Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics, including medical conditions.
Your genes are a set of instructions for the growth and development of every cell in your body.
For example, they determine characteristics such as your blood group and the colour of your eyes and hair.
However, many characteristics aren't the result of genes alone – environment also plays an important role.
For example, children may inherit "tall" genes from their parents, but if their diet doesn't provide them with the necessary nutrients, they may not grow very tall.
Genes are packaged in bundles called chromosomes. In humans, each cell in the body contains 23 pairs of chromosomes – 46 in total.
You inherit one of each pair of chromosomes from your mother and one from your father. This means there are two copies of every gene in each cell, with the exception of the sex chromosomes, X and Y.
The X and Y chromosomes determine the biological sex of a baby. Babies with a Y chromosome (XY) will be male, and those without a Y chromosome will be female (XX).
This means that males only have one copy of each X chromosome gene, rather than two, and they have a few genes found only on the Y chromosome that play an important role in male development.
Occasionally, individuals inherit more than one sex chromosome. Females with three X chromosomes (XXX) and males with an extra Y (XYY) are normal, and most never know they have an extra chromosome.
However, females with one X have a condition known as Turner syndrome, and males with an extra X have Klinefelter syndrome.
The whole set of genes is known as the genome. Humans have about 21,000 genes on their 23 chromosomes, plus 37 genes in their mitochondria.
Mitochondria are small structures inside cells that allow the cell to get energy from sugar and fat, and they're only passed on within eggs. This means mitochondrial DNA is only inherited from mothers.
Genes are made up of deoxyribonucleic acid (DNA). DNA is a long molecule made up of a combination of four chemicals: adenine, thymine, cytosine and guanine, represented as letters A, T, C and G.
These "letters" are ordered in particular sequences within your genes. They contain the instructions to make a particular protein, in a particular cell, at a particular time.
Proteins are complex chemicals that are the building blocks of the body. For example, keratin is the protein in hair and nails, while haemoglobin is the red protein in blood.
Genes and medical conditions
As well as determining characteristics such as eye and hair colour, your genes can also directly cause or increase your risk of developing a wide range of medical conditions.
Although not always the case, many of these conditions occur when a child inherits a specific altered (mutated) version of a particular gene from one or both of their parents.
Examples of conditions directly caused by genetic mutations include:
- muscular dystrophy – causes the muscles to weaken over time, leading to an increasing level of disability
- Down's syndrome – affects a child's normal physical development and causes learning difficulties
- cystic fibrosis – causes the lungs and digestive system to become clogged with thick, sticky mucus
- Lynch syndrome – causes an increase in the risk of certain cancers; for example, bowel cancer and womb cancer
There are also many conditions that aren't directly caused by genetic mutations.
These conditions can occur as the result of a combination of an inherited genetic susceptibility and environmental factors, such as a poor diet, smoking and a lack of exercise.
Read more about how genes are inherited.
Genetic testing can be used to find out whether you're carrying a particular genetic mutation that causes a medical condition.
This can be useful for a number of purposes, including:
- diagnosing certain genetic conditions
- predicting your likelihood of developing a certain condition
- determining if any children you have are at risk of developing an inherited condition
Testing usually involves taking a blood or tissue sample and analysing the DNA in your cells.
Genetic testing can also be used to find out whether a foetus is likely to be born with a certain genetic condition. A sample of cells from the womb is extracted and tested.
Read more about genetic testing and counselling.