Ohio Standards Connection: Life Sciences Benchmark C Explain the genetic mechanisms and molecular basis of inheritance. Indicator 6 Explain that a unit of hereditary information is called a gene, and genes may occur in different forms called alleles (e.g., gene for pea plant height has two alleles, tall and short). Indicator 8 Use the concepts of Mendelian and non- Mendelian genetics (e.g., segregation, independent assortment, dominant and recessive traits, sex-linked traits, jumping genes) to explain inheritance. Lesson Summary: In this lesson, students will learn about the general concepts of heredity. The teacher will explain that a unit of hereditary information is called a gene and genes may occur in different forms called alleles. Students will use classroom activities to explore the concepts of Mendelian and non-mendelian genetics. Estimated Duration: One hour and 30 minutes Commentary: This lesson, as originally published to the IMS, used the trait of tongue-rolling as an example of simple genetic dominance and recessiveness. Studies of identical twins conducted as early as the 1950 s by researchers including P. Matlock (1952) and A. H. Sturtevant (1965) suggested that tongue-rolling may not be a purely Mendelian inherited single-locus trait. Recent research by the Human Genome Project (HGP) confirms that tongue-rolling is not an illustrative example of dominance and recessiveness but is a more complex case of human inheritance. The HGP has thus far failed to identify this elusive gene s location or coding. For these reasons, the lesson has been modified to reflect current scientific understandings of genetics. All references to the trait of tongue-rolling in this lesson have been changed to the trait of attached/unattached earlobes, currently understood to be a simple case of dominance and recessiveness. This lesson and its subsequent modification provide an avenue for classroom discussion regarding the nature of science and scientific inquiry. For more information on the HGP, visit http://www.genome.gov or http://www.doegenomes.org Pre-Assessment: Copy and distribute Attachment A Pre-Assessment to students and have them respond to the questions. Scoring Guidelines: See Attachment B, Pre-Assessment Key to evaluate student responses. Discuss the questions with the students and monitor responses to guide instruction. Observe any students who have difficulty understanding genetic alleles and inheritance and provide intervention as necessary. 1
Post-Assessment: Distribute copies of Attachment C, Post-Assessment and have students answer the questions. Instructional Tip: You may choose to have students record answers in their journal. Scoring Guidelines: Use Attachment D, Post-Assessment Key to evaluate student knowledge of genetics. Instructional Procedures: Student Engagement: Begin by asking students to think about their physical features. Why do they look the way they do? Let several students answer before continuing on to the instructional procedures. The answer is because of genetics, the branch of biology that studies heredity. 1. Explain to the students that their parents transfer characteristics, or traits, to them by way of genes, pieces of DNA that carry genetic information from one generation to the next. Each parent carries two copies of an allele of a gene for each characteristic and is received by the offspring during reproduction. An individual receives one allele from each parent. Each allele is represented by an uppercase or lowercase of the same letter. The uppercase letter represents the dominant trait, or the expressed trait. The lowercase letter represents the recessive trait, or the unexpressed trait. For example, a person that has earlobes that are attached to the side of the head (recessive characteristic) would have the letters (or genotype) ee (homozygous recessive), while those that have earlobes that are unattached or hang freely at the side of the head would be either Ee (heterozygous) or EE (homozygous dominant). One s genotype (the set of alleles an individual has; represented by upper- and lowercase letters) directly affects one s phenotype (the physical appearance of expressed alleles). This discussion only addresses alleles in a dominant/recessive system. Codominance and incomplete dominance work differently. Instructional Tip: The original work performed by Mendel was observed in peas. The first plants used in Mendel s experiments are referred to as the P (parental) generation. The offspring created from the P generation are called F 1 or first filial generation. Offspring produced by crossing members of the F 1 generation with one another are called the F 2 or second filial generation. Remind students that all living things pass on heredity information via alleles and genes. Students usually know that homo means same, and hetero means other. The presence of an allele does not guarantee that a trait will be expressed in an individual. Dominant alleles are expressed; recessive alleles are only expressed when homozygous. 2. Use the following activity to further explain inheritance of traits. These activities reflect a monohybrid cross which examines the inheritance of one trait. A dihybrid cross would examine two traits at a time. 2
