Name
Date
Period
CODOMINANCE AND SICKLE-CELL ANEMIA
Codominance is a special type of inheritance in which heterozygous individuals show both homozygous
traits. For example, if you crossed a red flower with a white flower, the heterozygous would have both red and
white on the flower petals.
Sickle-cell anemia is an example of a human trait that is inherited through codominance. This condition is quite
common in countries that have a high incidence of malaria. Sickle-cell anemia is a blood disorder affecting
oxygen transport by hemoglobin. The sickled cells block capillaries, depriving the tissues of needed oxygen.
Individuals with sickle-cell anemia frequently die at an early age. Heterozygous individuals have sickle-cell
trait. These carriers are usually healthy, but experience some problems with intense exercise or under low
oxygen conditions. In the United States, sickle-cell trait affects 1 out of 10 African Americans.
The allele for normal hemoglobin is HbA and the sickle-cell allele is HbS. The following is a summary of the
possible sickle-cell genotypes and phenotypes.
HbA HbA
completely normal
HbA HbS
sickle-cell trait (this person has a combination or normal hemoglobin and the
abnormal, sickled form of hemoglobin)
HbS HbS
sickle-cell anemia (all abnormal hemoglobin)
1. What are the possibilities for children if both parents are heterozygous for sickle-cell trait?
2. What is the probability of a couple having a child with sickle-cell trait if one parent is normal and the
other has sickle-cell trait?
Name
Date
Period
A Family History of Sickle-Cell Disease
Preparation:
The foundation of our modern knowledge of sickle-cell anemia is based on the research of Dr. Angela Ferguson
and Dr. Roland Scott, prfessors at Howard University in Washington, D.C. The doctors became interested in
sickle-cell anemia because many of their friends and some family members were affected by the disease. They
published the first research paper on sickle-cell disease in the 1940’s ---- 25 years ahead of other researchers.
Imagine that you are a physician studying blood genetics in the laboratory of Drs. Ferguson and Scott. The
Minister if Health from Zaire has requested their help to investigate two health problems within the villages of
his country that seem to be related: sickle-cell disease and malaria. Drs. Ferguson and Scott send you to
investigate.
After spending a year looking into the problem, you have collecterd data about Family A and constructed a
pedigree and health profile for this family. Each individual on the pedigree is identified by a number.
Name
Date
Period
Table 14.2
Health Profile of Family A
HEALTH STATUS
Affected with malaria
Dead from malaria
Dead from sickle-cell anemia
Dead from causes unrelated to malaria or sickle-cell
anemia
In good health
AFFECTERD FAMILY MEMBERS
13, 32, 33
7, 9, 10, 16, 29, 30, 31, 34
8, 15, 25, 26, 35
3, 19
1, 2, 4, 5, 6, 11, 12, 14, 17, 18, 20, 21, 22, 23, 24, 27,
28, 36, 37, 38
Malaria is one of the world’s most serious diseases. It is transmitted by the bite of an infected mosquito. The
bite allows a parasitic protozoan to be passed from the mosquito to the bloodstream of humans. Once in the
body, the parasites live and multiply inside red blood cells.
Every two to three days, infected red blood cells burst, releasing thousands of new parasites which will infect
even more red blood cells. The infected person experiences bouts of chills and fever each time such a release
occurs.
1. On the pedigree chart in Figure 14.3, place an asterisk (*) next to the number of all persons who are
affected with malaria or who have died from malaria. List the pedigree number for each individual in
Family A who has died from malaria or is currently affected with this disease.
Table 14.3
Genotypes of Individuals Affected by Malaria
List Pedigree Number for Each Affected Individual
Genotype
(Separated by Genotypes)
HbA HbA
HbA HbS
HbS HbS
2. Of the total number of deaths in this family, what percentage died from each cause?
Table 14.4
Cause of Death
Malaria
Sickle-cell anemia
other
TOTAL
Causes of Deaths in Family A
Number Dead
Percentage of Total Deaths
Name
Date
Period
3. What is the leading cause of death among the members of Family A?
4. Go back to the pedigree and examine the genotypes of the individuals who are alive and in good
health. Among these individuals, what is the most frequent genotype for sickle-cell disease?
5. Write a hypothesis about the relationship between malaria and one of the three genotypes relaterd to
sickle-cell disease.
6. As you know, a hypothesis is an educated statement about a relationship. Explain how you developed
your hypothesis by listing facts from the pedigree of Family A.
7. Is the death patterns shown by Family A typical for the population as a whole? To answer this question,
make one graph comparing:
a. The percentage of deaths from each cause in Family A (as shown in Table 14.4)
b. The percentage of deaths from each cause in several neighboring communities (as shown in Table
14.5)
Table 14.5
Causes of Deaths in Neighboring Communities
Cause of Death
Number Dead
Percentage of Total Deaths
Malaria
1080
Sickle-cell anemia
600
other
320
2000
TOTAL
Name
Title:
Date
Period
Name
Date
Period
Comprehension Check
1. In Family A, family members 21 and 22 are expecting their ninth child. What is the probability this
child will have sickle-cell anemia? Show your work!!
2. Closer to home, Michelle has a brother, Charles, who died of sickle-cell anemia. She is concerned about
the chance of the condition appearing in her children. When blood samples were taken and placed under
low-oxygen conditions, some of her red blood cells sickled. Those of her husband James, however,
remained normal when tested. Show your work (by drawing Punnett squares) and list the genotypes
of everyone in the problem.
Michelle ________________
Charles _______________
James _______________
What is the probability that Michelle and James’s children will have sickle-cell anemia? _________________
Sickle-cell trait? ___________________