Circular membrane that delimits the pupil. The iris acts as a diaphragm that contracts according to the intensity of light. Pigmented tissue lying behind the cornea that gives color to the eye (e.g., blue eyes) Controls light by contracting and constricting the opening (pupil).
Fluid pressure exerted inside the eyeball (ocular globe), which keeps the wall taut.
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.