No one is overjoyed to find they're carrying a rare recessive gene that might cause a disability in their child.

But these very rare genes are responsible for just 2 percent to 7 percent of Alzheimer's cases and cause the disease in middle-aged people.

Dr. Haseltine said his method picked up rare genes too.

Red hair is caused by a relatively rare recessive gene, the expression of which can skip generations.

The second approach is to look for rare genes in unusually long-lived families.

This greatly increases the likelihood that both spouses will carry the same disease-causing rare recessive gene.

Blonde hedgehogs have a rare recessive gene giving rise to beady, button-black eyes and creamy-coloured spines; they are not strictly speaking albino.

Eventually the rarer genes will disappear and the genetic types leaving fewer offspring will become extinct.

In Europeans it is a rare gene.

A rare inherited gene disorder called Lesch-Nyhan syndrome.