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GRADE 12A: Biology 5 UNIT 12AB.5 9 hours Genetic inheritance About this unit Previous learning Resources This unit is the fifth of seven units on biology for Grade 12 advanced. To meet the expectations of this unit, students should already understand that changes in DNA bases cause variation. They should know some causes of mutation. They should understand that a mutation causes a change in DNA and that this can reduce the efficiency of, or block, an enzyme. They should know the difference between genes and alleles and that they are sections of DNA. They should understand how genetic variation occurs through the segregation of alleles and chromosome crossovers. They should understand how sex is determined in humans and the mechanism of sex linkage. They should understand the difference between dominant and recessive alleles and be able to calculate genotype and phenotype frequencies in monohybrid crosses. The main resources needed for this unit are: The unit is designed to guide your planning and teaching of biology lessons. It provides a link between the standards for science and your lesson plans. The teaching and learning activities should help you to plan the content and pace of lessons. Adapt the ideas to meet your students’ needs. For consolidation activities, look at the scheme of work for Grades 10A and 11A. You can also supplement the activities with appropriate tasks and exercises from your school’s textbooks and other resources. Introduce the unit to students by summarising what they will learn and how this builds on earlier work. Review the unit at the end, drawing out the main learning points, links to other work and real world applications. • overhead projector (OHP) or whiteboard • corn (Zea mays) cobs • coloured beads • chi-squared statistical tables • DNA autoradiograph from a genetic fingerprint • Internet access Key vocabulary and technical terms Students should understand, use and spell correctly: Expectations • incomplete dominance, co-dominance By the end of the unit, students calculate the frequency of different progeny from a cross with incomplete dominant alleles, from back crosses and from dihybrid crosses. They understand co-dominance and the inheritance of phenotypic traits through multiple alleles. They use the chisquared test to determine the significance of results of genetic crosses. They know about the Human Genome Project, genetic fingerprinting and genetic screening and counselling. • chi-squared test • dihybrid cross, Punnett square • Human Genome Project • genetic fingerprinting, polymerase chain reaction (PCR) • genetic screening, amniocentesis, chorionic villus sampling • genetic counselling Students who progress further are able to follow and understand the principles of the technological advances and applications of the Human Genome Project, genetic fingerprinting and genetic screening and counselling. 435 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005 Objectives for the unit 9 hours 2 hours Calculating ratios of phenotypes and genotypes SUPPORTING STANDARDS 11A.14.3 Explain the terms gene, allele, phenotype, genotype, dominant, recessive and co-dominant. Unit 12AB.5 CORE STANDARDS Grade 12 standards EXTENSION STANDARDS 12A.12.1 Calculate the ratios of the genotypes and phenotypes in the progeny of incomplete dominant monohybrid crosses, dihybrid crosses (9:3:3:1 ratio) and back crosses. 11A.14.4 Use genetic diagrams to solve genetic problems involving monohybrid crosses. 1 hour Co-dominance and multiple alleles 1 hour Using the chisquared test 1 hour The Human Genome Project 11A.14.5 Explain how variation occurs through segregation of alleles during gamete formation and through the crossing over of chromosome segments during meiosis 11A.14.6 Know how X and Y chromosomes determine sex in humans and the inheritance pattern of sex-linked characteristics. 12A.12.2 Explain co-dominance and the inheritance of phenotypic traits such as blood grouping through multiple alleles. 12A.12.3 Use the chi-squared test to determine the significance of observed and expected frequencies of different progeny in genetic crosses. 12A.12.4 Know the purpose of the Human Genome Project. 12A.12.5 Explain the basis of genetic fingerprinting and understand its advantages 2 hours Genetic fingerprinting and potential dangers. 12A.12.6 Explain the basis of genetic screening for alleles of disadvantaging inherited conditions; understand the advantages and potential dangers of such screening and the need for genetic counselling. 2 hours Genetic screening 436 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005 Activities Unit 12AB.5 Objectives Possible teaching activities 2 hours Reinforce previous knowledge by giving students a quiz on genes, chromosomes, monohybrid crosses, genetic variation and sex-linked characteristics (from Unit 11AB.5). Calculating ratios of phenotypes and genotypes Calculate the ratios of the genotypes and phenotypes in the progeny of incomplete dominant monohybrid crosses, dihybrid crosses (9:3:3:1 ratio) and back crosses. Tell students to use the library to read about, and make notes on, the work of Mendel and his recording of experiments conducted on the garden pea. These experiments included monohybrid crosses, dihybrid crosses and back crosses. Notes Enquiry skill 12A.2.1 School resources Use this column to note your own school’s resources, e.g. textbooks, worksheets. Use large diagrams on the board or OHP to display examples of the phenotypes of pea plants involved in a variety of crosses. Introduce an example of incomplete