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Learning Experience 3

N o M a t t e r Wh at Your S h a p e Prologue The easiest traits to describe in an organism are the visible traits. For instance, think about distinguishing between two people. You might use the texture of their hair as an easily identifiable trait. One might have curly hair, and the other straight hair. By using differences in hair texture, you have described variants of a single trait. In this case, that trait is hair texture. What is responsible for variants in traits? Are traits and their variants only characteristics that are directly observable? Or are there underlying causes for these variants? In this learning experience, you use variations in the shape of peas as a simple model for investigating the answers to these questions. You then explore a trait in humans, sickle-cell trait, as another example of variations in traits.

Brainstorming Your teacher will distribute two different kinds of peas to you and your partner. Examine them carefully. Discuss the following questions with your partner, and record your thinking in your notebook. Be prepared to discuss your ideas with the class. 1. List the traits you can observe in each kind of pea. 2. Describe the variations, if any, in each trait you have listed for the two kinds of peas. 3. What do you think might cause the variations in these traits? 4. What if you were to soak these peas in water? Do you think there would be a difference in the amount of water they absorbed? Explain your answer.

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A Pea by Any Other Name Is Still a Seed

A CTIVITY

How might you begin to answer the question, “Why is one pea wrinkled and another round?” Think about ways you could find out more about the differences between round and wrinkled peas. Are there differences in how the peas are constructed? How might these differences produce a wrinkled shape instead of a round shape? Are there differences in what the seed is made of? How would this affect the shape? In this learning experience, you will investigate the causes of variations in traits of organisms. Using these pea variants as a model, you will look at the differences between wrinkled peas and round peas (which are actually seeds). But first, you need to understand the general structure of a seed (shown in Figure 2.7). A seed is the part of a plant that results embryonic shoot tip from the fertilization of the female egg by the male pollen. Following fertilization, the embryonic plant develops within a embryonic stem protective seed coat. In addition to the embryonic plant and the seed coat, the seed also contains a source of food. The germinating seedling will use this food until it can carry out photosynthesis. In embryonic root one type of plant, this food source is in a separate structure within the seed called the endosperm. In another type of plant, the protein, starch, and fats are stored in two large seed leaves. Regardless of the system of storage, the newly sprouted plant seed leaves depends on these stored food sources until it can make its own. (cotyledon) Seeds have a very low water content. During the final stages of Embryonic Plant the development of the seed, cells within the seed dehydrate. In seed coat other words, most of the water in the seed is removed. The resulting low water content in the seed causes most cellular processes to slow down or stop. In this dehydrated state, the embryonic plant can remain dormant (inactive) but viable (alive) within the seed. It can stay this way Figure 2.7 for long periods of time without growing or developing. This process enables the Structure of a seed. seed to delay germination until environmental conditions are suitable for its growth. Germination is the start of growth and development of a plant. This is when the embryonic plant breaks out of its seed coat. When conditions become favorable, water enters the seed. Rehydration triggers the reactivation of normal metabolic processes, and germination begins (see Figure 2.8). As the plant begins to develop, it uses the starch stored in the endosperm or seed leaves as an energy source. A plant can use photosynthesis to provide its required food and energy only after breaking free of the soil and receiving sunlight. In this activity, you will determine why some seeds (peas) are wrinkled at the end of their development and some are round.

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(a) seed development water removed

(b) germination shoot

water added root

Figure 2.8 (a) During seed development, water is lost from cells. This dehydration slows down metabolic processes. (b) During germination, water enters the seed and the metabolic processes are reactivated. As a result, the embryonic plant begins to grow.

