A genetic disorder is a disease caused by changes or mutations in an individuals DNA sequence. These mutations can be due to an error in DNA replication or due to environmental factors such as chemical exposure, diet, certain medications, tobacco, or alcohol use. Genetic disorders can be divided into three main categories:

1. Single gene disorders: These disorders are caused by defects in one particular gene, often with simple and predictable inheritance patterns. There are over 10,000 human disorders caused by a change, known as a mutation, in a single gene. Individually, single gene disorders are each very rare, but as a whole, they affect about one percent of the population. Since only a single gene is involved, these disorders often can be easily tracked through families, and geneticists can predict the risk of them occurring in later generations. Single gene disorders can be divided into different categories: dominant, recessive, and X-linked.

2. Chromosome disorders: These disorders result from changes in the number or structure of the chromosomes that hold our genes. Down syndrome is an example of a chromosomal disorder.

3. Complex disorders: These disorders are caused by mutations in two or more genes, often in a complex interaction with environmental and lifestyle factors such as diet. Many common diseases of adult life, such as diabetes mellitus, hypertension, schizophrenia, and most common developmental abnormalities, such as cleft lip and congenital heart defects, have a strong genetic component and are caused by more than one genetic change. Because polygenic diseases involve more than one gene, inheritance patterns are diverse and complex. If a parent has a disease, it does not necessarily mean that their children will inherit it.