A genetic disorder is a genetic problem caused by one or more abnormalities in the genome, especially a condition that is present from birth. A genetic disorder is caused by an altered gene or set of genes. The four broad groups of genetic disorders are single gene disorders, chromosome abnormalities, mitochondrial disorders and multifactorial disorders. The four main ways of inheriting an altered gene are autosomal dominant, autosomal recessive, X-linked dominant and X-linked recessive. Genes are paired – one copy of each gene pair is inherited from the mother and the other copy from the father. Around 6,000 known genetic disorders are caused by inheriting an altered gene. A chromosome disorder means there is a change in either the structure or the number of chromosomes. This can happen in three main ways: The altered chromosome is passed from the parent to the child, the abnormality happens when either the sperm or egg (germ cells) is created and soon after conception. Mitochondria are like little batteries that make energy within each cell. The energy source is a chemical called adenosine triphosphate (ATP). Organs like the brain, heart and liver can’t survive without ATP. The symptoms of a mitochondrial disorder, depending on the genes involved. Multifactorial disorders, such as many common birth defects or diseases like high blood pressure, are disorders caused by the environment interacting with the action of several genes.