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Hereditary diseases are diseases or disorders that are genetically passed on from parents to offspring. Such diseases are caused by mutations or deformities in genes or in chromosome structure that can be passed down through generations. In many cases, a recessive form of the genetic disorder is passed down and the actual hereditary disease is not expressed at all. When such a "carrier" possessing the recessive form produces offspring with another carrier, however, it is possible for the offspring to express the disorder. Hereditary diseases, then, can be passed down through families for generations without anyone actually becoming ill.
There are many different inheritance patterns by which hereditary diseases can be inherited. The pattern of inheritance is generally based on the particular type of genetic aberration and its chromosomal location. The genetic basis for many hereditary diseases can, for instance, be either recessive or dominant. If it is recessive, both parents must possess at least one copy of the genetic aberration for the offspring to have the illness. Dominant genetic disorders, on the other hand, can cause disease symptoms if even a single copy of the aberration is present, so it is possible for a child to have the disease even if only one parent possesses the genetic aberration.
The inheritance patterns of hereditary diseases can also be affected by the chromosome on which a genetic aberration is located. Some hereditary diseases, for example, are sex-linked, meaning that they are present on the X chromosome. Males have only one X chromosome, so a single copy of the genetic aberration is enough to cause the expression of the disease. Females, on the other hand, have two X chromosomes, so two copies of the genetic aberration—one on each X chromosome—are necessary to cause disease expression. Some genetic disorders can also be Y-linked; this means that all male progeny with fathers possessing the Y-linked disorder will also have the disorder, as fathers pass the Y chromosome on to male progeny.
The expression of some hereditary diseases is not strictly based on inheritance patterns. Some diseases, for example, do tend to run in families but also tend to require some environmental factor. Heart disease, for instance, tends to run in families, but there are many other factors, including diet, environment, and lifestyle, that can aggravate or moderate the condition. There are also many hereditary diseases that are dependent on numerous different genes and therefore display more complex inheritance patterns.
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