Module 6 Mendelian Genetics

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Crash Course - Heredity

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Recall that Mendel worked with seven contrasting traits. Each trait was controlled by a single gene, and each gene had two alternate forms known as alleles. These alleles related to each other as either dominant or recessive. Dominant alleles are always expressed, and show up in the phenotype of either homozygous dominant or heterozygous individuals. Recessive phenotypes can only be displayed when there are only copies of recessive alleles present, thus only in homozygous recessive individuals.

In working with genetic problems, it is very important to understand how alleles move, and how they are expressed. Mendel’s first law: The Law of Segregation, states that each allele pair in the parents is separated in the creation of gametes. This separation is random. Further, these gametes unite with other gametes in an equally random way. This ensures that all possible combinations of gametes will show up in the offspring. To ensure that this is done, geneticists use a Punnett square. Read about how to represent alleles properly, and then how to create and use Punnett squares in your text on pages 589 and 590.

Watch and Listen

Let’s continue watching the following video on Classical Genetics and Monohybrid Crosses. (10 minutes). Begin the video at the section “Bio Challenge: Round vs. Wrinkled Peas," and watch until the end. Answer the following questions for your own understanding, and save your answers in your course folder.

1. Explain Mendel’s first law in your own words.
2. What does a Punnett square tell you in a cross?
3. Given two heterozygous parents for Pea type (Rr), what are the phenotypic and genotypic ratios expected in their offspring?
4. What does a test cross help determine?

Try This

Examine the sample on how to build a Punnett square on the bottom of page 590 of your textbook. Notice how they write the gametes of one parent along the top of the Punnett square, and the gametes of the other parent down the side. This way, the square can be filled in by carrying the appropriate letter across or down as shown.

Using this as a guide, answer practice problem #1 on the bottom of page 591. Create a Punnett square to illustrate the Parental cross, and another one to illustrate the F1 cross and the resulting F2 generation. Save your answers to your course folder.

Watch and Listen

The following video discusses Mendel's law of segregation. 

1. Did any of Mendel’s traits blend in the offspring?
2. What is the F2 ratio of dominant to recessive phenotypes in all of Mendel’s traits?
3. What are the phenotypic ratios for a test cross if the test plant is heterozygous? If it is homozygous?

Module 6: Lesson 2 Assignment—Lab

Crossing pure breeding plants or animals with contrasting traits or phenotypes can help demonstrate how dominant and recessive genes work.

You will complete a Gizmo on Mouse Genetics (One Trait) and all of the activities indicated in the lab. You will be prompted to complete the Module 6: Lesson 2 Assignment in the lab.