During the Activity
Activity Sequence in Brief
Students observe images of people with a variety of skin pigmentations.
Students simulate the combinations of pigmentation genes from two parents.
Students graph data and discuss the results of recombination.
Teacher explains melanin and the wide variability of skin pigmentation due to multiple genes controlling the trait and introduces the next activity.
Teacher collects and assesses the student activity sheets.
Engage (5 minutes)
- Collect students' work from the previous day's activity sheets: Hominid Migration Student Activity Sheet, Migration Comparison Student Activity Sheet, and Who, What, Where, When, and How? Student Activity Sheet.
- On the computer, display the slide show: Human Skin Pigments (3 slides). Comment to the students that it is obvious that people from different locations around the world vary in their degrees of skin pigmentation.
- Explain that today students are going to conduct an activity that will help them gain a better understanding of skin pigmentation and how it is inherited.
Explore (20 minutes)
- Organize students into teams of three. Give two cans of beans and a copy of the Genetics and Variation Student Activity Sheet to each team.
- Assign two students from each team to draw beans from the container and one student to record the information. Give each team a group number, from 1 to 11, and ask the recorder to write his or her group number in the title of the table in Part I of the activity sheet.
- Make sure students understand the directions for Part I on the activity sheet. Explain that three genes are responsible for skin pigmentation and that each parent contributes one each of these three genes to each offspring. In this simulation, the two cans represent the two parents. The beans represent the genes. Explain that the students are going to simulate the range of possible offspring of these two parents by randomly drawing six (three from each parent) beans from the cans.
- Instruct groups to complete Part I of the Genetics and Variation Student Activity Sheet. Display the transparency: Class Totals. Tell the groups to record their totals (from Step 7 of the activity sheet) next to their group number on the overhead transparency when they finish.
Explain (15 minutes)
- Add up the teams' totals and fill out the bottom row of the transparency: Class Totals.
- Direct students' attention to Part II of their Genetics and Variation Student Activity Sheets. Tell groups to use the Class Totals on the overhead transparency to construct a bar graph. Allow groups five minutes to complete their bar graphs.
- When all groups have finished, display the image: Interpreting Genetic Variation. Ask students to take notes on these five questions as the class interprets the results of Parts I and II of the activity. Tell them that they will need to refer to their graphs.
- What do the results indicate about the probabilities of drawing just one or two dark beans or just one or two light beans? (The probabilities are low.)
- What were the most frequent results? (Roughly an equal number of light and dark beans.)
- What are the odds for drawing either a dark bean or a light bean? (50:50, one in two, or equal, assuming there was an equal number of each color of bean in the can).
- Why are there so few all light or all dark bean groupings? (One must draw the same color bean all six times; there is only one way to draw all light or all dark beans. There are more ways to get combinations of 2/4, 3/3, and 4/2.)
- How can two parents, both with medium skin pigmentation, have offspring with skin pigmentation that varies from very light to very dark? (The offspring inherit different combinations of the genes for skin pigmentation.)
- Display the image: Combinations of Beans and briefly go over the combinations to ensure that all students understand why the numbers of possible combinations vary.
- Ask the students to consider why using a class set of trials is more accurate in preparing the graph than just using their individual group results. Refer to the transparency: Class Totals and point to the differences in student group data. (Larger sample sizes produce a more mathematically ideal representation.)
- Remind the students that they were simulating possible offspring of two parents, both of whom have medium skin pigmentation. If these were real parents, what would be the possible range of skin pigmentation in their children? (Like the beans; a few offspring would be very light or dark, but most would have medium skin pigmentation.) Explain to the students that most variations in offspring are due to genetic recombination, especially when there are multiple genes for a trait, and that most traits are controlled by multiple genes.
Elaborate (10 minutes)
- Tell students that skin color is influenced by a variety of substances, the most important of which is melanin (a molecule that is produced by specialized cells in the skin). In this activity, the beans represent the genes that are associated with the production of melanin. If the light bean represents the absence of the production of melanin and the dark bean represents the production of more melanin, the more dark beans you have in your sample the greater will be the melanin production and the darker the skin color.
- Explain that some individuals have the genetic ability to temporarily increase melanin production in response to a factor in the environment' sunlight or ultraviolet light. This adaptive process is called tanning, and is only a temporary change in the amounts of skin pigmentation. Other individuals cannot increase melanin production and burn rather than tan in the presence of sunlight.
- Show the slide show: Human Skin Pigments again. Ask students: Do all populations of people around the world show a range of skin pigmentation, from very light to very dark? (No.) Tell students that in the next activity they will be exploring why populations vary in skin pigmentation.
- As desired, collect and evaluate students' work on the Genetics and Variation Student Activity Sheet .