Genetics

Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics – including medical conditions.

About genes

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 due to 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.

Chromosomes

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, whereas 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 and 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, so the human genome contains two copies of those 21,000 (except for those on X and Y in males).

DNA

Deoxyribonucleic acid (DNA) is the long molecule found inside chromosomes that stores genetic information. It is tightly coiled into a ‘double helix’ shape, which looks like a twisted ladder.

Each 'rung' of the ladder is made up of a combination of four chemicals – adenine, thymine, cytosine and guanine – which are represented as the letters A, T, C and G.

These 'letters' are ordered in particular sequences within your genes and 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.

There are around six billion letters of DNA code within each cell.

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 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 – which causes the muscles to weaken over time, leading to an increasing level of disability
• Down's syndrome – which affects a child's normal physical development and causes learning difficulties
• cystic fibrosis – a condition in which the lungs and digestive system become clogged with thick sticky mucus

There are also many conditions that are not directly caused by genetic mutations, but 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

Genetic testing can be used to find out whether you are 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 and 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 carried to find out if a foetus is likely to be born with a certain genetic condition by extracting and testing a sample of cells from the womb.

Read more about genetic testing and counselling.