3. Make two copies of Attachment G, Activity Alleles. Cut the letters apart to represent the alleles of the cross between two heterozygous parents for the earlobe trait. The letters represent the trait for whether the earlobes are attached or unattached. The allele for attached earlobes (recessive) will be represented by e, and the allele for unattached earlobes (dominant) will be E. 4. Place 100 e and 100 E in a plastic bag to represent the alleles of one of the parents in 100 gametes. Label it male or female. Each bag represents one parent. Prepare enough bags for each student to participate. 5. Teams of two students will model the random pairing of alleles by choosing uppercase and lowercase letters from zippered plastic bags. These letters represent the trait for whether the earlobes are attached or unattached. The allele for attached earlobes (recessive) will be represented by e, and the allele for unattached earlobes (dominant) will be E. Instructional Tip: If the letters are used by several classes, make sure students count the letters to verify that the correct number of alleles is present in each bag. (100 e and 100 E ) Copy the alleles on pink or blue paper to help keep the correct number of alleles in each bag. 6. Since each parent contributes one allele at random to each offspring, teams will model a cross between these two parents by choosing a letter from each of the bags. Do this by simultaneously picking one letter from each bag randomly, without looking. Then place the pair of selected alleles together on the lab table. The pair of letters represents one offspring. Have students record a tally mark for each cross in Attachment E, Activity Summary Sheet, Table1. 7. Return the letters to a discard pile on the table and repeat step five, 99 more times. 8. After the team has completed all 100 crosses, have each pair of students determine the genotypic and phenotypic ratios among the offspring and record ratios in Table 2. Instructional Tip: The genotype is 1:2:1 and phenotype is 3:1. 9. Combine data for the whole class. Have each team compare their totals with those of the entire class. Have all students calculate the class genotypic and phenotypic ratios and record them in Table 3. Summarize data with the entire class. Instructional Tip: The class data can be collected by having each team record their team s numbers on an overhead or the black board. 3
10. Explain the role of the X and Y chromosome in gender determination. Explain that genetic diseases or conditions can be sex-linked. This means that a gene is located on a sex chromosome and not on any of the other 22 types of chromosomes. In humans, normal vision dominates colorblindness, which is sex-linked. The gene is located on the X chromosome, which is much larger than the Y and contains more genetic information. 11. Have students use a Punnett square to show a color-blind male marrying a woman who is heterozygous for normal vision (she is a carrier for colorblindness) and determine the genotypes and phenotypes of the offspring. Explain that the male is colorblind because his X chromosome has the colorblind allele and without another X chromosome to mask or compliment it, his cannot distinguish select colors. 12. Repeat the reproduction activity using the color blind alleles found in Attachment H, Sex-linked Alleles. 13. Use the genotypes X c Y for the colorblind male and X C X c for a carrier female. The outcome is (a or one) X C X c female carrier, X c X c color blind female, one X C Y normal vision male, and one X c Y color-blind male. Mother X C X c X c X C X c X c X c Y X C Y X c Y 14. Have students complete Attachment E, Activity Summary Sheet. 15. The inheritance of traits is not always as simple as it appears in the activity. Explain that some genes are scattered about randomly on chromosomes and may be repeated several times. Some genes, known as transposons or jumping genes have the ability to move from one chromosomal location to another. When a jumping gene moves to another location, it often inactivates a gene or causes a mutation, which may change the characteristics of the organism. Instructional Tip: You may find Indian corn labs available on-line and in other resources that may be used to help explain transposons, or jumping genes. 16. Summarize this lesson by explaining that all living things have genes and each gene in an organism is present as two alleles or copies. Genes are passed on to the offspring through sexual reproduction, for those diploid organisms that reproduce sexually. Have students respond by recording their reflection of the lesson in their journal. Differentiated Instructional Support: Instruction is differentiated according to learner needs, to help all learners either meet the intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the specified indicator(s). 4
Students who are having difficulty with the inheritance of alleles can repeat the activity with other traits to help them to understand how traits are passed from one generation to the next. Students can repeat the activity by changing the parents and predicting the outcomes, e.g. homozygous dominant x homozygous recessive, homozygous recessive x heterozygous etc. Extensions: Have the students use on-line resources to research genetic abnormalities or diseases. Have the students use on-line resources to research different breeds of dogs, cats, etc. Have the students use additional Punnett squares to show sex-linked genes. Have the students complete the activity using a dihybrid cross. Have the students examine skin color inheritance (use four alleles, although there are more). Have the students research Mendel s work and look at P, F 1 and F 2 generations. Have the students use on-line resources to look at the role of mitochondrial DNA. Have the students research hemophilia. Have the students use virtual labs exploring various genetic crosses. Homework Options and Home Connections: Students can research the genetic heritage of their pets. Materials and Resources: The inclusion of a specific resource in any lesson formulated by the Ohio Department of Education should not be interpreted as an endorsement of that particular resource, or any of its contents, by the Ohio Department of Education. The Ohio Department of Education does not endorse any particular resource. The Web addresses listed are for a given site s main page, therefore, it may be necessary to search within that site to find the specific information required for a given lesson. Please note that information published on the Internet changes over time, therefore the links provided may no longer contain the specific information related to a given lesson. Teachers are advised to preview all sites before using them with students. For the teacher: Plastic sealable bags, scissors. For the students: Prepared bags of allele cards. Vocabulary: alleles genes heredity genetics gametes Punnett square 5
