What is Sickle Cell Anemia?

Red blood cells with normal hemoglobin (HbA) move easily through the bloodstream, delivering oxygen to all of the cells of the body. Normal red blood cells are shaped like jelly- filled doughnuts with a depression in the center and they are soft and flexible. Sickle cell anemia occurs when an abnormal form of hemoglobin (HbS) is produced. HbS molecules tend to clump together, making red blood cells sticky, stiff, and more fragile. Red blood cells containing HbS can clog blood vessels, deprive the body’s tissues and organs of the oxygen they need, and are short-lived. Normal red blood cells last about 4 months in the bloodstream but sickle cells only last 10 to 20 days, which causes anemia (a low number of red blood cells). People who are anemic tire more easily and often feel weak (NIH, 2007).

Sickle cell anemia is not contagious and cannot be passed from one person to another like a cold or other infection. People with sickle cell anemia have inherited two sickle cell alleles, one from each parent. A child who has inherited the sickle cell allele from only one parent will not develop the disease, but they do carry the sickle cell trait. People carrying a single sickle cell allele can pass the trait to their own children.

One would think that there is little reason for sickle cell disease to remain in the any human population since natural selection should favor reduction of the frequency of the sickle cell allele to near zero. For example, individuals who carry two copies of the sickle cell allele typically die before reaching reproductive age, which will reduce the frequency of the allele in a population and minimize its transfer between generations. Why does the disease exist at all? An understanding of the origin of sickle cell disease and several other red blood cell disorders requires knowledge of a few of the basics about genetics and something about the process of natural selection. There is also another piece of the puzzle: malaria.

MalariaThe plasmodium parasite that causes malaria in humans is transmitted by mosquitos and the parasite spends part of their life cycle in mosquitos and part of it in human hosts. The parasite enters the human bloodstream via the saliva of an infected female mosquito obtaining a blood meal. Once in the blood stream and liver the parasites replicate to the point that the liver cells are filled with new copies of the parasite. These are then released into the bloodstream where they invade circulating red blood cells. After penetrating the red blood cells, the parasites consume hemoglobin in the red blood cells and enlarge until they fill the cell completely. During their growth these plasmodia particles reproduce forcing red blood cells to lyse (break apart) releasing new copies into the blood stream where they continue to infect new red blood cells. A mosquito taking a blood meal from a person whose red cells contain malaria-causing parasites then becomes a host furthering the transmission cycle.

Defenses Against Malaria

One point at which the life cycle of the malarial parasite can be stopped in humans is at the phase of red blood cell invasion and multiplication. Red blood cells are constantly created and destroyed as part of their life cycle. Any defense mechanism that could somehow destroy both the infected red blood cells and the parasite could potentially eliminate the malaria parasite because healthy red blood cells would eventually replace infected cells.

Interestingly enough, carrying just one copy of the sickle cell allele trait provides a survival advantage over people with normal hemoglobin in regions where malaria is endemic. People (and particularly children) infected with the malarial parasite are more likely to survive the acute illness if they possess just one copy of the abnormal allele than are people with two normal hemoglobin alleles. These individuals are therefore more likely to reach reproductive age and pass their genes on to the next generation. On the other hand, people carrying two copies of the abnormal allele encoding hemoglobin have a significant chance of dying of acute malarial infection in childhood. The precise mechanism by which sickle cell trait imparts resistance to malaria is unknown but a number of factors likely are involved.

References:

NIH. (2007). Sickle Cell Disease. Retrieved from http://ghr.nlm.nih.gov/condition/sickle-cell- disease.

Overview of the Activity

In this activity we will study the incidence of hemoglobin alleles and malaria using a simplified population genetics model. Our population genetics model will focus on a single gene, hemoglobin, and its two forms: the allele that codes for normal hemoglobin and the allele that codes for the sickle cell trait. We will use Hn to refer to the normal hemoglobin allele and Hsto refer to the sickle hemoglobin allele. In this activity we will use shorthand notations for frequency. For example, f(Hn) is the frequency of the Hn allele; f(Hs) is the frequency of the Hs allele; f(Hn Hn) is the frequency of people with two Hnalleles; f(Hn Hs) is the frequency of people heterozygous; and f(Hs Hs) is the frequency of people homozygous for the Hsallele.

You will explore three versions of this population genetics model. The first scenario models the how the frequency of this gene might change from one generation to the next in the absence of natural selection. The last two versions model the behavior of the hemoglobin gene in response to natural selection. In Natural Selection I the population is located in the United States and in Natural Selection II the population is located in equatorial Africa.

To mimic the frequency of each allele in a population you will beans of two differing colors to represent the allele coding for normal and abnormal hemoglobin. You will begin a hypothetical population with a certain allele frequency (the number of occurrences of each allele in the entire population) and ask what happens to the allele frequency under different selection environments. You will complete the exercises outlined below and submit your answers to the accompanying questions in the appropriate discussion board.

