A genetic disorder is a condition caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due to genetic abnormalities acquired in a few cells during life, the term "genetic disease" most commonly refers to diseases present in all cells of the body and present since conception. Some genetic disorders are caused by chromosomal abnormalities due to errors in meiosis, the process which produces reproductive cells such as sperm and eggs. Examples include Down syndrome (extra chromosome 21), Turner Syndrome (45X0) and Klinefelter's syndrome (a male with 2 X chromosomes). Other genetic changes may occur during the production of germ cells by the parent. One example is the triplet expansion repeat mutations which can cause fragile X syndrome or Huntington's disease. Defective genes may also be inherited intact from the parents. In this case, the genetic disorder is known as a hereditary disease. This can often happen unexpectedly when two healthy carriers of a defective recessive gene reproduce, but can also happen when the defective gene is dominant.
Currently around 4,000 genetic disorders are known, with more being discovered. Most disorders are quite rare and affect one person in every several thousands or millions. Cystic fibrosis is one of the most common genetic disorders; around 5% of the population of the United States carry at least one copy of the defective gene. Some types of recessive gene disorder confer an advantage in the heterozygous state in certain environments.
Genetic diseases are typically diagnosed and treated by geneticists. Genetic counselors assist the physicians and directly counsel patients. The study of genetic diseases is a scientific discipline whose theoretical underpinning is based on population genetics.

Multifactorial and polygenic disorders