Module 6 Mendelian Genetics

Read

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 he 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, and so on.

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 him to propose his second law: The Law of Independent Assortment. Read about this law, and about how to write out a large, 16 square Punnett square to track dihybrid crosses on page 593 of your text.

Watch and Listen

Consider the following video on Classical Genetics and Dihybrid Crosses (about 15 minutes). 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.” Answer the following questions for your own understanding, and save your work in your course folder.

1. What was the letter assignment used here for Widows peak / Straight hair line, and for Normal thumb / hitch hiker’s thumb?
2. What hypothesis (if/then statement) did the students create to test whether the attachment of ear lobes was a trait controlled by a dominant or a recessive allele?
3. Did they find many people with all three traits recessive?
4. In your own words, state or explain Mendel’s second law, the Law of Independent Assortment.
5. Starting with parents that are true breeding for two independent traits, what will be the resulting phenotypic ratio in the F2 generation?

        ___ dom / dom:         ___ dom / rec:           ___ rec / dom:           ___ rec / rec

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, search the Internet for dihybrid tutorials or speak with your teacher.

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

Watch and Listen

 Here is video to review the fundamentals of dihybrid crosses.

Punnett square creation for dihybrids

Module 6: Lesson 4 Assignment—Labs

From your practice in previous lessons, you should be strong in tracking 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. Now you need some practice to know how this looks in the lab, and to know how specific ratios in offspring help us 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.