dominant recessive segregation independent assortment jumping genes sex-linked homozygous heterozygous filial monohybrid phenotypes genotypes diploid Technology Connections: Students can use on-line resources to research genetic abnormalities or diseases. http://healthlibrary.stanford.edu Research Connections: Marzano, R., et.al. Classroom Instruction that Works: Research-base Strategies for Increasing Student Achievement, Alexandria, Va.,: Association for Supervision and Curriculum Development, 2001. Nonlinguistic representations help students think about and recall knowledge. This includes the following: Creating graphic representations (organizers); Making physical models; Generating mental pictures; Drawing pictures and pictographs; Engaging in kinesthetic activity. Attachments: Attachment A, Pre-Assessment Attachment B, Pre-Assessment Key Attachment C, Post-Assessment Attachment D, Post-Assessment Key Attachment E, Activity Summary Sheet Attachment F, Activity Scoring Guidelines Attachment G, Activity Alleles Attachment H, Sex-linked Alleles 6
Name Directions: Define the following terms. Allele- Attachment A Pre-Assessment Genes- Heredity- Punnett square- Can two tall pea plants produce a short pea plant? Explain your answer. 7
Directions: Define the following terms. Attachment B Pre-Assessment Key Allele- Any one of the alternate forms of a gene that can effect the phenotype of an organisms, e.g., hair color, height, eye color etc. Genes- A DNA sequence that codes produces a trait in an organism by coding for specific proteins. Heredity- Genetic information passed on from one generation to the next. Punnett square- A tool that illustrates the various genetic combinations that can result from a genetic cross of two individuals. Can two tall pea plants produce a short pea plant? Explain your answer. Yes, two tall plants can produce a short plant if both parents are heterozygous for the trait of tallness. Tall is dominant over short so the parents carry the trait for shortness even though it is not expressed. 8
Name Attachment C Post-Assessment Directions: Answer the following questions using sentences, Punnett squares or graphic organizers. 1. A homozygous dominant purple flowered pea is crossed with a homozygous recessive white flowered pea (this is the parental generation, P). What will the genotypes and the phenotypes in the next generation be (F 1, or fist filial generation)? What will the genotypes and phenotypes be in the second filial, or F 2, generation be (this assumes two individuals from the F 1 mate)? 2. In a cross between a plant that is homozygous dominant for two traits, yellow seeds with a smooth seed coat, and another plant that is homozygous recessive for green and wrinkled seeds, what are the genotypes and phenotypes for the next two generations? Show your work. 3. A sighted couple produces a color-blind boy. Colorblindness is sex-linked. Show the probable genotype of the parents. If they were to have other children what outcomes can be expected for color blindness? Remember to include the sex of the possibilities. 9
Attachment D Post-Assessment Key 10
Name Tables for Activity Table 1. Genotypes of offspring. Attachment E Activity Summary Sheet EE Ee ee Table 2. Offspring Ratios for Individual Teams Genotypes: Homozygous dominant (EE) Total (count tallies) Genotypic Ratio Heterozygous (Ee) Homozygous recessive (ee) Phenotypes: Unattached earlobes (EE and Ee) : : Phenotypic Ratio Attached earlobes (ee) Table 3. Offspring Ratios for Entire Class Genotypes: Homozygous dominant (EE) Total (count for class) : Genotypic Ratio Heterozygous (Ee) Homozygous recessive (ee) Phenotypes: Unattached earlobes (EE and Ee) Attached earlobes (ee) : : Phenotypic Ratio : 11
Attachment E (continued) Activity Summary Sheet 1. Construct a Punnett square showing the parents and their offspring from the activity completed in class. 2. What are the genotypes and phenotypes of the parents? 3. What does each letter in the bags represent? 4. Describe the genotypes of the parents using homozygous, heterozygous or both. 5. Did Tables 2 and 3 reflect a classic monohybrid cross phenotypic ratio of 3:1? 6. What trait is being studied in the activity? 7. When the class data was calculated, did the classic monohybrid ratio of 3:1 result? 12
1. Ee x Ee Mendelian and Non-Mendelian Heredity Grade Ten Attachment F Activity Scoring Guidelines E e E EE Ee e Ee ee 2. The genotype for both the mother and the father is Ee. The phenotype exhibits unattached earlobes. 3. An allele for the earlobe trait attached or unattached. 4. The parents are heterozygous. 5. Answers will vary, mostly yes. 6. Earlobe attachment 7. Yes 13
Attachment G Activity Alleles E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e 14
Attachment H Sex-linked Alleles X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X c X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C X C Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 15