Materials For each pair of students: • • • • • • • • • • • • •

2 pairs of safety goggles 10 round peas 10 wrinkled peas 1 balance 2 small beakers or containers (50-mL) 1 wax marking pencil 2 microscope slides with coverslips access to a compound microscope 1 razor blade or scalpel 1 forceps 1 dropping bottle of dilute Lugol’s iodine paper towels distilled water

PROCEDURE Part A

SAFETY NOTE

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1. STOP & Develop a hypothesis that explains why some peas are wrinkled T H I N K and some are round. Record your hypothesis in your notebook. 2. Read steps 3–8. Then create a data chart in your notebook. Fill it in as you carry out this part of the experiment. 3. Weigh all 10 round peas together. Record the weight of the peas on your data chart. Weigh all 10 wrinkled peas together. Record the weight of the peas on your data chart.

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Figure 2.9 Lab setup.

4. Use a wax marking pencil to label one beaker “R” and the other beaker “W.” Also mark the beakers with your group name. Place the dried round peas in the R beaker and the dried wrinkled peas in the W beaker. Add water until the beakers are three-quarters full (see Figure 2.9). 5. Place the beakers in the location designated by your teacher until the next class session. 6. After soaking the peas overnight, retrieve your 2 beakers. Label one paper towel “R” and another “W.” Pour off the excess water from each beaker carefully. Then empty the peas from each beaker in a pile on the appropriately labeled paper towel. 7. Weigh each pile of peas again to determine the weight after soaking. Record the weights in your table. 8. Determine the weight difference for each kind of pea. Calculate the percentage of increase in weight for each kind of pea. Record the results in your table. 9. STOP & Think about this experiment and your understanding of seed T H I N K formation. Do you want to change your hypothesis as to why some peas wrinkle and others remain round when they are dried? Record your response in your notebook.

Part B 1. Use a wax marking pencil to label one microscope slide “R” and another “W.” 2. Place 1 drop of dilute iodine on each slide. 3. Hold a soaked wrinkled pea with forceps. Cut the pea in half with a scalpel or razor blade. Cut a very thin segment or slice from the inside of the pea. Gently place the slice into the drop of dilute iodine on the slide labeled W (see Figure 2.10). 4. Carefully wipe the blade of the scalpel or razor blade clean with a paper towel. Repeat step 3 with a soaked round pea. Drop the slice on the slide labeled R. 5. Place a coverslip at an angle over each drop and gently lower it. Observe each slice under the microscope. 6. STOP & Describe and draw in your notebook what you see. Compare the T H I N K shapes, colors, patterns, and densities of the starch grains. Look at a few slides prepared by other class members and compare them with yours.

SAFETY NOTE

Avoid staining your skin with iodine. If iodine is accidentally ingested, seek immediate medical attention.

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forceps

a

pea

Figure 2.10

pea slice drop of iodine

b

Hold the pea firmly with the forceps. Carefully slice several sections (a) until a very thin section is obtained. Using the forceps, transfer the section to the drop of iodine on the microscope slide (b).

A NALYSIS Prepare a laboratory report for this experiment in your notebook. Be sure to include the following: a. your initial hypothesis (step 1 in the procedure); b. your revised hypothesis (step 9), if you changed it (if you changed it, explain why); c. your experimental procedure; d. the purpose of each part of the experiment; e. your data (show all your calculations where appropriate); and f. any conclusions you can draw, based on your data. Decide whether you have enough data and knowledge to identify the cause of the shape difference between the two different kinds of peas. Explain your answer.

R EADING

Adding a New Wrinkle to the Picture You have gathered a great deal of information about the differences between wrinkled and round peas. You know some things about these peas at both the visible and the biochemical levels. But you still may not be able to reach a conclusion about the exact cause of the difference in shape. To identify the exact cause of this difference, you need to understand more about the biochemistry of seed development.

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1 early seed development

2 starch formation

Figure 2.11

3 dehydration

round peas

H2O

wrinkled peas

H2O H2O

sucrose amylose amylopectin

Sucrose changes to starch in seed development: A comparison of round and wrinkled peas. (1) Sucrose is made during early seed development. (2) In round seeds, SBEI converts sucrose to amylopectin (branched starch). In wrinkled peas, the SBEI does not function correctly. So no amylopectin is made. Instead, a different enzyme converts some sucrose to amylose (unbranched starch). The high concentration of unconverted sucrose in the wrinkled seed causes the seed to take up water and swell. This stretches the seed coat. (3) When the seed undergoes dehydration, the coat of the round seed remains smooth. But the coat of the wrinkled seed wrinkles because of its stretched seed coat.

