Human Genetics: Phenotype and GenotypeHands-On Labs, Inc. Version 42-0101-00-03
Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.
Experiment Summary:
You will learn the concepts of phenotype and genotype and the difference between dominant and recessive traits. You will use phenylthiocarbamide (PTC) papers to test for the PTC trait and analyze human phenotypic features such as dimples, widow’s peak, and free ear lobes. You will create Punnett squares to determine if traits are homozygous or heterozygous. You will then explore karyotyping and the genotypes and phenotypes associated with chromosomal disorders.
Experiment Human Genetics : Phenotype and Genotype
MaterialsStudent Supplied Materials
Quantity Item Description1 Access to a printer1 Camera, digital or smartphone1 Mirror1 Pair of scissors4 Sheet of printer paper1 Tape or glue
HOL Supplied Materials
Quantity Item Description1 PTC Taste Strips – 2 in Bag 2”x 3”1 Possible Chromosomal Disorder #11 Possible Chromosomal Disorder #21 Karyotype Template
Note: To fully and accurately complete all lab exercises, you will need access to:
1. A computer to upload digital camera images.
2. Basic photo editing software such as Microsoft® Word or PowerPoint®, to add labels, leader lines, or text to digital photos.
3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.
Note: The packaging and/or materials in this lab kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your lab kit.
Who you are is a product of your heredity, your environment, and how you were nurtured and trained. Genetics is the science of heredity, or how specific characteristics (genes) are passed down from generation to generation. While genes are embedded in DNA and cannot be directly observed, observing and tracking physical features provides insight into characteristics that are passed down from previous generations.
The sum of the genetic makeup, which is passed down from parent to child, is referred to as genotype. The appearance of a person, resulting from their genotype is referred to as phenotype. Specifically, phenotypic characteristics are the physical manifestation of the genotype. As genotype is embedded in DNA and cannot be directly observed, scientists had to find a procedure that allowed them to identify the genotype and predict the likelihood that the offspring would inherit a particular trait. One of the first scientists to study this transfer of traits from one generation to the next was Gregor Mendel.
Gregor Mendel (1822–1884) is often referred to as “The Father of Genetics.” See Figure 1. Without any formal biology training, he developed the basis of modern genetics. He grew, cultivated, and studied tens of thousands of pea plants (Pisum sativum), carefully noting how the crossbreeding of different pea plant strains resulted in specific traits in the plants. His investigations showed that units of heredity would separate as each parent went through the process of forming sex cells. These units would recombine in the offspring, resulting in the unit of heredity passing from the parent to the offspring. While his work was not formally embraced until the twentieth century, he is credited with developing the Law of Segregation and the Law of Independent Assortment.
Figure 1. “Father of Modern Genetics,” Gregor Mendel (1822-1884). Image Courtesy Wikimedia Commons, 2013. Image is in the public domain.
Experiment Human Genetics : Phenotype and Genotype
Alleles
Today these units of heredity are called genes and each gene has two forms of alleles. Alleles, which are inherited from each parent, are the possible forms of a gene that produce distinguishable phenotypic effects (physical traits), such as eye or hair color. In simple Mendelian genetics, one of the alleles can be expressed or “turned on” while the other can be repressed or “turned off.” The expressed allele is referred to as dominant, while the repressed allele is referred to as recessive.
Alleles are usually written as follows:
● A capital letter, such as “A,” is used for the dominant allele.
● A lowercase letter, such as “a,” is used for the recessive allele.
Using “A” for the dominant allele, and “a” for the recessive allele, when combined, a person’s genotype could be “AA,” “Aa,” or “aa.”See Table 1. In this scenario:
● “AA” is referred to as homozygous dominant—both alleles are dominant.
● “Aa” is referred to as heterozygous—there are two different alleles, one dominant and one recessive.
● “aa” is referred to as homozygous recessive. This is the only scenario where the recessive trait would be expressed.
Continuing with this scenario, assume that the dominant allele was present. As phenotype is the physical appearance of the expression of the alleles, it is impossible to see, when viewing the dominant phenotype, if the genotype is “AA” or “Aa”. It is only possible to know that the dominant “A” allele is present in the genes.
Table 1. Genotype and Phenotype Comparison.
Allele Genotype Phenotype
AAA DominantAa Dominant
a aa Recessive
Punnett Squares
An English biologist named Reginald Punnett devised a shorthand method to determine the expected proportions of possible genotypes in offspring. This method is called a Punnett square (named for Reginald Punnett).
