These genes, which are contained on our 23 pairs of chromosomes, make up our genetic blueprint. Each gene codes for a particular set of instructions, and a gene's function is determined by its unique DNA code. DNA consists of four basic building blocks called bases that are linked in a specific order. When a change occurs in the ordering or number of bases, a gene may not function properly. A gene change which can cause a disease is called a mutation .
Genes come in pairs, with one copy inherited from each parent.
A condition is called dominantly inherited when only one copy of a disease gene is needed to lead to symptoms of that disease. A person affected by a recessively inherited condition inherits a particular disease gene from each parent. One example is cystic fibrosis in which both parents, by chance, have passed on a CF gene. Some diseases do not follow simple patterns of inheritance. Many factors influence how a gene works or who will get a disease and when.