In a developing seed, sucrose (a sugar) is converted to a highly branched form of starch. This starch is called amylopectin. It serves as a source of food for the developing plant. The conversion from sucrose to starch is facilitated by an enzyme. This enzyme is called starch-branching enzyme I (SBEI). Scientists used methods similar to the ones you carried out in class. They were able to demonstrate that the characteristic of wrinkled shape was the result of the pea’s inability to synthesize amylopectin from sucrose. As a result of a faulty SBEI enzyme, these peas cannot make amylopectin. The unchanged sucrose concentrates in the developing seed. Using a different enzyme, wrinkled peas can synthesize a different, unbranched form of starch. This form of starch is called amylose. Amylose serves as the seed’s food source. What does this inability to make branched starch from sucrose have to do with shape? During the seed’s development, the high concentration of sucrose caused water to accumulate inside the seed. The water, as in a water balloon, stretched the seed coat much more than it normally would stretch without the concentrated sucrose. During the final stages of seed development, dehydration takes place. The accumulated water is lost from the seed. This causes the stretched seed coat to collapse, somewhat creating a wrinkled pea. In the pea with the functional SBEI enzyme, sucrose did not accumulate. Thus, the seed coat was not stretched by excess water. So when the water was lost during dehydration, the seed coat remained round (see Figure 2.11). Do round peas have only functional SBEI? Do wrinkled peas have only nonfunctional SBEI? Would a pea having both kinds of enzymes be a little bit wrinkled? Biochemical analysis demonstrates that peas that have both kinds of enzymes still appear round. You cannot see any difference from those peas that have only functional SBEI. Some of the enzyme cannot make amylopectin from sucrose in these seeds. Even so, the enzyme that can function can convert enough sucrose to amylopectin to prevent water from being retained and stretching the seed coat. They therefore appear round. Learning Experience 3 No Matter What Your Shape

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Think about an enzyme that can function efficiently even in the presence of a nonfunctional or dysfunctional enzyme of the same kind. Its activity is considered dominant to the activity of the nonfunctional (or recessive) enzyme. An organism that has both kinds of enzymes and displays the trait of the dominant activity (in this case, round) is considered heterozygous for that trait. An organism with only one kind of enzyme is said to be homozygous. A wrinkled pea is always homozygous for the nonfunctional enzyme. But a round pea could be either homozygous or heterozygous. In this case, you cannot necessarily judge a pea by its cover.

C ASE S TUDY

Figure 2.12 Normal red blood cells are shaped like disks. Some red blood cells of sickle-cell patients become stiff and sickled (see arrows). The misshapen cells often get stuck in small blood vessels. This causes extreme pain and damage. © Dr. Gladden Willis/Visuals Unlimited

