Sometimes penetrance can be referred to in terms of a time scale. For example, Huntington's Disease has a lifetime penetrance of 100%, but has an age-related penetrance as follows:
(graph appears on screen)
According to this graph, at age 20 there is virtually no chance of showing signs of Huntington's and by age 70 everyone who is going to be affected will have got it. If an individual has a positive test result at a relatively young age, we cannot predict with certainty at what age that individual will develop the condition. This is one of the challenges associated with counselling for late-onset conditions.
Although why variations in penetrance occur is often unclear, the effects of penetrance likely depend on interaction of genes and environmental factors.
To view this interactive tutorial, you need the latest Flash player.