Classical Genetics

Overview

In this triangle students will develop a model for explaining how alleles move through families and interact with each other (simple dominance, sex linked and co-dominance).

Transition in: In our Gamete Formation (Meiosis) model we figured out that we have two pieces of information for each trait, one from each parent. How do those two genes work together to create the trait?

We know that genes get to offspring in gametes formed by meiosis. We still cannot explain how the two alleles we inherit from our parents interact to determine the characteristics we ultimately express (our phenotype). To begin understanding the role of alleles in phenotypic determination students will first study several family histories, create pedigrees for the each trait, and wonder about their patterns of inheritance. Students will build a model of simple dominance using Mendel’s data. They will practice applying the model to their own data collected “in the field” via the Virtual Genetics Lab (VGL), a free program that allows students to “collect” organisms, make crosses and observe the results. [This software will not run on Chromebooks nor iPads]. An alternative is to do a demo on your computer and project it to the class.

Once confident in our command of Mendel’s model of simple dominance we use it to try to make sense of the original pedigrees and notice, while it works for some, this model cannot explain all of the patterns observed. We come back to the families that cannot be explained by Mendel one at a time to see if we can figure out what might be going on. First, we try to explain the pattern in a family where there are 3 variations of a trait with 2 alleles. We experiment with a similar trait on VGL to help us figure out what is happening. Students conclude that neither allele is dominant - when both are present, both are expressed. We name this "co-dominance" or "incomplete dominance" and extend our model to include this pattern. Next we turn to a family with a trait that turns out to be sex-linked, and after that, one with multiple alleles. If time allows we can use VGL to help illuminate these examples as well. In each case, once students figure out the pattern we extend the model to include it. We emphasize that, in addition to the factors already identified (random assortment, crossing over, fertilization), the variety of ways that alleles interact also contributes significantly to variation within a species.

Transition out: At this point students understand various patterns of inheritance at a surface level, but we want them to explain them at the molecular (protein) level. What is happening with proteins that results in dominant/recessive alleles in one case, and co-dominant alleles in another? We go into an examination of what DNA is and what it does in the next triangle.