Scheme of Work Cambridge International as & a level

16.2.2 Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage.

16.2.3 Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of dihybrid crosses that involve autosomal linkage and epistasis.

16.2.4 Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses.

16.2.5 Use the chi-squared test to test the significance of differences between observed and expected results.

16.2.6 Explain the relationship between genes, proteins and phenotype with respect to the TYR gene, HBB gene, F8 gene and HTT gene.

Learners use coloured beads or sweets to represent different alleles, and then randomly select pairs of these items to create diploid genotypes illustrating the results of different genetic crosses. This helps learners appreciate that alleles are discrete (separate) entities that do not combine.

Learners research and present a short presentation about a disease or trait that interests them. Provide an opportunity to ‘flip’ the classroom: ask learners to pre-read the relevant section of their textbook, with further internet research, and be expected to offer mini-summaries of the concepts in a subsequent lesson. (I)

To practise using the chi-squared analysis, provide learners with tangible examples of characteristics that are not related to genetics. This worksheet directs learners to count the number of different coloured sweets in a bag and to calculate the chi-squared statistic to see if the deviation from equal numbers of each colour is attributable to chance: www.biologycorner.com/worksheets/chi_square_candy.html. Alternative examples include analysing the observed/expected number of lessons of a particular subject that a learner has during a term, or the observed/expected number of stripes or dots visible on the school tie that the learners are wearing.

Draw or display a large diagram of a Punnett square (ideally, showing an example of dihybrid cross or epistasis), but which has been covered by 12–15 small numbered ‘jigsaw’ pieces (this can be done virtually with computer software, or by affixing A3 sheets to the whiteboard). Ask learners to choose which pieces to remove, thus gradually revealing the image, and to identify what type of inheritance is shown, and the proportion of individuals with the various genotypes and phenotypes.

Write a passage that summarises the wide range of concepts that learners have encountered in this subtopic, in which between five and ten mistakes have been intentionally included. These include spelling mistakes, but also conceptual errors, e.g. ‘There is a 0.25 probability of the offspring of a cross between a male of blood group AB and a female of blood group O of a child being born with blood group A.’ (F)

Learners make a guide for a younger learner to explain the relationship between genes, proteins and phenotype. Ask different learners to research the different genes listed in the syllabus and provide a short presentation for the benefit of the rest of the class.