosis), each pair of chromosomes is split up randomly, and the two members of each pair can even swap chunks in a process called recombination. This means siblings born to the same parents inherit different mixes of their parents' genes. It is why your brother or sister looks similar to you but never exactly the same.
Deeper dive: junk DNA, and why we have so much of it
One of the great surprises of the Human Genome Project (finished in 2003) was that only about 2% of human DNA actually codes for proteins. The other 98% was initially nicknamed "junk DNA", because biologists assumed it had no function.
Over the past 20 years it has become clear that "junk DNA" is not really junk at all. Most of the non-coding DNA falls into several useful categories:
- Regulatory regions: short DNA sequences that switch other genes on and off at the right times. These are essential for development.
- Non-coding RNA genes: produce short pieces of RNA that help control how other genes are used.
- Structural elements: provide the framework that keeps chromosomes the right shape.
- Ancient viral DNA: roughly 8% of the human genome is the leftover DNA of ancient retroviruses that inserted themselves into our ancestors' DNA millions of years ago. Some of this old viral DNA has even been repurposed for useful jobs (the placenta in mammals depends on a gene of viral origin).
Even genuinely "useless" DNA does no harm: it just gets copied along with the rest each generation. Some scientists think the leftover, non-functional DNA is part of why mutations are mostly safe, since most random changes hit DNA that does not matter.
For where genes come from, see inheritance. For DNA changes, see mutations. For the early scientist who worked out the rules, see Gregor Mendel and heredity.