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 to
p and from left to right appear as a half-shaded circle labeled 1 next to a half-shaded square labeled 2. 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 labeled 1, half-shaded circle labeled 2, another half-shaded circle labeled 3, and a completely shaded square are shown labeled 4. All of these are also connected by a horizontal straight line. What best describes offspring 4?
