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The fact that genetics is playing a larger role in the cause and potential treatment of all diseases makes knowledge about families important. Doctors are now taking a more extensive family history, and if it seems warranted, they construct a genetic diagram (family tree; see Figure 3-2 for an example).

FIGURE 3-2: Diagram of genetic history.

Many people have at least one disease-causing genetic mutation if it occurs on both chromosomes. Fortunately, people have two chromosomes, so the mutation does not cause a clinical problem. But if two people have children and they both have the same mutation, then one out of four children may have the disease. A family tree may show relatives that had symptoms of the diseases which would put the couple on alert to test for the mutation.