Genetic diseases are caused by changes in a persons DNA sequence, which can be due to an error in DNA replication or environmental factors such as chemical exposure, radiation exposure, smoking, or UV exposure from the sun. These changes can affect the protein that the DNA codes for, and may result in a genetic disorder. Genetic disorders can be caused by a mutation in one gene (monogenic disorder), by mutations in multiple genes (multifactorial inheritance disorder), by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that carry genes) . Some genetic disorders are inherited from parents and are present in an individual at birth, while others occur only in the child and are not present in the parents. Genetic disorders can be grouped into three main categories:
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Single gene disorders: These are disorders caused by defects in one particular gene, often with simple and predictable inheritance patterns. Examples of single gene disorders include dominant diseases (occur when an individual has one altered copy of the relevant gene and one healthy copy) such as Huntingtons disease, and recessive diseases (occur only when an individual has two altered versions of the relevant gene) such as cystic fibrosis.
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Chromosomal disorders: These are disorders caused by missing or changed chromosomes (or parts of chromosomes). Examples of chromosomal disorders include Down syndrome, which results from an extra chromosome 21.
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Multifactorial disorders (complex diseases): These are disorders caused by changes in multiple genes, often in a complex interaction with environmental and lifestyle factors such as diet or cigarette smoke. Examples of multifactorial disorders include cancer.
Genetic testing can help identify genetic disorders, and doctors can use genetic maps to find and treat or cure some kinds of genetic disorders.
