A<unk>  is a condition caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due to<unk>  abnormalities acquired in a few cells during life, the term<unk>  disease" most commonly refers to diseases present in all cells of the body and present since conception. Some<unk> s 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<unk>  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<unk>  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 about 4,000<unk> s are known, with more being discovered. Most<unk> s are quite rare and affect one person in every several thousands or millions. Cystic fibrosis is one of the most common<unk> s; around 5% of the population of the United States carry at least one copy of the defective gene. Some types of recessive gene<unk>   confer an advantage in the heterozygous state in certain environments.<unk>  diseases are typically diagnosed and treated by<unk> ists.<unk>  counselors assist the physicians and directly counsel patients.  The study of<unk>  diseases is a scientific discipline whose theoretical
underpinning is based on population<unk> s.
Genetic disorder
Congenital abnormality
Autoimmune disease
Neurodegenerative disease
Vascular injury
Neuromuscular disease
Physical trauma
Immunodeficiency