In a simple Punnett square, where only one trait is being studied, the Punnett square is two boxes tall and two boxes wide, as each parent can contribute two different types of alleles for the trait. The alleles from Parent 1 are shown across the top of the square, one allele above each box; while the alleles from Parent 2 are shown along the side of the square, one allele to the left of each box. See Figure 2.
Experiment Human Genetics : Phenotype and Genotype
Figure 2. Possible examples of a simple, one-trait Punnett square. A. The possible offspring are 50% AA and 50% Aa. B. 25% AA, 50% Aa, and 25% aa. C. 50% Aa and 50% aa. For example, in Figure 2 B: the genotypic ratio is 1:2:1, yet the phenotypic ratio would be 3:1 as both the AA
and Aa genotypes are expressed showing the dominant trait.
One genetic test, which is used in this experiment, is to determine if a person can taste
phenylthiocarbamide (PTC). The fact that not everyone is able to taste PTC was identified by accident in 1931. A scientist, Dr. Arthur Fox was working with powdered PTC, when a colleague walked into the room and was shocked by
the bitter taste that surrounded him. Dr. Fox was taken aback because he did not taste anything at all. As a result of his inability to taste the bitter PTC, Dr. Fox began to research the link between PTC and taste, eventually determining
that the ability to taste PTC is a genetic trait.
Karyotyping
An extension of the analysis of genotypes and phenotypes is karyotyping. Karyotyping is the analysis of chromosomes, the threadlike structures that carry genes. Chromosomes are found in the nucleus of a cell and each chromosome contains one very long DNA molecule. Healthy humans have 23 pairs of chromosomes, which are numbered 1 through 22, decreasing in size, followed by X and/or Y chromosomes. See Figure 3. A healthy male karyotype contains an X and a Y chromosome, while a healthy female karyotype contains two X chromosomes. A trisomic cell contains an extra chromosome (2n+1), while a monosomic cell is missing a chromosome (2n-1).
Experiment Human Genetics : Phenotype and Genotype
Figure 3. Karyotype of a human contains 23 pairs of chromosomes. Note how the chromosomes physically decrease in size from 1 (the largest) to 22 (the smallest). This karyotype is for a
In karyotyping, the genotype is analyzed by simply counting the number of, and observing the size of the chromosomes, rather than specific traits. The phenotype is the physical and physiological manifestations that occur as a result of chromosomal abnormalities. For example, Down syndrome (Trisomy 21) occurs when there is a third copy of chromosome 21. The genotype is the presence of the chromosomal abnormality (additional 21st chromosome), and the phenotype is the outward manifestation of the chromosomal disorder, as seen by delayed cognitive ability, oblique eye shape, and flat nasal bridge. See Figure 4.
Experiment Human Genetics : Phenotype and Genotype
Chromosomal Disorders
While karyotyping is typically performed in utero through amniocentesis (the testing of amniotic fluid during pregnancy), it may be performed on the majority of tissues, including blood and bone marrow. The tissue sample is grown and subsequent cells are removed and stained for microscopic observation. From this stained sample, the chromosomes are arranged in order, producing the karyotype that is used to study the number, size, and shape of the chromosomes. A variety of chromosomal genotypes and their resulting physical manifestations (phenotypes) are described in Table 2.
Table 2. Chromosomal Disorders.
Chromosomal Disorder Genotype Phenotype
XYY SyndromeAn extra copy of the Y
chromosome (XYY)
Male specific
Increased height, reduced cognitive ability, normal fertility
Patau Syndrome An extra copy of chromosome 13
Cleft palate, as well as: eye, brain, and circulatory defects
Turner’s syndromeOnly one X chromosome
Female specific
Only viable monosomy not resulting in death. Reduced height, sterile, normal
Experiment Human Genetics : Phenotype and Genotype
Exercise 1: Observing Phenotype CharacteristicsIn this exercise, students will observe hypothetical genetic traits. The goal of the exercise is to determine the possible genotypes and phenotypes of offspring for a variety of hypothetical genetic traits.
Important Note: This exercise does NOT follow true Mendelian genetics. This exercise is meant to be used as a learning tool to understand the properties and background of phenotype and genotype characteristics. The examples were chosen as they are easily identified on a person and serve the goal of teaching and learning about the topic.
ProcedureData Table 1 of your Lab Report Assistant contains a series of genotypes and phenotypes for a variety of hypothetical genetic traits. The genotype and phenotype for “Parent 1” are provided in Table 1. For each of the traits in Data Table 1, determine your phenotype and possible genotype(s).
