PGD for highly penetrant adult disorders such as Alzheimer’s or Huntington’s disease prevents the birth of a child who will be healthy for many years, but who in her late 30s or early 40s will experience the onset of progressive neurological disease leading to an early death.

Although these indications do not involve diseases that manifest themselves in infancy or childhood, the conditions in question lead to substantial health problems for offspring in their thirties or forties.

Avoiding the birth of children with those conditions thus reflects the desire of parents to have offspring with good prospects for an average life span.

By being able to identify these genes through particular "markers" associated with the gene, doctors will know which individuals are more susceptible to late-onset disorders.

The extremely difficult life experience of families affected by inherited Alzheimer's disease or any other catastrophic late-onset disorder--being unable to help their family members suffering from the disease and being afraid they will soon develop the disease themselves--makes them responsible for insuring that their children will not face the same difficulties.