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 the next row of circles and squares which are the offspring. Starting from the left an unshaded square, half-shaded circle, another half-shaded circle and a completely shaded square are shown. All these are also connected by a horizontal straight line. What percentage of the offspring have sickle cell anemia?