Module 6

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

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Mendel was interested in determining whether the movement of one trait’s alleles affected the movement of another trait’s alleles. To test this out, Mendel conducted crosses between plants that were true breeding for two traits. When a cross is carried out to observe two traits at the same time, it is called a dihybrid cross. If three traits were being analyzed at once, it would be a trihybrid cross.

One of the most important discoveries Mendel made when conducting dihybrid crosses was the discovery that the movement of alleles for different genes did not affect each other. This led Mendel to propose his second law: the Law of Independent Assortment. Read “The Law of Independent Assortment” and how to develop a 16-square Punnett square to track dihybrid crosses on pages 591 to 593 of your textbook.

Watch and Listen

Watch the video “Classical Genetics and Dihybrid Crosses.” You may choose to watch the entire video, or, using the navigation bar on the right edge of the video, start watching the section titled “Objectives.” Continue to watch until the end of “Bio Simulation: Mendel’s Dihybrid Cross.” To master the concepts, answer the following Try This questions and save your work in your course folder.

TR 1. What was the letter assignment used here for widow’s peak/straight hairline, and for normal thumb/hitchhiker’s thumb?

TR 2. What hypothesis (if/then statement) did the students create to test whether the attachment of earlobes was a trait controlled by a dominant or recessive allele?

TR 3. Did the students find many people with all three traits recessive?

TR 4. In your own words, state or explain Mendel’s second law, the law of independent assortment.

TR 5. Starting with parents that are true breeding for two independent traits, what will be the resulting phenotypic ratio in the F2 generation for each of the following?

1. dom/dom
2. dom/rec
3. rec/dom
4. rec/rec

TR 6. Does the movement of the alleles for plant height affect the movement of the alleles for flower colour?

Try This

Review again how to create gametes from parents, and then how to build a 16-square Punnett square by examining “Figure 17.10” on page 593 of your text. If you are still unsure about how the gametes fill in the square, ask your teacher to suggest questions from the textbook that you could practise, or you may choose to search the Internet for dihybrid tutorials.

TR 7. Answer question 6 on page 598 of your textbook. Use a Punnett square to find all of the genotypes and phenotypes in part (b). Save your answers to your course folder.

For additional resources, you may go to the McGraw-Hill Ryerson online learning centre. On this website are many resources for practising and for testing yourself.

Watch and Listen

Think back to the video “Classical Genetics and Dihybrid Crosses.” Mastering dihybrid crosses and Punnett Squares is an essential skill in Biology 30. Ensure your mastery by answering the following questions. If you have difficulty with these questions, you may choose to review the video or consult with your teacher.

TR 8. Did the parental alleles remain together when the F1 generation self crossed, or did they separate?

TR 9. What is the F2 ratio of phenotypes in Mendel’s dihybrid crosses?

TR 10. How many different gametes can be formed from AaBbCc?

Module 6: Lesson 4 Assignment—Labs

In previous lessons, you tracked the movement of alleles for one gene, such as plant height. In this lesson you have been introduced to the independent movement of four alleles; two for each separate gene, such as plant height and seed colour.

In this lab simulation, you will practise tracing dihybrid traits in a Punnett Square and you will examine ratios in the offspring that help you understand the genotypes of the parents.

You will complete a Gizmo on Mouse Genetics (Two Traits) and all of the activities indicated in the lab. You will be prompted to complete the Module 6: Lesson 4 Assignment as part of the lab.