Unit Overview
8.5 Genetics

Why are living things different from one another?

Unit Summary

This unit on genetics starts out with students noticing and wondering about photos of two cattle, one of whom has significantly more muscle than the other. The students then observe photos of other animals with similar differences in musculature: dogs, fish, rabbits, and mice. After developing initial models for the possible causes of these differences in musculature, students explore a collection of photos showing a range of visible differences.

In the first lesson set, students use videos, photos, data sets, and readings to investigate what causes an animal to get extra-big muscles. Students figure out how muscles typically develop as a result of environmental factors such as exercise and diet. Then, students work with cattle pedigrees, including data about chromosomes and proteins, to figure out genetic factors that influence the heavily muscled phenotype and explore selective breeding in cattle. In the second lesson set, students use what they’ve learned from explaining cattle musculature to help them explain other trait variations they’ve seen. They investigate plant reproduction, including selective breeding and asexual reproduction (in plants and other organisms) and other examples of traits that are influenced by genetic and environmental factors. Students figure out that environmental and genetic factors together play a role in the differences we see among living things.

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Unit Examples


Additional Unit Information

Building Toward the Following Standards and Practices
Performance Expectations

This 8th grade science unit on genetics builds toward the following NGSS Performance Expectations (PEs):

  • MS-LS1-5*. Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.
  • MS-LS3-1. Develop and use a model to describe why structural changes to genes (mutations) located on chromosomes may affect proteins and may result in harmful, beneficial, or neutral effects to the structure and function of the organism.
  • MS-LS3-2. Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.
  • MS-LS4-5. Gather and synthesize information about technologies that have changed the way humans influence the inheritance of desired traits in organisms.
  • MS-LS1-2*. Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function.
  • MS-LS1-4*. Use argument based on empirical evidence and scientific reasoning to support an explanation for how characteristic animal behaviors and specialized plant structures affect the probability of successful reproduction of animals and plants, respectively.

*PEs marked with an asterisk are partially developed in this unit and shared with other units, as explained in the DCI column. Also, regarding the strike through LS3.B–“and some are neutral to the organism”–in middle school there is a boundary of scale in working with genetic information. Generally, we stick with the scale of working with chromosomes and do not get into the more-detailed model of genetic information at the DNA level. We do not learn about base pairs, the four nucleotides, or the sequencing of those nucleotides because those are reserved for high school. Because of this boundary, it makes building an evidence-based model of neutral mutations on an organism very difficult. This is because the evidence the class needs that a change in the structure of genetic information occurs without seeing an effect at the phenotype level happens at the nucleotide sequence level. For this reason, we limited the effect of mutations to those where students can detect a phenotypic difference.

