glossary

Far-sightedness, an eye that is too short and/or insufficiently powered. The image forms behind the retina, which explains why the hyperopic subject has better eyesight in far vision than in near vision. In cases of mild hyperopia, the subject sees correctly in far vision by compensating the hyperopia through accommodation. In cases of severe hyperopia, the eye can no longer compensate in this way. Focusing defect in which an eye is underpowered. Light rays coming from a distant object strike the retina before coming to sharp focus, causing blurred vision. Corrected with plus (convex) lenses.

Long-sightedness, requiring a convex lens to focus the vision.

The concept that even though several members of a family or population may have the same DNA mutation, not everyone with that mutation will have or develop the disease. This is in contrast to variable expressivity which means that people with the same disease have varying disease severity. Penetrance, in contrast, is more of a yes/no question ? does or does not the person have the disease.