Materials Included In Kit

Safety Precautions

PTC paper is paper soaked with phenyl thiocarbamide (PTC). The LD₅₀ (Rat) for PTC is 3 mg/kg. Such a low LD₅₀ suggests that this is a very toxic substance. However, the solution used to make the taste test papers contains approximately 50 mg of PTC per liter of water. Using crude arithmetic we calculate that each strip of PTC paper would contain approximately 0.03 mg of PTC and that a person would have to lick and ingest 5000 strips of PTC paper to reach the LD₅₀ for a body weight of about 50 kilograms (110 pounds). A few sensitized individuals may have an allergic reaction to PTC.

The solutions used in the blood testing simulation are considered non-hazardous but normal safe laboratory procedures should be followed. Please consult Safety Data Sheets for additional safety and handling techniques. Wear chemical splash goggles and chemical-resistant gloves whenever laboratory chemicals are used.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Dispose of all simulated blood products down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period, if the pre-laboratory assignment is completed prior to the lab and if the data compilation and calculations are to be completed the day after the lab or as homework.
• The traits listed on the Dominant vs. Recessive Traits Chart are classic examples. Recent genetic testing has determined that most of these are not simple single gene traits. John McDonald of the University of Delaware has consolidated much of these findings. See http://udel.edu/~mcdonald/MythsHumanGenetics.pdf (Accessed May 2018)
• You may either assign students an unknown blood sample (Y or Z) or allow them to choose which one they will test.
• Step 5 of Activity 1 may be included in the homework portion to save lab time (deciding if the traits appear to be dominant or recessive using the class totals).

Teacher Tips

• The U.S. Department of Energy was the agency responsible for the Human Genome Project (HGP). They continue to host the HGP website as an archive. Many teachers have found the Human Genomes Landmark poster particularly interesting to students. Free printed copies of the poster are no longer available but it may be downloaded as a pdf using the following link—https://public.ornl.gov/site/gallery/highres/GenomePoster2009.pdf (Accessed May 2018). The pdf may be printed but it will no longer be updated. The last version is the 2009 version. Also hosted at the website are individual chromosome viewers, a timeline for the project, and other free education resources.

• Stress to students that the dominant trait is not always observed in higher frequency than the recessive counterpart.
• It may be interesting to keep class totals each year and continually add in the new class totals.
• Although taste is not technically a simple genetic trait, it does appear at a similar frequency in a typical classroom.
• The gene that primarily affects the ability to taste PTC is called TAS2R38. It is part of a family of genes called TAS2R all of which are related to the ability to taste bitter substances. TAS2R38 is located on chromesome 7, location q35–q36. Three amino acid substitutions are the typical differences between tasters and non-tasters. Other TAS2R genes play a minor role in the ability to taste PTC. Certain TAS2R genes contribute to those individuals called “super-tasters.”
• One study found that many people who react strongly to PTC do not like to eat foods such as broccoli or coriander nor are they likely to smoke cigarettes or drink coffee or tea.
• The ability to taste PTC is present in about 70% of the overall human population, varying from 58% for Australians to 98% for Native American populations.

Answers to Prelab Questions

1. Fill in the following Punnett squares and list the respective probabilities for the offspring.

Square 1
Homozygous dominant = __0__%
Heterozygous = __50__%
Homozygous recessive = __50__%

Square 2
Homozygous dominant = __50__%
Heterozygous = __50__%
Homozygous recessive = __0__%

2. Use the Hardy-Weinberg equation to find the following genotype probabilities for a population of 100, where 10 people are observed to have the recessive phenotype. Hint: See Background section.

q² = 10/100 = 0.1
q = √0.1 = 0.316
p = 1 – 0.316 = 0.684

Homozygous dominant: p² = (0.684)² = 0.468 or 47%
Heterozygous: 2pq = 2(0.684)(0.316) = 0.432 or 43%
Homozygous recessive: 0.1 or 10%

Do these percentages add up to 100%?
0.47 + 0.43 + 0.10 = 1.0 or 100%

3. If a person’s blood clumps with Anti-b sera, but not with Anti-A or Anti-Rh, what blood type does this person appear to have?

B–

Sample Data

Student Data Table results will vary depending on which blood unknown they have.

Answers to Questions

1. A mother and father both have a phenotype of attached earlobes (a recessive trait). Is it possible for them to give birth to a child with free ear lobes (a dominant trait). Explain why or why not. Draw out the Punnett square if needed.

No, they cannot have a child with a dominant phenotype if both parents are recessive for the trait. Since a recessive trait requires homozygous alleles, we know that both parents must be homozygous recessive, meaning 100% of their children will have the trait.