Disciplinary Core Ideas
  • LS1.B: Genetic factors as well as local conditions affect the growth of the adult plant. Students specifically investigate the combination of local environmental effects and genetic influences on plant growth in Lessons 13, 15, and 17. In Lesson 3, students investigate environmental effects on musculature and the combination of environmental effects with genetic influences on other trait variations in Lessons 15 and 16. Students are focused on determining how genetic factors influence the growth of organisms in Lessons 4, 5, 6, 7, 8, 9, 10, 11, 12, and 14. This DCI element is shared with Unit 7.3 Metabolic Reactions.
  • LS1.B: Organisms reproduce, either sexually or asexually, and transfer their genetic information to their offspring. Transferring genetic information via sexual reproduction is the focus of Lesson 5 specifically (in animals) and Lesson 13 (in plants), where students also encounter asexual reproduction, and they continue to explore how organisms reproduce asexually in Lesson 14.
  • LS3.A: Genes are located in the chromosomes of cells, with each chromosome pair containing two variants of each of many distinct genes. Each distinct gene chiefly controls the production of specific proteins, which in turn affects the traits of the individual. Changes (mutations) to genes can result in changes to proteins, which can affect the structures and functions of the organism and thereby change traits. Students investigate cattle karyotypes in Lesson 5 and then connect chromosomes to genes, alleles, and proteins in Lesson 6, where they also encounter an example of genetic modification that resulted in changes to the myostatin protein, thereby affecting the structure of animals’ musculature. Students continue to explore the gene-to-protein-to-trait story in Lessons 7 and 8, where they hear about the original mutation that led to a new allele, which gave rise to heavy musculature. Additional mutation examples are added in Lessons 13 and 16.
  • LS3.A: Variations of inherited traits between parent and offspring arise from genetic differences that result from the sub-set of chromosomes (and therefore genes) inherited. Students contrast the phenotypes of parents and offspring in Lesson 5 and connect those differences to chromosomes (genes) in Lesson 6. In Lesson 14, students specifically contrast the variation between parent and offspring due to sexual reproduction with the inheritance of identical genetic information due to asexual reproduction. 
  • LS3.B: In addition to variations that arise from sexual reproduction, genetic information can be altered because of mutations. Though rare, mutations may result in changes to the structure and function of proteins. Some changes are beneficial, others are harmful, and some are neutral to the organism. Students are introduced to the natural mutation that leads to extra-big muscles in Lesson 8, and they explore the benefits and drawbacks of this mutation in Lesson 9. In Lesson 16 students encounter other examples of mutations and again consider their effects and how rare they are.
  • LS3.B: In sexually reproducing organisms, each parent contributes half of the genes acquired (at random) by the offspring. Individuals have two of each chromosome and hence two alleles of each gene, one acquired from each parent. These versions may be identical or may differ from each other. In Lesson 5, students discover that sex cells contain single copies of chromosomes, and these combine so offspring have two sets of chromosomes. They connect genes and alleles to this pattern in Lesson 6, and in Lesson 8 students use their understanding of random assortment along with probability calculations, and Punnett squares to determine the chances of possible genotypic outcomes of various parent crosses. They apply that algorithm to other organisms in Lesson 10, Lesson 13, and Lesson 16.
  • LS4.B: In artificial selection, humans have the capacity to influence certain characteristics of organisms by selective breeding. One can choose desired parental traits determined by genes, which are then passed on to offspring. Students explore selective breeding in Lessons 9 (in animals) and 13 (in plants).
  • LS1.B: Plants reproduce in a variety of ways, sometimes depending on animal behavior and specialized features for reproduction. Students investigate plant reproductive structures in Lesson 13, focusing on how certain pollinators interact with specialized flower parts. The PE related to this DCI element is shared with Unit 8.6.
  • LS1.A: Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell. In Lesson 2, students investigate the structure and function of muscle cells, building on knowledge they bring from Unit 6.6, which shares this DCI element. Then in Lesson 5 and beyond when students investigate chromosomes, they are able to build on their previous understanding of the structure and function of the nucleus specifically.
Focal Science & Engineering Practices
  • Obtaining, Evaluating, and Communicating Information: This unit intentionally develops this practice. Students work with a new element (evaluating competing accounts in Lesson 9) and gradually become more independent in their use of all the other elements of this practice. Throughout the unit students obtain and evaluate information from a variety of sources, including articles, audio interviews, videos, charts, graphs, and images. They have formal and informal opportunities to communicate information orally and in writing. To support their use of this practice, in Lesson 3 students co-construct a version of the checklist tool they’ve used in prior units and use it in Lessons 6, 10, 13, 14, and 15 with opportunities to obtain, evaluate, and communicate increasingly complex information. Students are formally assessed on this practice in Lesson 10, and then they reflect on their use of this practice in a self-assessment in Lesson 14.   
  • Using Mathematics and Computational Thinking: This unit intentionally develops the practice of using mathematics and computational thinking. Students calculate the probability of offspring phenotypes from various parental crosses in Lessons 8, 10, 14, and 16. In Lesson 8 students take note that the series of ordered steps they’re using is an algorithm (an element of this practice they have not used previously). In addition, students use digital tools to analyze very large data sets for patterns and trends in Lesson 16. 
  • Developing and Using Models: Modeling is key to the sensemaking in this unit. Although no new elements of this practice are introduced, students use models to make sense of and explain almost every aspect of what they figure out in this unit. Students have frequent opportunities to develop models with a partner, in small groups, or as a class when they are making sense of new science ideas. Students then use models independently to explain those science ideas and relationships on assessments in Lessons 7, 10, and 17.


