Download Biology Exam Review: Sickle Cell Anemia Practice Problem

Biology Exam Review: Sickle Cell Anemia Practice Problem
1.
The term anemia refers to a deficiency in the oxygen-carrying capacity of the blood.
In some forms of anemia, the number of a particular type of blood cell has decreased significantly. Which type of
blood cell is this?
2.
Within body cells, which organelle is least likely to function properly in an individual who is anemic? Explain
your answer.
3.
What types of symptoms might you expect to see in someone suffering from anemia? Explain your answer.
4.
Sickle cell anemia is a hereditary hemoglobin defect. In a common form of the disease, there is a substitution of a
single amino acid in the β-chain of the hemoglobin protein. A portion of the amino acid sequence for normal
hemoglobin (HbA) and the sickle-cell hemoglobin (HbS) is shown in the diagram below. Use these sequences to
explain how a single amino acid substitution can cause a change in the structure of the hemoglobin protein.
5.
Sickle-cell anemia is caused by a recessive allele for HbS. Individuals who are heterozygous are carriers. While
they are usually healthy, a fraction suffer some symptoms of sickle-cell anemia when there is an extended
reduction of blood oxygen. At the molecular level, it has been observed that carriers produce both HbA and HbS
hemoglobin.
Which pattern of inheritance is this an example of?
6.
A boy named Timmy has sickle cell anemia (i.e., he is homozygous recessive), although neither his sister Priscilla
nor his parents (Burt and Lily) suffer from the disease. Neither of his maternal grandparents suffered from the
disease, although his paternal grandfather did.
Draw a pedigree for this family and identify the genotype of each individual involved.
7.
What was the probability of Timmy inheriting sickle cell anemia? Show your work.
8.
What is the probability of Priscilla being a carrier?
9.
Make a sketch that shows what one of Burt’s cells would look like in metaphase I. (Do not attempt to include all
of Burt’s chromosomes; rather, include only the genetic information relevant to this problem.)
10.
Where in the body would you find the cell drawn in question 9?
What is the name of the process occurring in the cell in question 9?
Outline the role of hormones in this process. Include all hormones that have a direct or indirect effect.
11.
Timmy’s parents read an article from the New England Journal of Medicine, in which they learn the following.
“Among children and adults with sickle cell anemia (homozygous for sickle hemoglobin), the median age at
death was 42 years for males and 48 years for females.”1
Timmy’s parents are troubled by this news. During a late-night discussion they make the decision to not have any
more children, understanding that future offspring might inherit the disease as Timmy did. They then discuss
which sort of contraception they should use in the future. Burt suggests that Lily see her doctor to obtain a
diaphragm. Lily suggests that Burt see his doctor for a vasectomy.
What follows is a heated argument, the details of which will not be recorded here. Who has made the better
suggestion? Why?
12.
While Burt and Lily are arguing, Priscilla sneaks into Timmy’s bedroom and attempts to steal his teddy bear.
Timmy awakes, and what follows is a lively fistfight, the details of which will not be recorded here either. After
several minutes, Timmy has recovered his teddy bear, but he is out of breath.
At low oxygen concentrations (as in Timmy’s blood post-fistfight), HbS molecules latch onto each other and cause
erythrocytes (red blood cells) to be elongated and pointed at the ends. Sickled erythrocytes are sticky and tend to
clump together.
What problems could this cause for Timmy?
Normal erythrocytes
1
Timmy’s erythrocytes after fistfight with Priscilla
Orah S. Platt, Donald J. Brambilla, Wendell F. Rosse, Paul F. Milner, Oswaldo Castro, Martin H. Steinberg, and Panpit P. Klug. “Mortality In Sickle Cell
Disease -- Life Expectancy and Risk Factors for Early Death” New England Journal of Medicine, 330 (June 9. 1994): 1639-1644.
13.
Despite the health problems associated with sickle cell anemia (and the dangers associated with living with
Priscilla), Timmy enters high school. He particularly enjoys his biology class. While learning to use the light
microscope, he decides to take a look at his sickled blood cells. He pricks his finger with a pin and collects a drop
of blood on a slide.
(It should be noted that Timmy’s biology teacher clearly stated that students should not look at their own blood in
class, due to the possible transmission of blood-borne pathogens. Timmy’s behavior is blatantly insubordinate.)
Unfortunately for Timmy, his lab partner Dirk often does not pay attention to directions. Dirk was supposed to
add a drop of distilled water to an Elodea leaf on a slide; however, Dirk adds this drop of water to the slide
containing Timmy’s blood cells.
What would Timmy observe if he looked at this slide? Explain the observation.
14.
As mentioned previously, heterozygous individuals are generally unaffected by sickle cell symptoms except in low
oxygen environments. People with two recessive alleles (HbS), however, suffer severe illness and even death. For
this reason, the recessive allele is often considered a lethal allele.
The recessive allele, however, does provide considerable protection against malaria. Sickled blood cells have low
potassium levels, which causes the parasite inside the cells to die. Individuals with two dominant alleles (HbA)
are quite susceptible to malaria, but heterozygotes are much less so.
How would you expect natural selection to act on a population in a region of the world where the incidence of
malaria is high?