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Am I a Carrier, and What Does That Mean? It was Health Day at Denzel Jones’s high school. “Career Day, Health Day, Environment Day,” fumed Denzel. “When am I going to hear about stuff that matters to me?” Denzel’s class filed into the auditorium. The air was buzzing with conversation about music, friends, the last biology exam—everything except the topic of health. Who cared, anyway? Well, at least it got them out of fifth period. As several individuals from the local health clinic talked, Denzel found himself drawn in by some topics. These included exercise, smoking, and methods for the prevention of infectious diseases. One topic in particular caught his attention because he actually knew a couple of people with the problem. A physician’s assistant began to talk about something called “hemoglobinopathies.” He described one in particular, sickle-cell anemia. Denzel’s uncle Jamal (his father’s brother) had the disorder. He suffered from fatigue and bouts of intense joint pain. Because it bothered him to watch his uncle suffer, Denzel was curious about the cause. Denzel learned that sickle-cell anemia is a disorder of red blood cells that can run in families. It causes the red blood cells to collapse into shapes resembling sickles (see Figure 2.12). This happens when the oxygen level of the blood is low. Red blood cells sickle because they contain hemoglobin that is biochemically a little different from the normal hemoglobin protein. Normal hemoglobin (or hemoglobin A) is found in solution in red blood cells. It binds oxygen and transports it throughout the body. Once it releases the oxygen, the hemoglobin remains in solution in the red blood cell. Sickling hemoglobin (designated S) is a variant form of hemoglobin. It differs from normal hemoglobin by only a single amino acid. That slight difference in structure, however, alters its function. Hemoglobin S binds oxygen and carries it to where it is needed. But a problem arises when the oxygen is released and the concentration of oxygen around the hemoglobin is reduced. Normal hemoglobin remains in solution under these conditions. But the sickling hemoglobin comes out of solution. Its molecules bind together into long fibrous chains (crystallizes). These fibers push out against the inside of the membrane of the red blood cell. This produces the characteristic sickle shape (see Figure 2.13).

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Because of their shape, these cells cannot flow easily through the tiny capillaries. (Capillaries are the smallest passageways of the circulatory system.) The cells get stuck and clog the flow of blood. This blockage decreases the blood supply to the vital organs—such as the heart, spleen, kidneys, and brain. These organs can be damaged. The buildup of pressure behind the blockage also can cause small blood vessels to burst. This results in internal bleeding and pain. The symptoms of sickle-cell anemia are quite variable. But some general features include jaundice, anemia, and pain. (Jaundice is yellowing of the skin a and other tissues due to the breakdown products of red blood cells.) Infants and children may have a predisposition to infection. In later years, blood-rich organs such as the heart, spleen, and liver are damaged by the restricted blood flow. The disease may cause leg ulcers, anemia, kidney failure, stroke, and heart failure. The severity of the symptoms varies from individual to individual. Some show few symptoms; others die young. The physician’s assistant explained that sickle-cell anemia is an inherited disease. The variant can run in families. Individuals can pass the variant to their children without having symptoms themselves. Parents b who do not have sickle-cell anemia can have children with the disorder and children without the disorder. About 2.5 million, or one in every 12 African Americans carry the sickling trait without having the disease. (This group is the most affected population in the United States.) They have both kinds of hemoglobin in their red blood cells. Individuals who have both kinds of proteins are called carriers. Approximately 80,000 African Americans have only sickling hemoglobin. These people demonstrate the characteristics or symptoms of sickle-cell anemia. Denzel began to wonder whether anyone else in his family besides Uncle Jamal had sickle-cell anemia. Could he be one of the individuals who had the sickle hemoglobin variant but didn’t show it? The physician’s assistant told the group that an easy test for sickling hemoglobin could be done at the clinic. He encouraged the students to have it done. Denzel decided he wanted to be tested. After the assembly, Denzel approached the physician’s assistant to ask questions about the test. He told Denzel that there is a test to distinguish normal hemoglobin (hemoglobin A) from sickling hemoglobin (hemoglobin S). This test is based on the understanding that the difference between the two types of hemoglobin is only one amino acid. This amino acid changes the electrical charge on the molecule. This charge difference causes the two different forms of hemoglobin to separate in an electric field. In a solution through which an electrical current is passed, hemoglobin A will move in one direction; hemoglobin S will travel the opposite way. Denzel was amazed. The difference between being healthy and having the symptoms of sickle-cell anemia was a single amino acid. And, through a fairly simple blood test, Denzel could learn whether he had any hemoglobin S.

