Sickle cell anemia is known to run in a family. A pedigree chart for this family is shown below.

The parents are shown at the top and from left to right appear as a half-shaded circle next to a half-shaded square. The two are connected by a straight line. Below the parent pair is a vertical straight line connecting to the next row of circles and squares which are the offspring. Starting from left an unshaded square, half-shaded circle, another half-shaded circle, and a completely shaded square are shown. All of these are also connected by a horizontal straight line.

What percentage of the offspring are carriers of the disease but do not have it?
0%
25%
50%
100%