2. Use the Hardy-Weinberg equation to find the probable genotypes of your classmates for the trait of PTC tasting.

Answers will vary by class but are likely to show that PTC tasting is dominant over non-tasters.

Do these percentages add up to 100%?
They should. However, if they do not but are close, it is likely due to rounding.

3. Complete the following Punnett square for

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Square 3
Homozygous dominant = __25__%
Heterozygous = __75__%
Homozygous recessive = __0__%

4. For the blood types above, what will be the observed phenotype for each genotype?

5. The person from which the unknown blood sample came from is having a child. Is there any possibility this person could have a child with type O blood? Explain.

For both samples the answer is yes.
For sample Y all we know is that this person has blood type A+. They could have a genotype I^Ai. If the other parent had genotype I^Ai, I^Bi or ii, they stand a chance of giving birth to a child with O blood.
For sample Z all we know is that this person has blood type B–. They could have a genotype I^Bi. If the other parent had genotype I^Ai, I^Bi or ii, they stand a chance of giving birth to a child with O blood.

Introduction

Learn about the human genome by exploring dominant and recessive traits on a very exciting specimen—you! Analyze class data to see how it compares with the normal distribution of traits in the general population.

Concepts

• Genetics

• Heredity
• Human genome
• Dominant traits
• Recessive traits
• Co-dominant traits
• Genotypes and Phenotypes
• Hardy-Weinberg equation
• Homozygous and heterozygous

Background

The sequencing of the human genome revealed a wealth of information regarding human genetics and heredity. Heredity is the passing of genetic information through generations from parent to child. Genes contain the genetic codes necessary to create traits, such as the ones which make us similar in appearance to our family members. Alternative forms of the same genes are called alleles. Each human has a pair of alleles for each gene, where each allele may be either dominant or recessive. Dominant phenotypes only require the presence of one allele in order for the trait be observable, regardless of the other allele present (hence the name). Recessive phenotypes require the presence of two copies of the same allele. Homozygous (homo = same) refers to a gene for which both alleles are of the same type, whether both dominant or both recessive. Heterozygous (hetero = different) is the term used to describe a genotype is which an individual has both a dominant allele and a recessive allele. An individual’s genotype is a series of 2 letters, indicating their alleles. The dominate allele is represented by a capital letter and the recessive allele is represented by a lowercase letter. For example, the inability to taste the bitter chemical phenylthiocarbamide (PTC) is a recessive phenotype and the alleles are represented using two lowercase letters, such as “tt.” The ability to taste PTC is a dominant phenotype and could be due to either “TT” or “Tt” genotypes. A genotype represents the alleles contained in the gene and can only be determined through laboratory testing, whereas a phenotype is the observable characteristic. If a trait is dominant, only a genetic test is able to determine if the genotype is “TT” or “Tt.” However, when a recessive phenotype is observed, it always indicates a homozygous recessive genotype since two copies must be there in order for the trait to be observable.

Punnett squares are used to relate the genotypes of the parent organisms to the probability of each genotype of the offspring. Figure 1 demonstrates that two heterozygous parents have a 25 percent chance of having a child that is homozygous dominant (TT) for the PTC, a 50 percent probability that the child’s genotype will be heterozygous (Tt) and a 25 percent chance the genotype will be homozygous recessive (tt).

The Hardy-Weinberg equation is used to determine the frequency of genotypes in a population.

p is the frequency of the dominant allele (e.g., T)
q is the frequency of the recessive allele (e.g., t)

p² is the predicted frequency of the homozygous dominant (TT) genotype
2pq is the is the predicted frequency of the heterozygous (Tt) genotype
q² is the is the predicted frequency of the homozygous recessive (tt) genotype

The only known factor is q², since the recessive phenotype indicates an exact genotype. To determine q, first divide the number of students having the recessive trait by the total number of students in the class. For example, if 5 students cannot taste PTC and there are 30 students in the class then the q2 value is 5/30 = 0.167. The square root must be taken to find q, √0.167 = 0.409. This q value can then be used in the first equation to find p, p = 1 – 0.409 = 0.591.