Figure 2.13 (a) Normal hemoglobin (A) remains dissolved in the cell after the release of oxygen. Cells remain disk-shaped. (b) Sickling hemoglobin (S) comes out of solution after the release of oxygen. It forms long crystals and distorts the cell shape. © Dr. Stanley Flegler/Visuals Unlimited

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Table 2.1 Hemoglobin Data Individual

Hemoglobin A

Hemoglobin S

Grandpa Jones

+

+

Grandma Jones

+

+

Grandpa Beausejour

+

–

Grandma Beausejour

+

–

Mr. Jones

+

+

Mrs. Beausejour-Jones

+

–

Uncle Jamal Jones

–

+

Tara Jones

+

+

Tabitha Jones

+

–

Denzel Jones

+

–

Carlos Jackson

+

+

At dinner that night, Denzel told his family what he had learned that day about the sickle-cell trait. He said he would like to be tested. He also thought that it might be a good idea for everyone to be tested, to know whether they carried the trait. Denzel’s father was not so sure. He worried that if he carried the trait and someone at work found out, they might think he wasn’t healthy enough to operate the forklift he drove every day. And what if he applied for more health insurance? What impact would being a carrier have on that? Denzel assured him that the physician’s assistant said that individuals who carried the trait rarely exhibited any symptoms of the disease and were never considered “sick.” Anyway, the results of the test were confidential. No one was ever supposed to know. Tara, Denzel’s older sister, was worried for a different reason. She was planning to be married soon and very much wanted to have children. What if she and her fiancé, Carlos Jackson, were both carriers? What would that mean for the children they might have? She wasn’t sure she wanted to know. In the end, everyone in the family decided to be tested. This included Denzel’s four grandparents, Grandpa and Grandma Jones and Grandma and Grandpa Beausejour; his sisters, Tara and Tabitha; and Uncle Jamal. Even Tara’s fiancé, Carlos, wanted to find out whether he carried the trait. Everyone nervously waited a week for the blood test results. The data in Table 2.1 were collected on the Jones and Beausejour families. (A + sign indicates the individual has that form of hemoglobin; a – sign indicates it was not present.)

A NALYSIS Record your responses to questions 1–5 in your notebook. 1. Earlier in this learning experience, you found out that the difference in the shape of peas is the result of a difference in a single enzyme that functions during pea development. Explain the differences in the biochemistry of sickling and normal hemoglobin. How do these differences result in the visible trait (as seen under the microscope)? 172

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2. The results of Tara’s test, as well as some of the others tested, indicate that some of her red blood cells carry hemoglobin S as well as normal hemoglobin. These individuals are carriers of this variant form of protein. Yet none of them has shown any symptoms of sickle-cell anemia under normal circumstances. How do you explain this? 3. Carlos enjoys mountain climbing. On occasion, at very high altitudes he has suffered fatigue and severe cramps in his joints. What do you think is the reason for this? Base your answer on his test results. 4. Scientists use family trees or pedigrees as a tool to record and track inherited characteristics in families. What specific characteristics have you seen in members of a family that help identify them as belonging to that family? 5. Create a pedigree for Denzel’s family. Indicate how members are related and how the sickle-cell trait runs in the family. Use the test results shown in Table 2.1. To help you diagram the trait of sickle cell in Denzel’s family, you will need to use the symbols shown in Figure 2.14 to create a pedigree of his family. The generations of a family are marked with Roman numerals. Begin with the first generation listed in Table 2.1. Each individual within a generation is labeled with an Arabic numeral (1, 2, 3, 4, etc.). Within the children of a particular couple, the first born child is usually placed to the far left. Subsequent children follow to the right. Figure 2.15 is one example of a pedigree. Examine the pedigree. What can you tell about the relationships in this family? Who has the disease? Who are the carriers? 6. Tara and Carlos hope to marry soon and to have children. What do you think the test results mean for them? Write responses to the following: a. List all of the choices that Tara and Carlos have with respect to having children. b. Describe all of the consequences for each of the choices you listed in item 6a. c. Describe in a short paragraph what choice you might make if you were in the same situation as Tara and Carlos. Include your reasons for making that choice. d. What do you think would happen if everyone who was confronted with this situation made the same choice you made? Write a short paragraph describing what this future might look like. e. List four important values that influenced your decision. Explain how they influenced you. For example, some values might include religious reasons, your view of community, your sense of responsibility, your own personal health issues, and your sense of family. male female

marriage

divorce

affected individual female twins carrier deceased individual adopted into a family

Figure 2.14 Pedigree symbols. Learning Experience 3 No Matter What Your Shape

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1

I

II

1

Maria

2

Al

III John

2

John

1

2

Tanya

3

Isabelle

4

John

5

Sandy

Elizabeth 3

Elaine

4

Sally

5

Dan

6

Sue

7

9

Eric

Jack 2

Joshua

10

Charles

Lucy

1

IV

8

Luke

3

Joan

4

5

Evelyn Derrick

6

Phil

Figure 2.15 An example of a pedigree.

EXTENDING In the 1970s, Susan Perrine was a young doctor working in Saudi Arabia. She observed that many of the Arab patients who came to her clinic had surprisingly mild cases of sickle-cell anemia. In fact, many of them displayed no symptoms, even though their blood showed the characteristic sickling effect under conditions of low oxygen. When their hemoglobin was examined, the patients displayed high levels of fetal hemoglobin. Fetal hemoglobin is the kind of hemoglobin that all humans produce before birth but generally is replaced after birth by adult hemoglobin. Fetal hemoglobin has a higher affinity for oxygen. That means it binds oxygen more tightly than adult hemoglobin does. Apparently in these Arab patients for some reason the red blood cells had not completely switched from making fetal hemoglobin to adult hemoglobin. And surprisingly, the presence of this fetal hemoglobin reduced or eliminated the problem found when an individual makes only hemoglobin S. Explain why the presence of fetal hemoglobin may mask or dominate the effects of the sickling hemoglobin. Describe how this information might be used to treat sickle-cell patients. The study of genealogy, that is, tracing a family’s history, can be fascinating. Some people track their ancestors when an unfortunate illness shows up in the immediate family. They are concerned about whether they or their children may inherit the disease. Others search for the names and places of origin in their mother’s and father’s pasts for clues to their heritage. Create your own family tree. You may want to interview your oldest relatives. Ask them for their views of life and of family in past times to help you recapture family history that is often lost.

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It’s early in the morning, but the hospital is already busy. Metal trays covered with vials, syringes, tourniquets, and doctors’ orders are being wheeled from room to room. One of these trays is followed closely by Arzu, a phlebotomist. On her morning rounds, she has orders to draw blood from an elderly woman being treated for a blood clot. She will also take a blood sample from a middle-aged man needing tests to find out why he has been feeling so ill and a young girl who is in the hospital for gall bladder surgery. Arzu loves meeting new people. Many of those she deals with aren’t too happy to see her, because it is common for people to be afraid of needles. But Arzu comforts them by educating them about what is going to happen and describing each step. She can usually quell patients’ fears and take samples of their blood without any problem. Long-time patients are relieved when they see her face in the morning. They know she cares. Patients are thankful for Arzu’s gentle touch, but they are often unaware of her great range of knowledge. She is very skilled. To complete her certificate program in phlebotomy, Arzu was trained in collecting, transporting, handling, and processing blood samples; identifying and selecting equipment, supplies, and additives used in blood collection; recognizing and adhering to infection control and safety procedures; and recognizing the importance of each step from drawing blood to analysis and seeing how her part fits into the whole picture of a specific person’s medical care. Arzu’s expertise in the field is in drawing blood for analysis. She translates the doctors’ orders for the lab technicians who do the analysis. When doctors, physician’s assistants, and nurses receive the results, they use the data from these blood tests to prescribe medication and a plan for care. By being specifically trained in bloodletting procedures, Arzu allows doctors and nurses the time to complete important paperwork, update records, and continue patient care toward a speedy recovery.

CAREER Focus

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