Hereditary diseases are passed down genetically, but it's not guaranteed that every child will have every disease; it depends on the genetic pattern. Genes are contained within chromosomes, which are inside the nucleus of every cell in the human body. You have 46 chromosomes in each cell, with the exception of your sex cells, or eggs and sperm. Of those chromosomes, 44 are called autosomes. Each autosome has a partner chromosome; half of the pair is from your mother and half is from your father. That means you have two genes for every human trait, although the two genes aren't necessarily identical. Typically, one of the genes is considered dominant and one is recessive.
If you have one gene for a disease, you may be sick with it and you may not, depending on whether the gene you have is dominant or recessive. If it's recessive, you might pass it on to your children but not actually have any of the disease's symptoms yourself. In autosomal dominant diseases, the gene for a hereditary disease is a dominant one, which means it can't be canceled out by a healthy gene. If a parent has an autosomal dominant disease, his children have a 50/50 chance of contracting the disease. An example of an autosomal dominant disease is Huntington chorea, which is a brain disorder that causes abnormal movement and midlife mental deterioration. Compare that to an autosomal recessive disease, in which children have only a 25 percent chance of ever having the disease.
Sickle cell anemia is an example of a hereditary disease passed down through autosomal recessive inheritance. If both parents are carriers of the sickle cell anemia gene, their children have a one-in-four chance of having the disease. Unfortunately, there is no cure for sickle cell anemia, but with modern treatment, children who have the disease often make it to adulthood. Sickle cell anemia is more prevalent among blacks than others - - about 8 percent of blacks carry the gene - - so genetic screening is recommended for black couples.
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