Virtual Genetics Lab: Sickle-Cell Inheritance

Virtual Genetics Lab: Sickle-Cell Inheritance Worksheet

Learning Goal: To determine the inheritance pattern of sickle-cell disease.

Prerequisite Knowledge: Before beginning this lab, you should be familiar with these concepts:

 the definitions of allele, dominant, and recessive, and how these apply to genetic traits

 the use of Punnett squares to understand patterns of inheritance

Introduction: In the United States, the sickle-cell allele is carried by approximately 1 in 12 people of African ancestry. The sickle-cell allele is expressed in hemoglobin, the molecule that carries oxygen in red blood cells.

 Sickle-cell disease (also called sickle-cell anemia) is the most common genetic disorder among people of African descent. In this disease, hemoglobin molecules clump together in long fibers, forcing the red blood cells, which are normally round, into long shapes that look like sickles. These sickle-shaped red blood cells tend to get stuck in capillaries, blocking blood flow to cells, tissues, and even entire organs.

 Sickle-cell trait is a related condition, although people have few, if any, harmful symptoms. People with this condition actually have a heightened resistance to malaria, a disease that has killed millions.

In this lab activity, you will act as a genetic counselor and help parents determine their likelihood of having children with sickle-cell disease.

Part A A husband and wife come into your office. The husband is homozygous for the sickle-cell allele. The wife is homozygous for the normal allele. They want to know if their children would be at risk for having sickle-cell disease.

Enter the Genetics Lab Room by clicking the button. Then, follow the Lab Procedure.

Lab Procedure

1. Choose Human from the Species Selector, and Sickle-Cell Anemia from the menu of traits. (To review how to choose these settings, watch this video.) 2. Set the parents’ alleles according to the information above. Then, set the number of offspring to the maximum of 6.

3. Click the Cross button repeatedly until the parents have produced about 100 F1 offspring—a large enough sample size to determine the pattern of inheritance of the trait. Make sure the Include Male/Female box in the lower left corner of the results box is not checked.

What percentage of the F1 offspring have sickle-cell trait, sickle-cell disease, and normal blood? ANSWER:

Part B What does this result suggest about the pattern of inheritance of sickle-cell disease and sickle- cell trait?

ANSWER:

Part C How would you counsel this couple about their would-be offspring? What about their offspring’s offspring?

ANSWER:

Part D Complete the three Punnett squares below to illustrate how likely it would be for this couple’s children (who are all carriers of the sickle-cell allele) to produce offspring with sickle-cell disease or sickle-cell trait. (A represents the normal allele; S represents the sickle-cell allele.)

AA AS SS

Scenario 1 If spouse is homozygous for sickle-cell allele

S S A Heterozygous S (carrier)

Scenario 2 If spouse is heterozygous (carrier)

A S Heterozygous A (carrier) S Scenario 3 If spouse is homozygous normal

A A Heterozygous A (carrier) S

Part E After completing the Punnett squares in Part D, try each cross in the Genetics Lab Room. 4. For Scenario 1, set the parents’ alleles according to the information provided. Then, set the number of offspring to 6 and click the Cross button repeatedly until these parents have produced about 100 offspring. Record the results.

5. Repeat step 4 for Scenario 2 and Scenario 3. Record the results. What percentage of the offspring in each scenario would have sickle-cell disease? What percentage would have sickle-cell trait?

ANSWER:

Part F Think It Over It’s harmful to have two sickle-cell alleles, but in a region where malaria is prevalent, having one sickle-cell allele can be beneficial.

Review the phenotype breakdown of the offspring in each of the three scenarios above. Which phenotype is most common overall? Explain how a population in a region affected by malaria could derive more benefit than harm from the presence of the sickle-cell allele.

ANSWER: