The Genetics of Codominance
Instructions
Activity 1. PTC Tasting and Pedigree Analysis
Not all alleles produce visible traits like skin color or height. Most alleles control physiological traits, such as
production of digestive enzymes, hormones, and antibodies. Alleles are responsible for invisible traits such as blood
type, ability to carry out metabolic pathways (such as producing proteins or storing blood sugar), color vision, and
many others. One example of a physiological trait controlled by a single gene is the ability to taste a harmless
chemical, PTC (phenylthiocarbamide).
The story of the discovery of the “bitter taste gene,” which controls the ability to taste PTC, is quite interesting. It
happened in 1931 at Dupont Chemical Company. Some PTC crystals accidentally blew into the air in the room,
and into the mouths of scientists working in that lab. Some of the scientists complained of a bitter taste, but others
said the chemical has no taste! After this observation, the scientific method took over. The scientists tested friends,
family members, and co-workers, and found similar results. Some tasted PTC as bitter and others couldn’t taste it at
all. Seventy years later, these results were linked to a single gene located on chromosome #7.
Procedure
1.
You have been given an index card with the one piece of control taste paper and one containing PTC.
2.
Taste the control paper first (to establish the taste of the paper itself ). Then taste the PTC paper.
If you’re a taster, you’ll detect a very unpleasant, bitter taste. If there’s little difference between the PTC and
the control papers, you’re not a taster.
Are you a taster or a nontaster? __________________________
3.
Record your taste-test results on the white board.
Answer Questions 1 and 2 on the Lab Report.
4.
The inheritance pattern of traits like PTC tasting can be diagrammed in a chart called a pedigree. A pedigree
illustrates the marriages for several generations within a family and the children produced.
•
Females are shown with circles and males are shown with squares.
•
A black square or circle shows the presence of the condition being studied. A white square or circle
means the condition is absent in that person.
•
A marriage or mating is shown by a line connecting the parents.
•
Children from a mating are shown by a vertical line between the parents.
•
All individuals from the same generation are shown along the same horizontal line.
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Fig. 1. PTC Pedigree
5.
After examining the pedigree key, determine the genotypes of all the people in this family. In a few cases, there
may not be enough information to determine a person’s second allele. In this situation, enter a blank (__) in
place of the second letter (i.e., T_).
Answer Questions 3-6 on the Lab Report.
Activity 2. Exploring the Genetics of Codominance
In humans, the ABO blood types are inherited in a codominant fashion. There are four phenotypes: type A, type B,
type AB, and type O. They are controlled by three alleles: the codominant alleles A and B and the recessive allele O.
Blood types can be determined with a simple test that use antibodies that recognize the presence or absence of
particular carbohydrates on the surface of red blood cells. If a blood sample agglutinates (clumps) when treated with
a given antibody, then those blood cells have that carbohydrate (Table 1).
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Table 1. Reactivity of red blood cells with antibodies against “A” and “B” carbohydrates.
If blood sample clumps when treated with
anti-A
anti-B
then the blood type is…
+
–
A
–
+
B
+
+
AB
–
–
O
In this activity, you and your lab group will visit “scenario stations” around the room and solve case studies based on
your understanding of blood type inheritance.
Procedure for Testing Blood
1.
Visit each of the four “scenario stations” set up on the white trays around the room.
2.
At each station, you will find the following items:
•
an index card labeled with the name of each person being tested
•
clean blood typing slides
•
plastic toothpicks: blue, yellow and white
•
artificial blood samples
•
dropper vials with antibodies that bind to the “A” carbohydrate (anti-A; blue vial), the “B” carbohydrate
(anti-B; yellow vial), and the Rh factor (anti-Rh or anti-D; clear vial).
3.
Place a clean blood typing slide on each index card. (We’ll do this instead of writing directly on the slides,
since they can be a pain to get clean.) Use one slide per person you are testing.
4.
With the vials capped, gently invert (tip upside-down several times) the artificial blood to resuspend the
plastic “cells.”
5.
Place one drop of “blood” in all three wells on each patient’s slide. That means three drops of blood on each
slide.
6.
Please make sure that you put the caps back on the appropriate vials so you don’t cross-contaminate the
samples.
7.
Add a drop of anti-A to the well labeled A on each slide. Replace the cap.
8.
Add a drop of anti-B to the well labeled B on each slide. Replace the cap.
9.
Add a drop of anti-Rh to the well labeled Rh on each slide. Replace the cap.
10. Use the toothpicks to stir the reactions for about 30 secs each, and then STOP!
•
Use a clean toothpick for each reaction and don’t cross-contaminate!
•
Use a blue toothpick for each “A” reaction.
•
Use a yellow toothpick for each “B” reaction.
•
Use a white toothpick for each “Rh” reaction.
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11. Carefully examine each slide for results.
•
In each well, a thin film should appear on the surface of the liquid.
•
If the film remains uniform in appearance, no reaction/agglutination has occurred. This is a negative (–)
result.
•
If the film appears granular (”chunky”), a reaction has occurred. This is a positive (+) result.
Scenario 1
Mrs. Johnson was one of two mothers in a maternity ward at a hospital. When she was given baby #1, she denied it
was hers, claiming baby #2 instead. The other mother also claimed baby #2.
Mrs. Johnson is blood type O. Unfortunately, Mr. Johnson died just before the baby was born, so we can’t find out his
phenotype or genotype, but the Johnsons had three other children whose phenotypes are known. Ryan is blood type
A, Logan is blood type B, and Jenna is blood type B.
Blood type the two babies and use the results of the test to determine which baby (if any) belongs to Mrs. Johnson.
Scenario 2
During a freak April snowstorm, two planes collide in mid-air over farmland in Kentucky. Two sets of badly-burned
remains are found. Mr. and Mrs. Palmer are certain that one set of remains is that of their 28-year old daughter Tara.
Blood samples are collected from both sets of remains, as well as Mr. and Mrs. Palmer. Which set of remains—set 1
or set 2—belongs to Tara?
Scenario 3
A famous paternity battle arose when Joan Barry claimed that Charlie Chaplin, a star of silent movies, fathered her
child. Blood typing in paternity cases was gaining widespread use in the 1940s, so Charlie’s lawyers requested that
Joan, Charlie, and the child (a daughter named Carol Ann) be blood typed.
In the end, Chaplin was ordered by the jury to pay child support.
Bloodtype all three people and determine whether the verdict was appropriate.
Scenario 4
The Duke and Duchess of Massasoit are in possession of a large blue diamond that they plan to sell in order to raise
funds for their yearlong safari in Africa (the economy isn’t what it used to be, after all). They awake one morning to
find that someone has broken into their jewel sanctuary and has attempted to steal the diamond. They immediately
suspect that one of their three children, all of whom stand to lose if the jewel is sold, is behind the attempted theft.
The would-be thief was injured at the scene on a broken window and left behind some type A blood. Both the Duke
and Duchess know their blood types. The Duke is blood type B and the Duchess is blood type O. Their three
children—Ferdy, Roxanne, and Cristophe—each display varying degrees of innocence, provide samples for testing.
Answer the remaining questions on the Lab Report.
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The Genetics of Codominance
Lab Report
Name ________________________________________
Activity 1. PTC Tasting and Pedigree Analysis
1.
In the general population, approximately 75% of people can taste PTC. The remaining 25% aren’t able to taste
this chemical.
Were your class results close to the expected percentage? __________________________
If not, suggest a possible reason why your class results differed from the expected outcome:
2.
The ability to taste PTC comes from a dominant allele (T). Using this information, fill in the appropriate
genotypes for tasters and nontasters.
Tasters ______________________________________
Nontasters ___________________________________
3.
Referring to the pedigree in Fig. 1, if Jill marries Howard, what is the probability that their children will be
tasters? __________ non-tasters? _____________
Show your work.
4.
If Pam marries Howard, what is the probability that their children will be tasters? _____________
Show your work!
5.
After performing the PTC experiment, do you think that the ability to taste certain chemicals can explain why
your children’s food preferences might be different from your own?
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6.
Barium sulfate is tasteless to some people, but bitter to others. Two sisters are having their gastrointestinal
tracts x-rayed. They are both asked to drink a barium sulfate “milk shake” before the procedure. One sister
drinks the “milk shake” without complaint. The other sister complains that it tastes terrible. Using the
information gained in today’s lab activities, suggest an explanation for this difference.
Activity 2. Exploring the Genetics of Codominance
Please check your bloodtyping results with me before you answer the questions!
Scenario 1.
7.
Record your results in the table below.
Sample
anti-A
anti-B
anti-Rh
blood type
Baby #1
Baby #2
8.
What must have been Mr. Johnson’s blood type? Construct a pedigree of this family to support your answer.
9.
Which baby—if any—belongs to Mrs. Johnson?
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Scenario 2.
10. Record your results in the table below.
Sample
anti-A
anti-B
anti-Rh
blood type
anti-B
anti-Rh
blood type
Mr. Palmer
Mrs. Palmer
Remains #1
Remains #2
11. Which remains—if either—belong to Tara?
Scenario 3.
12. Record your results in the table below.
Sample
anti-A
Joan Barry
Charlie Chaplin
Carol Ann
13. What is the probability that Charlie is the father of Carol Ann? Show your work and circle your answer.
14. This scenario (and these results and verdict) are historically accurate. Do you agree with the verdict? Use
scientific evidence from your test to support your answer.
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Scenario 4.
15. Record your results in the table below.
Sample
anti-A
anti-B
anti-Rh
blood type
Ferdy
Roxanne
Cristophe
7.
Who committed the break-in? Show your work and circle your answer.
8.
What was the other mystery uncovered by this scenario? Construct a pedigree to support your answer.
9.
Which of the children has a potential claim to the title of the real heir to the diamond? Why?
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