Any changes may be small 'misprints' or bigger alterations, with a corresponding effect of the 'readability' of the instructions. The two copies of each gene may not be identical. In general, there are only two copies of each gene and in all people they are always on the same chromosome pair. Apart from the X and Y chromosomes, the order of the genes on one chromosome exactly matches the order on the other chromosome of the same pair (and on chromosomes of the same number in everyone else). The genes, thousands on each chromosome, are spaced along the length of the DNA molecule, in a definite order. When seen down a microscope the chromosomes lie in a haphazard manner, but when they are photographed, cut out and arranged in pairs, a picture of an individual's chromosomes known as a karyotype is obtained. Each pair has a different staining pattern. In chromosome tests, the chromosomes (usually from blood cells) are stained with dye. The sex chromosomes are different from the others. Girls have two X chromosomes and boys have an X and a Y. The 23rd pair are the sex chromosomes which are called the X and Y chromosomes. Each chromosome has a long arm and a short arm. The chromosome pairs are numbered 1-22 according to size. A person's characteristics are determined by the interaction of the copies of the genes from each parent. One chromosome of each pair comes from the mother and one from the father. Each pair of chromosomes has different genes. There are 46 chromosomes (in 23 pairs) in all body cells (except the sperm and egg cells). Thus chromosomes carry our genetic information. The genes are grouped together in long, thin, thread-like structures called chromosomes. There are also long stretches of DNA, the precise role of which is still a mystery. These chemical instructions tell the body what components to make, and how, when and in what quantity to make them, in order to ensure normal working of the body. The arrangement of chemicals in the DNA is different for every gene. The genes are made of DNA (deoxyribonucleic acid). Most people have two copies of every gene - in general one copy comes from the mother and the other from the father. Genes are passed from parents to children. The body's supply and use of building and maintenance materials.The timing of the milestones of growth and development.

The way babies and children grow and develop, even before they are born.Genes therefore control or influence such things as: Genes contain the body's instructions for making its parts and for its day to day running. The cell then divides in two with one copy of the genes in each new cell. It does this by making an exact copy of the genes in a cell. The active genes in a cell are only those appropriate to the cell type and its functions. Not all the genes are active in any one cell at any one time, however. The genes are the same in every cell of the body. The production and output of each cell is controlled by genes. Different parts of the body have specialised cells with special tasks, for example muscle cells are different from nerve cells, heart cells are different from brain cells. Each cell is like a tiny factory which makes the materials needed for growth and maintenance of the body. This is enough to change the finely-tuned balance of the body and produce the physical and intellectual characteristics found in people with Down's syndrome. People with Down's syndrome have the same genes as anyone else they just have 1% extra. They provide the chemical information that is needed to maintain our body and keep it working well. They are inherited from our parents and are responsible for our development from fertilized egg to fully grown adult. Genes are present in every cell of the body.

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