Genetics 

Genetics is the branch of science that deals with how you inherit your physical and behavioural characteristics. The genetic information that controls these characteristics, such as the colour of your hair and eyes, is located in genes (single units of genetic material), which are found in chromosomes.

However, it is important to remember that characteristics are not just due to genes alone – the environment is very important.

A child may inherit tall genes from their parents, but if the child is malnourished (has a diet that does not give them the nutrients they need to maintain good health), they may not grow very tall.

Chromosomes

Each cell in the body contains 23 pairs of chromosomes. These carry the genes that you inherit from your parents. In this way you can inherit a health condition or disease, or a tendency to develop a particular condition. One in each pair is inherited from each parent, so with one exception, there are two copies of each gene in each cell.

The exception is with the X and Y chromosomes, which determine your sex (see the box to the left). People with an X and a Y chromosome develop as males. Those without a Y chromosome develop as females. This means that males only have one copy of each X chromosome gene.

Each chromosome contains hundreds or thousands of genes. In total, there are about 21,000 genes, all written into the DNA contained in the chromosomes. The whole set of genes is called the genome.

DNA

Deoxyribonucleic acid (DNA) is the long molecule that stores genetic information. This information is written in the DNA using an alphabet of only four 'letters', or smaller molecules: A, C, G and T.

Each gene consists of a precise sequence of letters. This is the genetic code. Each gene contains the instructions to make a particular protein, in a particular cell, at a particular time. Proteins are complex chemicals that make up the:

• blood
• muscles
• tendons (fibrous cords that join bones to muscle)
• skin
• nerves
• organs

DNA is made of two strands coiled together, each one a mirror image of the other (see the image to the left). Because of this, it can be divided easily when a cell divides and the genetic code is passed on exactly. The process of DNA copying when a cell divides is called replication.

There are about 3 billion letters of DNA code in the 23 chromosomes.

The Human Genome Project

The Human Genome Project is an international scientific project that involves thousands of scientists around the world.

The initial project ran from 1990 to 2003. Its objective was to map the immense amount of genetic information that is found in every human cell.

As well as identifying specific human genes, the Human Genome Project has enabled scientists to gain a better understanding of how certain traits and characteristics are passed on from parents to children.

It has also resulted in a better understanding of the role of genetics in inherited conditions, such as:

• muscular dystrophy – a condition that gradually causes the muscles to weaken over time, leading to an increasing level of disability
• Down's syndrome – a condition that affects a baby's normal physical development and causes mild to moderate learning difficulties
• cystic fibrosis – a condition that causes the internal organs to become clogged with thick, sticky mucus