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Every person's body is made up of millions of tiny structures called cells.
Each cell comes with a full set of instructions which tell the cell what to
do and how to make our bodies work. The instructions are called genes, and
they are made from a chemical called DNA. Genes usually come in pairs, and
they determine everything about our bodies. For example, certain genes
determine the color of our eyes, while other genes determine our blood type.
Genes are often
called the units of heredity because the information they contain is passed
from one generation to the next. We all get one gene in each pair from our
mothers and the other gene in the pair from our fathers. In this way our
bodies work with a combination of instructions inherited from both our
parents. Parents have no control over which genes get passed to their
children.
Ataxia
Telangiectasia is called a recessive genetic disease, because parents do not
exhibit symptoms, but they each carry a recessive gene which may cause A-T
in their offspring. The genetic path of A-T is therefore impossible to
predict, and an A-T child is almost always a shock to parents. The recessive
gene may lie dormant for many generations until suddenly two people with the
defective gene have children.
Each time two such
A-T carriers have a child together, there is a 1-in-4 chance (25% risk) of
having a child affected with A-T. And every healthy sibling of an A-T
patient has a 2-in-3 chance (66% risk) of being a carrier, like his parents.
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