References:

NIH. (2007). Sickle Cell Disease. Retrieved from http://ghr.nlm.nih.gov/condition/sickle-cell- disease.

Hemoglobin and Fitness Instructions

Directions: Neutral Evolution

1.            Obtain 20 beans of two different colors (e.g., white and red). Count out 16 white and 4 red beans. The white beans represent the Hn allele and the red beans represent the Hs allele. This is the genetic makeup of your starting population. (Note: You can use any objects that can readily be categorized into two groups, such as coins, colored rocks, or paper clips.)2.            Calculate the frequency of both alleles [f(Hn) and f(Hs)] and record them in Table 1. In our experiment frequency is a measure of how many copies of a given allele exist in the gene pool (i.e., a proportion). Use decimal values.

3.  Arrange the beans into pairs. These pairs represent the genotype of each of 10 individuals in the population. Record the number of individuals with each genotype [f(Hn Hn), f(Hn Hs), and f(HsHs)] in Table 1.

4. Now imagine that the individuals are living and reproducing with each individual reproducing at the same rate (i.e., all individuals produce two copies of each of their alleles into the next generation). Obtain enough beans to represent the next generation— the offspring generation—and then let the parental generation “die”.

5.Calculate the frequency of each allele in the offspring generation and record it in Table 1.

6.Answer the questions that follow in Table 1.

Table 1

Answer the following questions

1. What happened to the frequency of the common allele?

1. What happened to the frequency of the rare allele?

1. What happened to the frequency  of the common and rare alleles when the starting frequencies were different from yours (Ask a neighbor)

1. What happens to allele frequencies from one generation tot eh next if there are no evolutionary forces acting on the population?

Directions: Natural Selection I (United States)

1. Obtain 40 beans: 32 white and 8 red. This is your original population.

2.  Calculate the frequency of both alleles [f(Hn) and f(Hs)] and of the three genotypes

[f(HnHn), f(HnHs), and f(HsHs)] and record them in Table 2. Use decimal values.

Use Table 2 for the remainder of this exercise.

3. Arrange the beans into 20 pairs (individuals within the population) in any way you wish.

4. Apply natural selection by allowing only the following proportions of individuals

exhibiting each genotype to “survive”:

HnHn                  1.0            100%

HnHs                  0.7            70%

HsHs                  0.2            20%

 (Note: Interpret this table by keeping all individuals with the HnHn genotype but only 70% of HnHs individuals and 20% of HsHs individuals.) Round off the number of surviving individuals to whole numbers. These proportions represent the fitness of the different genotypes in the US. In so doing we are making a simplifying assumption that all fitnesses are due to survival differences alone.

5. Calculate the new allele frequencies after selection and record in Table 2. Have allele frequencies changed? If so, what allele is more common? Do these changes make sense in the light of what you did in step 4? Also, calculate the genotype frequencies and record.

6. This step is a little complicated so please be patient. You will use the new allele frequencies (after selection) to tell you the expected frequencies of each genotype in the next generation. This should be familiar if you remember p2 +2pq + q2.

To calculate the expected frequencies of HnHn square the frequency (expressed as a decimal) of Hn and multiply by 20. This is the expected number of HnHn individuals in the new generation. To calculate the frequencies of HnHs first multiply the frequency of Hn (expressed as a decimal) by the frequency of Hs, then multiply this value by 2, and finally multiply by 20. This is the expected number of HnHs individuals in the new generation.

To calculate the expected frequencies of HsHs square the frequency (expressed as a decimal) of Hs and multiply by 20. This is the expected number of HsHs individuals in the new generation.

7.            Calculate the new allele frequencies (before selection) for the new generation. These should be the same as the allele frequencies after selection in the previous generation. Can you understand why? Also, calculate the new genotype frequencies.

8.            Repeat steps 3 – 7 for two more cycles.

9.            Answer the questions that follow Table 2.

Table 2

Answer the following questions

1. What happens to allele frequencies with natural selection?

1. Specifically, what happened to the frequency of H_n? Of H_s?

1. What would happen to allele frequencies if natural selection stopped acting?

Directions: Natural Selection II (Africa)

1.

Proceed with steps 1 – 7 in Natural Selection I but use the following values for fitness, which represent the fitness of each genotype in Africa:

HnHn                  0.9            90%

HnHs                  1.0            100%

HsHs                  0.2            20%

Put your results in Table 3.

Table 3

Answer the following questions:

1. What happens to allele frequencies with natural selection?

1. Specifically, what happened to the frequency of H_n? Of H_s?

1. What would happen to allele frequencies if natural selection stopped acting?

Summary Questions:

Respond to the following questions in a few paragraphs and submit it in the form of a discussion question response.

1.            What advantage does the sickle cell trait offer to people living in areas where malaria is prevalent?

2.            What have you learned about changes in allele frequencies with and without natural selection? What impact does this have on evolution in populations?

3.            Why is the sickle cell allele relatively common in parts of Africa but relatively rare in the US?

4.            Why isn’t the sickle cell allele eliminated from human populations entirely?