Note: If you have a dominant phenotype, you may have either a recessive allele and a dominant allele, such as “Aa,” or two dominant alleles, such as “AA.” If your phenotype is recessive, your genotype is always homozygous recessive such as “aa.”
1. Use the mirror to determine if you have a dimpled chin.
2. Record either “Dimpled Chin” or “No Dimples” under “Phenotype & Genotype(s) from Parent 2” in the “Dimpled Chin” row.
3. If you have a dimpled chin, record “Dd” and “DD” under “Phenotype & Genotype(s) from Parent 2” in the “Dimpled Chin” row to represent the 2 possible genotypes for your observed dominant phenotype.
4. If you do not have a dimpled chin, record “dd” under “Phenotype & Genotype(s) from Parent 2” in the “Dimpled Chin” row to represent the 1 possible genotype for your observed recessive phenotype.
5. Repeat steps 1 through 4 for the remaining 9 traits in Data Table 1.
Note: For the PTC test, place the PTC strip on your moist tongue and determine whether or not you can taste the chemical.
Experiment Human Genetics : Phenotype and Genotype
To Determine the “Offspring”
6. Use a Punnett square to show both the genotype and phenotype ratios possible for the “Dimpled Chin” trait.
Note: If you have a dominant phenotype then you will create two Punnett squares, if you have a recessive phenotype then you will create one Punnett square.
a. Place alleles in a Punnett square as shown in the example template below. Then fill in the offspring. The example template shown below is for “Dimpled Chin.”
Note: There are 2 Punnett squares provided for each trait in your Lab Report Assistant.
b. Record the possible offspring genotype(s), percent of phenotype, and genotypic ratio in Data Table 1 under the “Offspring” column.
7. Repeat step 6 for the remaining 9 traits in Data Table 1. When the exercise is finished, you will have created 10 to 20 different Punnett squares.
Note: All Punnett squares, for all 10 traits, need to be included in your report and turned in to your instructor.
QuestionsA. As “Parent 2,” how many recessive traits did you express? What were they?
B. As “Parent 2,” how many dominant traits did you express? What were they?
C. While the traits studied in Exercise 1 were hypothetical genetic traits, what type of genetic traits do you think are important to study and predict? Give an example of a genetic trait that is carefully monitored when two parents are creating an offspring and why it would be important to monitor.
Experiment Human Genetics : Phenotype and Genotype
Exercise 2: Genetic Screening: Phenotype/GenotypeIn this exercise, you will perform genetic screening of chromosomes through karyotyping. You will karyotype two sets of chromosomes to determine if the genotype of the chromosomes results in a genetic disease with a specific phenotype.
ProcedurePart 1: Karyotyping
1. Review the idiograms (photograph or drawing of chromosomes) shown in Figure 5.
5. Using the printed copy of Figure 8 as a template, create a karyotype for the cut and aligned chromosomes. Tape the chromosomes in place on the template.
Figure 8. Karyotype template.
6. Take a photo of the completed karyotype.
7. Resize and insert the photograph in Data Table 2 of your Lab Report Assistant under “Possible Chromosome Disorder 1 Karyotype.” Refer to the appendix entitled “Resizing an Image” for guidance with resizing an image.
8. Determine the chromosomal disorder present in the karyotype and record in Data Table 2. Use Table 2 in the background as necessary to help determine the chromosomal disorder.
9. Repeat steps 2 through 8 for the chromosomes in Figure 9 (print 1 copy of the “Possible Chromosomal Disorder #2” supplemental file) and record in Data Table 2 under “Possible Chromosome Disorder 2 Karyotype.”
Experiment Human Genetics : Phenotype and Genotype
Figure 10. Karyotypes of Disorders
Note: Use Table 2 in the background section as necessary to help you with this process.
11. Describe the genotype of each chromosomal disorder and record in Data Table 3.
12. Describe the phenotype of each chromosomal disorder and record in Data Table 3.
13. Return all items to your kit for future use.
14. When you are finished uploading photos and data into your Lab Report Assistant, save your file correctly and zip the file so you can send it to your instructor as a smaller file. Refer to the appendix entitled, “Saving Correctly,” and the appendix entitled, “Zipping Files” for guidance with saving the Lab Report Assistant correctly and zipping the file.
QuestionsA. Describe a karyotype and explain how it is performed.
B. Would a normal karyotype mean that a person would not have any disorder or disease in their life? Explain your answer, incorporating the definition of genetics.
