Our bodies are made up of millions of cells, which all arise from one single cell, formed when a baby is first conceived. A chromosome is a strand of DNA. Each cell contains 46 chromosomes made up of 23 pairs. These pairs of chromosomes are numbered from 1 to 23 according to size as shown below.

Pairs 1 to 22 each contain two identical chromosomes. One of the chromosomes in each pair is inherited from a person's mother and the other from a person's father. Pair number 23, the sex chromosomes, determine the sex of the baby. Males have an X and an Y chromosome and females have two X chromosomes.

In order to produce a child with 46 chromosomes in each cell, eggs and sperm cells only contain 23 chromosomes, one of each pair. When an egg is fertilised by a sperm, the 23 chromosomes from the sperm come together with the 23 chromosomes from the egg and produce the full complement of 46 chromosomes. This cell then divides into two identical copies, which then divide again and again. In this way 2 cells become 4, then 8 then 16 and so on. This bundle of cells then continues dividing and becomes a baby.

Chromosomes carry our genes. Humans have approximately 30,000 genes, and it is these genes that provide the instructions telling our bodies how to grow, develop and function. Because our chromosomes come in pairs, we have two copies of each gene.

Sometimes when a baby is conceived, the first cell may contain extra or missing chromosomes. This can occur when the egg or sperm that produces the baby contains too few or too many copies of a chromosome. Down's syndrome is caused when there are three copies (trisomy) of chromosome 21. Klinefelter's syndrome and Turner's syndrome are caused when there are more or less than two sex chromosomes.

In other cases, mistakes can occur when the cells divide so that some cells end up with too many or too few chromosomes. In individuals where this happens, some of the cells will contain more or less than 46 chromosomes. People who have some cells with 46 chromosomes and other cells with more or less than this, are referred to having mosaicism. The percentage of cells with the extra or missing chromosomes depends on how early the mistake occurred. The earlier it happened the more cells will be affected.

As well as having too few or too many whole chromosomes, cells may contain extra or missing pieces of chromosomes. These changes are called translocations.

Very rarely, the DNA on the chromosome may contain a mutation or 'spelling mistake'. This can lead to a genetic condition such as cystic fibrosis. There are over 5,000 conditions resulting from a change in a single gene, but most of them are very rare. Cystic fibrosis is a recessive genetic condition, which means that individuals are only affected if they inherit two copies of the faulty gene. Some conditions (for example achondroplasia or restricted growth) are dominant, which means that an individual is affected even if they only inherit one copy of the faulty gene.

Changes in chromosomes or genes alter the usual situation in which we have 2 correct copies of each gene. Such changes may lead to differences in the way in which a person grows, develops and functions.