Huntington's disease, a neurological disorder of adulthood that eventually leads to complete disability and death, passes from parent to child in a dominant fashion. Someone born with one copy of the Huntington's gene probably will develop the disease if he or she lives long enough. If the gene is recessive, then the child would have to receive two copies - one from each parent. Cystic fibrosis, the most common genetic disease among people of European descent, works this way. 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.