dominance (which Mendel did not meet). Show students large diagrams on the board or OHP to illustrate the typical cross between red and white snapdragon (Antirrhinum ) flowers. The F1 offspring are all pink. Ask students to explain this observation. Ask them to predict what will happen in the F2 when the F1 plants are selfed. Confirm that when the pink plants are selfed, then a ratio of 1 red : 2 pink : 1 white is produced in the F2. Ask students to explain the results using a Punnett square. Make sure students appreciate that the alleles remain discrete and that they do not blend together. Enquiry skill 12A.3.3 Get students to predict the outcome of a back cross on the pink plants. Provide students with an example of a dihybrid cross. Use large diagrams on the board or OHP showing the phenotypes of the parents and offspring for both F1 and F2. For example, use one of Mendel’s crosses in which he crossed plants with tall purple flowers with plants with short white flowers, and produced all F1 tall purple flowers. These were selfed and he finally obtained the following F2 plants: 96 tall purple, 31 tall white, 34 short purple and 11 short white flowers. Get students to explain the results using a Punnett square and work out the dihybrid ratio of 9:3:3:1. Enquiry skill 12A.3.3 Provide students with a number of cobs of maize (Zea mays). Maize kernels display a number of easily recognisable characteristics, such as colour and shape. Ask to students to examine the kernels and make deductions about the genotypes of the parent plants. Use a computer simulation to investigate genetic crosses. ICT opportunity: Use of computer simulation. Get students to predict the outcomes of dihybrid crosses and compare their predictions with collected data Enquiry skill 12A.1.2 Simulate a dihybrid cross. Provide four bags, each containing the same number of a different colour of bead (e.g. 50 red beads in one bag, 50 blue in the second, 50 green in the third and 50 blue in the fourth). Arrange students in pairs and get each student to take two bags of the beads, pour them into one bag and mix thoroughly. Tell students to decide which colour will be the ‘dominant’ and which the ‘recessive’ allele. Each student then takes two beads out of their bag and puts them together to represent the first of the dihybrid’s F2 progeny. This should be repeated at least 16 times and the ‘genotypes’ of the progeny identified. Ask students to compare their ratios with those of other pairs of students and discuss the results together. 437 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005 Objectives Possible teaching activities 1 hour Distinguish co-dominance from incomplete dominance by asking students to find out, using the library, what the difference is between these genetic features. Co-dominance and multiple alleles Explain co-dominance and the inheritance of phenotypic traits such as blood grouping through multiple alleles. Notes School resources Ensure students understand that, in co-dominance, both of the alleles are expressed in the phenotype, whereas, in incomplete dominance, the phenotype has an expression of a feature some way between the phenotypes of the two parental varieties. A human example of co-dominance is in blood grouping. The three blood groups M, N and MN display the genotypes MM, NN, and MN, respectively. Note that MN is not intermediate between the M and N phenotypes, since both these factors are expressed on the membrane of a red blood cell. Explain multiple alleles by using the other example of human blood groups: the ABO system. Use the OHP or whiteboard to set up the examples and ask students to show the possible outcomes of parents with different blood groups producing children. Get a student to come to the front of the class and explain their answer, by adding the details such as a Punnett square. For example, ask them to explain how parents, one of blood group A and one of blood group B, can produce children with blood group A, B, AB or O. 1 hour Using the chi-squared test Use the chi-squared test to determine the significance of observed and expected frequencies of different progeny in genetic crosses. Explain the purpose of the chi-squared test to students. It is a basic statistical test of experimental data that is used to indicate whether the observed data is significantly different from the expected values. If a difference has been established, then the probability of this occurring by chance can be determined from statistical tables. Work through an example with students, using data from phenotypes resulting from a dihybrid cross. Show them how to interpret the equation to calculate the chi-squared value. Provide students with a worksheet containing results from a dihybrid cross displaying the numbers of the progeny. Ask them to work out the chi-squared value to see if the difference between the observed and expected data is significant. They will need access to chi-squared tables. Prepare worksheets on the appropriate dihybrid crosses. Students will need access to chi-squared tables. Enquiry skills 12A.3.3 Using other examples, get students to calculate the probability of obtaining the progeny of genetic crosses by chance. 