Effects of natural selection on a population

This exercise demonstrates the effects of natural selection on a population. To begin, working in groups, count out 70 red beans and 30 pink beans for a total of 100 beans. The red bean represents the allele for normal hemoglobin, which carries oxygen on the surface of the red blood cells. The pink bean represents the sickle cell gene, which has a mutation that damages the hemoglobin and causes distortion of the red blood cell and an inability to carry oxygen. Sickle cell anemia is a recessive disorder. Therefore, in this population, there is a strong selection against the homozygote recessive.

Red/Red—homozygous dominant (healthy)

Red/Pink—heterozygous (healthy)

Pink/Pink—homozygous recessive (anemia)

1. Put all beans in your can and mix them together. 2. Without looking in the can, randomly pull out pairs of beans until the can is empty. Place each pair separately on your desk. These represent the genotypes of the 50 individuals you are studying (50 individuals, 100 alleles). 3. Record the number of each type of bean pair in the generation 1 row, and record the frequency of the 2 alleles. 4. Because your pink/pink individuals have sickle-cell anemia, they have a higher chance of not surviving to the next generation. To demonstrate this, remove 90% of the pink/pink pairs from your gene pool. 5. Place all the remaining beans in the can and mix them thoroughly. 6. Repeat steps 2 through 5 three more times. In the following table, record for each generation the number of each genotype and the frequency of each allele.