At the top of the page, draw shapes representing the original two members of the first generation. Represent the males of the family as squares and females as circles. This carries throughout the chart.
Draw horizontal lines between males and females to represent any mating or marriage. Draw a vertical line between the male and female and branch it with angled lines to represent the next generation. Shade the shapes representing family members in whom the gene is active.
Label each generation with Roman numerals to the left of the family tree. Add children by their birth order -- looking from left to right -- and represent them with Arabic numerals. For example, if the second child of the third generation expresses the gene, it should be represented as III2.