Module 6 Mendelian Genetics

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Larissa's tomcat Smokey was her pride and joy; dusty grey with chocolate brown spots, he was unique to the neighbourhood. One day a new family moved in next door. To Larissa's surprise they had a female cat, also grey with chocolate spots. When the neighbours discovered Larissa's cat, they immediately requested its siring services. They wanted to breed her cat to their cat. Ten weeks later, the female gave birth to six kittens. Hopes of the unique grey and chocolate combination were dashed as only one kitten, a male, showed the parental colors. Four kittens were solid grey, and a fifth was completely white.

Why are there three phenotypes in this case instead of two as was the case in all monohybrid crosses studied so far? Here is a curious genetic phenomenon inconsistent with Mendel's observations. What is the explanation?

Read the section of your text entitled “Incomplete Dominance and Co-dominance” on pages 594 and 595 of your text. When finished your reading, answer the following questions for your own understanding and save the results in your course folder.

1. Four-o'clock flowers occur in two pure-breeding varieties, one with white flowers and the other with red flowers. If these are crossed, all of the F1’s have pink flowers. Give the genotype of each parent, and the genotypes of the F1's of such a cross. Use R1 and R2 to represent the alleles for red and white respectively.

2. The phenotype of the F1's is pink flowers that appear to be a blend of the two originals. It is an intermediate type.

3. How does the phenotypic ratio change for a monohybrid cross with no dominance, as compared to a Mendelian cross where there is dominance?
4. In the case of co-dominance and incomplete dominance, pure-breeding individuals are easily identified. Explain this statement.
5. Use a Punnett square to find the probability of producing a blue roan horse if you cross a roan with a black stallion.
6. Can you generally explain the results of breeding Larissa's tomcat with the neighbour's spotted cat?

Watch and Listen

Consider the following video on Alternate Patterns of Inheritance: the Potential for Diversity. (about 15 minutes). Begin the video and continue watching until the section “Bio Simulation: Inheritance Case”. Skip a few sections and then watch the “Incomplete Dominance” section. Answer the following questions for your own understanding and save the results in your course folder.

1. What are multiple alleles?
2. How many alleles can one organism have for a gene with multiple alleles?
3. Define or explain co-dominance?
4. What is incomplete dominance?
5. How do the phenotypic and genotypic ratios compare for an incomplete dominant trait?

Module 6: Lesson 3 Assignment—Labs

Breeding chickens has never been easier! You will complete a Gizmo on Chicken Genetics and all of the activities indicated in the lab.