The nineteenth-century Czech monk Gregor Mendel, studying pea plants over many years, figured out the mathematics of inheritance. Females have two X chromosomes. So with an X-linked disorder, men have a much greater chance of being affected by the disease. Hemophilia, a bleeding disorder, was the first human trait discovered to show X-linked inheritance. Even minor cuts can bleed uncontrollably.
People sometimes undergo carrier screening well before they decide to have children.
In the Orthodox Jewish community, for example, young couples may check whether they both are carriers for Tay Sachs, a lethal hereditary disease, before deciding to marry. If they have a family history of a particular disease, they may undergo tests that look specifically for the responsible gene variation. Or, if such a test doesn't exist, the laboratory may look for tell-tale proteins or enzymes in their blood. Next, they may decide to do further testing to see whether the fetus itself is at risk. Early in the Gene Hunt Genetic testing presents deeply personal choices.