Phenomenon
- Family histories: Students observe a number of different patterns of trait inheritance in various families.
Phenomenon Specific Cases:
- 2 variations of a trait
- 3 variations of a trait
- More than 3 variations of a trait
- Offspring with traits not shown by either parent.
- Traits that show up in every generation of a family.
Question
BIG QUESTION How are traits determined and passed from parents to offspring?
Specific Question for this model:
- How can we explain these different patterns of inheritance?
Model Ideas
1. Traits are characteristics of organisms. Traits can by physical or behavioral.
2. Sexually reproducing organisms have two alleles that determine each trait (or characteristic), one from each parent.
- A parent passes only one of his/her two genes for a trait to each offspring.
- Random chance determines which of the two alleles is passed to each offspring.
3. A gene for a trait can occur in different forms called alleles.
4. When there are two variations for a trait (=two phenotypes) in a population, there are two alleles (1 and 2) and three possible allele combinations (genotypes): (1,1), (2,2) and (1,2):
- If (1,1) and (1,2) have one phenotype and (2,2) has the other, then 1 is the dominant allele. It always shows when present. 2 is the recessive allele and will only show if no dominant allele is present
Codominance Extension:
. When there are three variations of a trait and two alleles, each genotype [(1,1), (1,2), (2,2)] has a different phenotype. BOTH 1 and 2 are expressed so neither is dominant (co-dominance).
Sex-linkage Extension:
. Males receive only one allele for traits on the unmatched part of the X chromosome so that allele alone determines their phenotype, even if it is recessive. Such traits are said to be sex-linked.
Multiple Alleles Extension:
There can be more than 2 alleles for a trait aka multiple alleles. This can result in more than three phenotypes. The alleles can be dominant/recessive or codominant.
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.
Advanced Planning
For the Virtual Genetics Lab software:
Secure computer lab time or laptops (does not work on Chromebooks or iPads). Install software and practice with problems.
1.
Overview: We observe the variations of several human traits, realize that trait expression is not so simple as receiving ½ from each parent so we then ask: How do the instructions we get from our parents work together to make us to look the way we do?
What we figured out... We observed variations of several human traits. We know that we receive ½ of our information from our mother and ½ from our father but we are still wondering, how does the DNA from 2 parents work together to make us to look the way we do?
2.
Overview: We asked: How does the DNA from 2 parents work together to determine an offspring’s traits?
What we figured out... We have several initial ideas about how the DNA from 2 parents come together and make us look the way we do.
3.
Overview: Now we have an initial set of ideas to explain how traits are expressed, but before we explore those ideas we look at more phenomena from 5 family pedigrees.
What we figured out... We have made and analyzed pedigrees and have seen 5 different patterns of inheritance in 5 different families.
4.
Overview: Next, we look Mendel’s data to see if it will help us explain the patterns we saw in the 5 families.
What we figured out... We have added to our model: When there are 2 different versions for a trait and when one version is dominant over the other, the dominant version of the trait is expressed. We also figured out Mendel’s 3:1 ratio.
5.
Overview: We now use our model to explain how the trait of albinism is inherited in the Kendrick family. Once we have a better understanding of how our model works to explain the phenomenon, we introduce science terminology.
What we figured out... We have successfully used our model to explain how the grandma and the grandson are the only family members with albinism. We have also added science terminology to our model statements.
6.
Overview: In this learning segment we read about Mendel and his laws from a Biology Text or other sourced reading on Mendel. We use a reading guide to help us look for connections between Mendel’s ideas and our own.
What we figured out... We figured out that our model ideas are very similar to Mendel’s ideas.
7.
Overview: We now use our model to make sense of simple dominance in the fictitious Dragonbug. With the help of a VGL simulation and a set of MBER problem sets we breed several generations of Dragonbugs to help us determine which trait variation is dominant. We keep track of our observations, findings and explanations in our field notebook.
What we figured out... We can use our model and data from several crosses of Dragonbugs to determine which variation of a trait is dominant.
8.
Overview: We return to our pedigrees to see if we can use our model to explain the pattern of inheritance for each family. For the pattern that we can explain we assign alleles and then genotypes to make sense of who has the trait variation and why. For those patterns that we can’t explain, we think about how we could extend our model.
What we figured out... We figured out that we can only explain what is happening with the Summers & Reed families. Our simple dominance model does not explain what is happening with the other 3 families.
9.
Overview: We take the role of a genetic counselor and write a formal letter to the Summers family explaining why their daughter has PKU, but neither parent not her brother is affected.
What we figured out... Students have written a letter to the Summers Family which entails a complete explanation of how Jane has PKU when neither her parents nor her brother has the disease. We have used our model statements to support our claims. We may have also practiced how to write a formal letter.
10.
Overview: We return to the Marcus, McCann, and Medeiros family pedigrees. We work with data form the VGL simulator and MBER Problem set II, to figure out similar patterns in the Dragonbugs and then apply those ideas to the family patterns. We extend our model beyond simple dominance.
What we figured out... We figured out that simple dominance is not the only explanation for patterns of inheritance. The ideas of co-dominance, multiple alleles, and sex-linked traits can be used to explain other patterns of inheritance.
11.
Overview: Finally, we write a letter to Gregor Mendel explaining how we have extended the model of simple dominance to help explain other inheritance patterns.
What we figured out... Students have written a letter to Mendel to explain how we extended his model to explain different patterns of inheritance beyond simple dominance.
Activity
OPTIONAL TOPIC: PROBABILITY (click for materials) Probability is a tool of geneticists. Throughout Model 14 we use laws and language of probability to discuss and predict genetic outcomes. Making sure students have a basic understanding of probability before introducing genetics concepts is helpful, especially for students who are less mathematically inclined. The activity provided is a quick lab using coin toss as the "random chance event". It is structured so students derive the basic laws of probability themselves by analyzing the data from the lab. The lab itself takes 55 minutes. It will take 30 minutes the following day to discuss and have groups "finalize" the laws of probability. It makes more sense to do this activity before developing the Mendel model, either right before the family histories or immediately after.