Traits and Reproduction Unit
Inside virtually every cell in every organism on Earth, genes provide instructions for making proteins that govern all the functions of an organism’s body. An organism inherits its genes from its parent or parents, but different combinations of genes can lead to striking variation even among closely related organisms. Understanding the role of genes and the process of inheritance has allowed researchers to explain variation in life on Earth, breed plants and animals with new traits, and develop cures for devastating diseases. In the Traits and Reproduction unit, students take on the role of student genetic researchers, working with the fictional bioengineering firm, Bay Medical Company. Bay Medical Company is attempting to breed spiders with the type of silk that can be used for medical applications (e.g., to create artificial tendons). The student genetic researchers are faced with the challenge of explaining how the silk flexibility traits of closely related spiders can vary, which serves as the anchor phenomenon for the unit. To explain this mystery, students create physical models, read articles, and observe genetics in action, using the Traits and Reproduction Simulation. This powerful and engaging digital tool allows students to observe and breed spiders, making connections between what happens inside cells and how this affects the traits of an organism. Through their research, students learn about the role proteins, genes, and sexual reproduction play in trait variation. They are able to apply what they have learned about spiders to a human context.
This unit helps students connect ideas about genes, proteins, traits, and sexual reproduction to form a deep understanding of the causes of variation. Students consider intriguing examples of variation drawn from both human contexts (e.g., athletic ability, fraternal and identical twins, and genetic diseases such as hemophilia) and nonhuman contexts, particularly spider silk flexibility. This unit, in comparison to a more traditional heredity unit, gives greater attention to protein molecules and the connections between genes, proteins, and traits. Spider silk flexibility was carefully chosen as the central example because it provides an accessible illustration of the structure–function relationship between proteins and traits: Less flexible silk is formed from protein molecules with a greater number of connections. This focus on the role of proteins allows for a deeper, more mechanistic understanding than would an approach that glosses over the role of proteins in making the connection between genes and traits. After establishing an understanding of the relationship between proteins and traits, students move on to learn about how genes influence traits. They learn genes determine which protein molecules are produced for particular traits. Finally, students learn how sexual reproduction results in trait variation. This unit is designed to help students integrate their understanding of how genes affect traits with an understanding of how genes are inherited, topics that are sometimes presented in a disconnected way. In addition to being an excellent context for building this deep understanding, spider silk genetics and the possible uses of spider silk as a biomaterial are actual areas of cutting-edge scientific research. Throughout the unit, students have regular opportunities to apply what they have learned from their investigations of spiders to human contexts. By relating what they have learned about spider genetics to humans, students are challenged to think more deeply about the connections between genes, proteins, traits, and sexual reproduction.
Students begin the Traits and Reproduction unit by learning that Bay Medical Company has been researching a species of spider: the Darwin’s bark spider. This spider makes the strongest spider silk in the world. The silk of a Darwin’s bark spider has many possible medical uses as well. It may, for example, be used to repair human tendons. However, the researchers at Bay Medical Company cannot explain why their efforts to breed Darwin’s bark spiders with medium silk flexibility (the best silk characteristic for constructing artificial tendons) are not working. Researchers at Bay Medical Company present students with a specific spider family to investigate throughout the unit.
In Chapter 1, students focus on the question: Why do traits for silk flexibility vary within this family of Darwin’s bark spiders? A fascinating video about a real spider researcher, Dr. Cheryl Hayashi, introduces students to the spider research context. Then, after reading a compelling text about variation in spiders, making physical models of proteins, and investigating traits in the Traits and Reproduction Simulation, students discover the connection between the protein molecules produced in cells and the traits of an organism. As they investigate this concept, students learn that variation in the structure of protein molecules affects their function and how they interact with other molecules, which leads to variation in observable traits.
In Chapter 2, students explore why the Darwin’s bark spiders’ offspring make different proteins for silk flexibility. By reading about hemophilia research and engaging with a physical model of genes and proteins, students learn that genes are instructions for building protein molecules. Using the Sim, students discover that organisms have two copies of each gene. These copies can be the same version or different versions of the gene. This means that the cell could produce one type of protein or two different types of proteins, depending on the instructions from the genes. Students synthesize the ideas from this chapter by creating a visual model that explains what they have learned about how different gene combinations result in the production of different proteins inside cells, which leads to trait variation.
In Chapter 3, students investigate why the Darwin’s bark spider offspring have different gene combinations even though they have the same parents. As they investigate how genes are inherited, students consider the case of identical twins and how they compare to fraternal twins. Students learn that sexual reproduction leads to variation in the genes of offspring because an organism receives one copy of each gene from each of its parents. Students use the Sim to conduct experiments related to spider breeding. By the end of this chapter, students are able to write a complete explanation for why the Darwin’s bark spider offspring in the lab had different traits for silk flexibility compared to each other and to their parents.
In Chapter 4, students apply their understanding of genes, proteins, and traits to a new anchor phenomenon about a difference in traits among members of a human family. They focus on a family of runners, investigating why one person in the family is an elite distance runner when no one else in her family has that trait. Students consider claims about whether or not the distance runner’s trait could be due to gene inheritance, her environment, or a mutation in the runner’s genes. After analyzing a range of evidence, students present their ideas to the whole class in a Science Seminar before constructing their final written argument.
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NOTE: Homework answers are reviewed IN class, as a WHOLE class