Then, using Equation 2, the predicted frequency of each genotype may be determined:

Homozygous dominant = p² = (0.591)2 = 0.349 or 35%
Heterozygous = 2pq = 2(0.591) x (0.409) = 0.484 or 48%
Homozygous recessive = q² = (0.409)2 = 0.167 or 17%

The sum of these values will add up to 1 (or 100%):

0.35 + 0.48 + 0.17 = 1

ABO blood typing is genetically a bit more complex than PTC because blood type has 3 possible alleles. In the ABO blood typing system, the presence or absence of the A and B proteins on the red blood cells determines an individual’s blood type. Individuals whose red blood cells contain protein A and lack protein B have type A blood. Those with protein B and no protein A are blood type B. Individuals with both protein A and protein B are type AB and individuals lacking the presence of both proteins are blood type O. The three alleles for ABO blood type are I^A, I^B and i. I^A codes for protein A, I^B codes for protein B and i codes for neither protein A nor protein B (type O). Within this multiple allele pool, the gene interactions illustrate both simple dominance as well as codominance. (Remember each individual has only two alleles for each trait even if there are multiple alleles in the gene pool.) When the I^Ai allele combination occurs, the individual is blood type A because i is recessive. When the I^AI^B combination occurs, the I^A and I^B alleles are codominant and the individual is blood type AB. Figure 2 illustrates the allele combinations, resulting blood type, proteins on the red blood cells and antibodies in the blood for the four blood types in the ABO system.

As an immune defense, our bodies attack substances that are recognized as foreign. Anti-a sera, found in the serum of people with type B blood, cause clumping of red blood cells that have protein A on the surface, since this protein is foreign to their body. Anti-b sera will clump type B blood. This clumping is referred to as agglutination. Agglutination will occur in both sera with type AB blood and in neither sera with type O blood. In the ABO blood typing procedure, drops of blood are first obtained using sterile procedures. A drop of blood is placed in a drop of Anti-a sera and another drop is placed in a drop of Anti-b sera. The drops are then observed for agglutination. The pattern of clumping or non-clumping is interpreted and the blood type determined. The following patterns occur for the various blood types:

The Rh factor (+ or –) is another component of blood type. Rh is an abbreviated term used to describe the presence of a particular surface protein. If an individual is Rh positive (Rh+), their red blood cells have the protein on their surfaces and will agglutinate with the Anti-Rh sera. Conversely, if the red cells lack Rh antigens, the blood is Rh negative (Rh–). Rh factors are typically used in conjunction with the ABO blood typing system (e.g., A+, O–, AB+).

Materials

Prelab Questions

1. Fill in the following Punnett squares and list the respective probabilities for the offspring.

Square 1
Homozygous dominant = _____%
Heterozygous = _____%
Homozygous recessive = _____%

Square 2
Homozygous dominant = _____%
Heterozygous = _____%
Homozygous recessive = _____%

2. Use the Hardy-Weinberg equation to calculate p and q and determine the following genotype probabilities for a population of 100, where 10 people are observed to have the recessive phenotype. Show your work. Hint: See Background section.

q =
p =

Homozygous dominant: _____%
Heterozygous: _____%
Homozygous recessive: _____%

Do these percentages add up to 100%?

3. If a person’s blood clumps with Anti-B sera, but not with Anti-A or Anti-Rh, what blood type does this person have?

Safety Precautions

PTC test papers contain chemicals and should not be swallowed. Handle the test papers as little as possible before use. Once any taste is detected remove the test paper from the mouth and discard. Do not reuse any test papers; always use a fresh test paper for each student. Wash hands before touching the test papers and handle them as little as possible. Wash hands with soap and water before leaving the classroom.

Procedure

Activity 1

1. Obtain a Dominant vs. Recessive Traits Chart and a Student Data Table for you and your partner.
2. Fill in the “Group Totals” column of the Student Data Table by observing the traits listed in the Dominant vs. Recessive Traits Chart on yourself and your partner (the only additional materials required are two PTC test papers).
3. After filling in the chart for each trait, pool your data with the class data per your teacher’s instructions.
4. After all groups have completed, fill in the Class Totals column of the Student Data Table using your class data.

Activity 2

1. Record the letter that corresponds with the Unknown Simulated Blood Sample (Y or Z) on the data table.
2. Place 3–4 drops of the Unknown Simulated Blood Sample in each of the three wells in the well plate.
3. Add 3–4 drops of Anti-a sera to the Anti-a well.
4. Similarly, add Anti-b Sera to the Anti-b well.
5. Add Anti-Rh to the Anti-Rh well.
6. Using a fresh toothpick for each, stir the mixtures in all 3 wells with a toothpick taking care not to scratch the plastic.
7. Mix each solution thoroughly and let the slides sit for at least two minutes.
8. Observe each well against a white background, such as paper, and record the agglutination results on the data table.
9. PTC papers may be placed in the trash, and the simulated blood samples may be washed down the drain with water.