This is a brief discussion on chromosomes and some other general biology information that you may find helpful when reading about Arran and his medical situation.
Humans are made up of millions of tiny cells. There are many different types of cells in your body that function differently and make up different organs in your body. Dispute all of the differences the cells all have a nucleus. The nucleus acts as the cell's "brain" or "information center". It is made up of deoxyribonucleic acid (DNA). This DNA is a long spiral-shaped molecule which acts as a blue print for building us. Our genes are contained within the DNA. The genes carry specific instructions for our bodies on how it should function and grow.
Chromosomes are tiny thread like packages of genes present in every cell of the body. Using a microscope, scientists can distinguish one chromosome from another. Humans usually have 23 pairs of chromosomes. pairs 1-22 are numbered and they are called autosomes (non-sex chromosomes) The last pair are the sex chromosomes, labeled "X" and "Y". Since Arran is male he is XY (girls are XX).
Chromosomes are shaped with an upper and a lower portion. These segments are called arms. The arms are separated by centromeres. The shorter arm of the chromosome is called the "p" arm and the longer is called the "q" arm. When looking at a chromosome you are able to see different banding patterns within the arms. These patterns also help to distinguish what genes may be missing if there is a deletion.