Geneticists group genetic disorders into three categories: 1. Single gene disorders caused by a mistake in a single gene. The mutation may be present on one or both chromosomes of a pair. Sickle cell disease, cystic fibrosis and Tay-Sachs disease are examples of single gene disorders. 2. Chromosome disorders caused by an excess or deficiency of the genes. For example, Down syndrome is caused by an extra copy of a chromosome, but no individual gene on the chromosome is abnormal. 3. Multifactorial inheritance disorders caused by a combination of small variations in genes, often in concert with environmental factors. Heart disease, most cancers and Alzheimer’s disease are examples of these disorders.