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Pure genetic diseases are caused by a single error in a single gene in our DNA. The effects of the disease depend on what that gene was supposed to do. Everyone has two copies of each gene, one copy on each of the chromosome pair, and some diseases require both copies to be damaged (recessive diseases), whereas others need only one gene copy damaged (dominant diseases). Thus, there are two main classes of pure single-gene genetic diseases:
However, there is an extra wrinkle in this pattern with regard to the sex-determining genes X and Y. Not everyone has two copies of the genes on the sex chromosome. Men are XY and women are XX. Hence, men have only one copy of the X chromosome, whereas women have two. Note that genetic errors on the Y chromosome are extremely rare because it is very small. Because of this anomaly, genetic diseases are also classed as follows:
With this in mind, there are actually four major classes of genetic diseases with different inheritance patterns:
Y-linked diseases: Note that diseases of the Y chromosome are almost non-existant. See Y-Linked Genetic Diseases.
Sporadic genetic diseases: Non-inherited exceptions: random mutations: Most genetic diseases are inherited from the parents, with different inheritance patterns depending on its dominance versus recessiveness and whether it is located on an autosome or the X chromosome. However, not all genetic diseases are inherited. Genetic diseases can also arise naturally via genetic mutations, which is presumably how the diseases arose in the first place. Hence, rare cases of genetic diseases are possible even if neither parent has any genetic history of the particular disease. In such cases, the risk of a reoccurrence in a second child is very low, but genetic testing is required to check whether the parents really do not have the genetic condition.
Rare exceptions: germinal mosaicism: Sometimes even an autosomal dominant disease can arise surprisingly when neither parent has the disease. This can be due to a mutation, but need not be, and it can also be due to "germinal mosaicism". This is a condition where one parent's gamete cells have the genetic defect, whereas their normal cells do not. These conditions are very rare.
Non-contagiousness of genetic diseases: All types of genetic diseases occur at birth. You cannot catch the disease from someone else who has the disease. You are either born with the genetic disease or not. So the disease can be inherited from your parents through the DNA, but cannot be caught later in life. Genetic tests can determine whether or not a person has the disease, even as early as in the fetus by antenatal testing for genetic diseases.
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