Draw a grid of perpendicular lines that looks like you are about to play a tic-tac-toe game using the pen and ruler. Next, enclose the squares by drawing four lines around each side. The result should look like a nine-paned window in a frame. Put an X in the top left square to block out that space, since it should remain blank.
Put both possible allele types of one parent on the squares on the two remaining squares on the top row. Only one letter should be put in each box, but it doesn't matter which parent is on the side or top row. For example, one parent may have a dominant widow's peak allele and a recessive straight hairline and would be assigned an "A" (dominant) in one box and an "a" (recessive) in the other. Assign a label, such as "Parent One," above the top row.
Put both possible allele types of the other parent in the two boxes on the row along the left-side of the framed boxes. Write a "Parent Two" label along the left side.
Chart four possible offspring genes into the remaining squares, using the alleles from "Parent One" for the first two and the alleles from "Parent Two." For instance, if both parents had one dominant allele and one recessive one, then one of their children would be assigned "AA" (double dominant) indicating they will have a widow's peak, two would have "Aa" a widow's peak with a recessive straight hairline allele and another would have "aa" (double recessive) indicating a straight hairline.