Question: How will allele frequencies change if a dominant allele is deleterious?

Student Exploration: Microevolution

Vocabulary: allele, cystic fibrosis, deleterious, dominant allele, fitness, genotype, heterozygote superiority, heterozygous, homozygous, incompletely dominant, malaria, predator, recessive allele, sickle cell anemia

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

Many people from warm regions carry an allele that provides resistance to malaria. Two copies of this allele, however, causes a deadly blood disorder called sickle cell anemia.

Suppose two parents are healthy carriers of the sickle-cell allele. The genotype of each parent is AS (A is the normal allele, and S is the sickle-cell allele).

How likely is it for a child of these parents to have sickle cell anemia (SS)?

Do you think the allele for sickle cell anemia would be common in regions where malaria did not exist? Explain why or why not.

Gizmo Warm-up

If natural selection does not favor one trait over another, the frequencies of various alleles in a population will tend to be stable over long periods of time. But what happens when one allele confers an advantage or disadvantage to an individual? These scenarios and others can be explored with the Microevolution Gizmo™.

The feather color of the parrots in the Gizmo is controlled by two alleles, D and d. The D allele is incompletely dominant over d. What is the feather color of each parrot genotype?

The fitness of parrots is determined by how well they blend into the background trees. Set the Fitness of DD slider to 100% and the other fitness sliders to 60%.

Click Begin, and then click Predator. Which parrots were killed by predators? Light Green and Yelllow because they did not blend in with the tree.

Activity A: Deleterious dominant alleles

Get the Gizmo ready: Click Reset. Set the DD and dd sliders to 34%.

Introduction: A deleterious allele is one that significantly lowers the fitness of an individual. If the deleterious allele is a dominant allele (D), then both homozygousDD and heterozygous Dd individuals will be at a disadvantage.

Question: How will allele frequencies change if a dominant allele is deleterious?

Observe: To model the effects of a deleterious dominant allele, set the Fitness of DD and Fitness of Dd sliders to 60%. Set the Fitness of dd to 100%.

Based on the color of the trees, which parrots will be easiest for predators to spot and kill?

Predict: How do you expect the proportions of D and d alleles to change in five generations?

Record: Click Begin and Predator. Record the results of hawk predation in the table below.

Calculate: To calculate the percentage of survivors for each genotype, divide the Ending population by the Starting population. Multiply each result by 100 to convert to a percentage, and record the percentages below.

Analyze: How do the percentages of survivors relate to the fitness of each parrot genotype?

(Activity A continued on next page)

Activity A (continued from previous page)

Gather data: Click Breed, and then click Hatch. Click Continue, and then click Predator. Repeat this sequence to play the simulation for five generations. Select the TABLE tab and record the genotype populations over time in the spaces below.

Analyze: What patterns do you see in your data? DD decreases every time, Dd fluctuates but eventually decreases, dd increases every time.

Interpret: Select the ALLELE GRAPH tab. What does this graph show?

Interpret: Select the GENOTYPE GRAPH tab. What does this graph show?

Think and discuss: Why do you think deleterious dominant alleles are not very common?