Chromosomes

The following information on Chromosomes is provided by DDC, the UK's leading expert on DNA, Paternity and Genetic Testing.

Simply put, chromosomes are the structures that hold our genes. Genes are the individual instructions that tell our bodies how to develop and keep our bodies running healthy. In every cell of our body there are 50,000 to 100,000 genes that are located on 46 chromosomes. These 46 chromosomes occur as 23 pairs. We get one of each pair from our mother in the egg, and one of each pair from our father in the sperm. The first 22 pairs are labeled longest to shortest. The last pair are called the sex chromosomes labeled X or Y. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY).

Everyone should have 46 chromosomes in every cell of their body. If a chromosome or piece of a chromosome is missing or duplicated, there are missing or extra genes respectively. When a person has missing or extra information (genes), problems can develop for that individual's health and development.

Each of the chromosomes has a "p" and "q" arm; "p" (petit) is the short arm and "q" is the long arm. Some of the chromosomes, such as 13, 14, and 15, have very small "p" arms. When a karyotype is made (see below), the "q" arm is always put on the bottom and the "p" is on the top. The arms are separated by a region known as the centromere (red in picture), which is a pinched area of the chromosome.

The chromosomes need to be stained in order to see them with a microscope. When stained, the chromosomes look like strings with both light and dark 'bands'. Each chromosome arm is defined further by numbering the bands, the higher the number, the further that area is from the centromere.