1 hour The Human Genome Project Know the purpose of the Human Genome Project. Get students to visit the Human Genome Project website to gather information and to find out the purpose of this ambitious research project. ICT opportunity: Use of the Internet. The project is producing evidence of the sequence of the bases of the DNA in the entire human genome. Ask students how this knowledge will help people. Ensure they understand the huge potential benefits, for example: • health care – identification and mapping of the genes responsible for genetic diseases will help in the diagnosis, treatment and prevention of those conditions; • science – knowledge of the genome will give insight into the control of gene expression, cellular growth and differentiation; • evolutionary biology – enabling clarification of genetic relationships between species. Show students a video of the Human Genome Project. Get students to write an article for a magazine about the Human Genome Project. 438 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005 Objectives Possible teaching activities 2 hours Before beginning this topic, get students to collect newspaper and magazine articles (e.g. New Scientist) on genetic fingerprinting. Get them to use these and the Internet to find out about the development of genetic fingerprinting by Alec Jeffreys and colleagues at the University of Leicester, and to explain the principles of the procedure of genetic fingerprinting. Ask them to produce a poster showing the main stages of genetic fingerprinting and its applications. Examples of applications include: Genetic fingerprinting Explain the basis of genetic fingerprinting and understand its advantages and potential dangers. Notes School resources ICT opportunity: Newspaper and magazine articles can be obtained from the Internet. Enquiry skills 12A.2.1 • settling paternity disputes; • settling disputes in hospitals where newborn babies have been accidentally switched; • revolutionising forensic work (using DNA extracted from cells in traces of blood, saliva, hair roots or, in rape cases, semen); • animal identification (e.g. establishing the variation of the whale population). Ask students to find out about the dangers or shortcomings of genetic fingerprinting, for example: • discovering that a child may not be the natural child of a parent may create problems for family relationships; • relatives show many similarities in their genetic fingerprints, so if more than one family member is a suspect of a crime, it may be difficult to be certain who is the culprit; • the use of the polymerase chain reaction (PCR) to amplify the amount of DNA for forensic work means the technique is now extremely sensitive to contamination; anyone who has shed dandruff or sneezed at the scene of a crime may become a suspect! Show students autoradiographs of genetic fingerprints of individuals from a murder case and ask them to select the possible suspect. Get students to write an article for a magazine about genetic fingerprints. 2 hours Genetic screening Explain the basis of genetic screening for alleles of disadvantaging inherited conditions; understand the advantages and potential dangers of such screening and the need for genetic counselling. Ask students what they understand by the term genetic screening. Explain that everyone probably carries several genetic defects, and detecting the mutant genes in an individual is known as genetic screening. Ask students to find out from the library the situations in which genetic screening is particularly relevant. These include: prenatal diagnosis, carrier diagnosis (e.g. CF, sickle cell), and predictive diagnosis (e.g. Huntington’s disease). Ask students to investigate and compare the advantages and dangers of prenatal diagnosis by amniocentesis and chorionic villus sampling. Enquiry skills 12A.2.2 Debate the ethics of genetic screening with students. Should the mother be able to choose to abort her foetus? Which genetic disorders should result in a foetus being aborted? Where do you draw the line? Will we be able to breed genetic abnormalities out of the human race in the future? Ask students what they understand to be the role of a genetic counsellor. Ask students to find out whether their local hospital has a genetic counselling department and why only certain people may need to visit a genetic counsellor. Discuss the nature of a conversation that a counsellor might have with a husband and wife, one of whom thinks they are carrying an allele for a disadvantaging condition. 439 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005 Assessment Examples of assessment tasks and questions Assessment Set up activities that allow students to demonstrate what they have learned in this unit. The activities can be provided informally or formally during and at the end of the unit, or for homework. They can be selected from the teaching activities or can be new experiences. Choose tasks and questions from the examples to incorporate in the activities. Examine the kernels of cobs of maize (Zea mays). Make deductions about the genotypes of the parent plants. Justify and support your deductions by drawing diagrams to show the parental cross and a Punnett square to show the offspring. Unit 12AB.5 Notes School resources Provide suitable cobs of maize for this question. Distinguish co-dominance from incomplete dominance by using examples. Examine the worksheet containing results from a dihybrid cross and displaying the numbers of the progeny. Work out the chi-squared value to see if the difference between the observed and expected data is significant. Provide a suitable worksheet and chi-squared tables. Write an article for a magazine about the Human Genome Project. Explain the principles of the procedure of genetic fingerprinting. Explain the advantages and dangers of genetic screening by prenatal diagnosis by amniocentesis and chorionic villus sampling. 440 | Qatar science scheme of work | Grade 12 advanced | Unit 12AB.5 | Biology 5 © Education Institute 2005