  • The following practices are also key to the sensemaking in the unit:
    • Asking questions and defining problems
    • Planning and carrying out investigations 
    • Constructing explanations and designing solutions
Focal Crosscutting Concepts
  • Cause and Effect: This crosscutting concept is key to the sensemaking students do in this unit. Students’ more independent use of cause-effect thinking is supported by removing scaffolds and applying these ideas to explain increasingly complex phenomena. They begin by using a cause-and-effect framing tool from [material: DF] to predict outcomes and summarize investigation findings in Lesson 3, and this tool is revisited with less scaffolding in Lessons 4, 6, 12, and 14. In Lesson 6 students consider whether the relationship among alleles, proteins, and phenotype is causal or correlational. The class develops an initial model in Lesson 1 to explain the possible causes of extra-big muscles, and then in Lesson 4 students revise that model to represent multiple causes contributing to the phenotype they see. In Lesson 10 they revise a cause-effect chain tool to include multiple causes, carefully considering the language involved. In Lessons 15, 16, and 17 students develop a model to explain how both genetic and environmental factors contribute to the variation we see in living things. In those final models, as well as in their work calculating the probability of offspring phenotypes from various parental crosses in Lessons 8, 10, and 14, students apply the idea that some cause-effect relationships can only be described using probability. 
  • Structure and Function: This crosscutting concept is key to the sensemaking in this unit. Students explicitly use a structure-function lens to consider several of the ideas they’re developing in this unit. Specific wording in videos and readings as well as guiding questions on handouts and slides and in discussions scaffold students as they explore structure-function relationships in complex biological systems. They investigate how proteins have specific structures to do their jobs (Lessons 2 and 6), and they learn that if there is a change to the structure of a gene it can affect the protein produced (Lesson 7). Students also investigate the specialized structures of plants that affect the probability of successful reproduction (Lesson 13). 


  • The following crosscutting concepts are also key to the sensemaking in the unit:  
    • Patterns
    • Scale, proportion, and quantity
Unit Information
How is the unit structured?

The unit is organized into two lesson sets. 

  • Lesson set 1 consists of Lessons 1-10. The class decides at the end of Lesson 1 to focus on figuring out how animals can get extra-big muscles, expecting that making progress on understanding that phenomenon will help us be able to explain other variations we’ve noticed in other organisms. We first explore possible causes for building muscles that we have in our background knowledge, but after hearing from a farmer that heavily muscled cattle do not exercise nor eat a special diet and see pictures of baby cattle born with extra-big muscles, we investigate how animals can inherit this trait variation. We end this lesson set by applying the model we figured out about cattle muscles to explain selective breeding in goldfish. 
  • Lesson set 2 consists of Lessons 11-17. The focus of this lesson set is to explain variations we see in other living things. We are first curious to see how our model will work with plants, since they seem very different from cattle, and then we go on to investigate other examples of organisms whose trait variation is caused by multiple genetic and environmental factors. In the final lesson, students apply this more-complex model to explain height variation in redwood trees.


Where does this unit fall within the OpenSciEd Scope and Sequence?

This unit is designed to be taught after students have experienced Unit 6.6 and Unit 7.4. As such, work in this unit can leverage ideas about cell structure and function as well as understanding about how our bodies break down food molecules into smaller pieces that can be used for different functions. 

Additionally, this unit uses and builds on ”cause and effect” supports that were established in Unit 8.3, and the checklist for ”obtaining, evaluating, and communicating information” that can support students’ work in this unit was previously used in Unit 7.4 and Unit 6.6.

This unit is designed to be taught prior to Unit 8.6, in which students investigate structures that modern animals do or do not have in common with ancient animals and how certain traits in a population increase some individuals’ probability of surviving and reproducing. In that unit, students will build on ideas they developed in this unit about mutations, sexual reproduction, and inheritance. 

The Scope & Sequence document has additional information about the sequence of the courses.

What additional strategies are available to support equitable science learning in this unit?

OpenSciEd units are designed to promote equitable access to high-quality science learning experiences for all students. Each unit includes strategies which are integrated throughout the OpenSciEd routines and are intended to increase relevance and provide access to science learning for all students. OpenSciEd units support these equity goals through several specific strategies such as (1) integrating Universal Design for Learning (UDL) Principles during the unit design process to reduce potential barriers and provide more-accessible ways in which students can engage in learning experiences; (2) developing and supporting classroom norms that provide a safe learning culture; (3) supporting classroom discourse to promote students in developing, sharing, and revising their ideas; and (4) specific strategies to supporting emerging multilingual students in science classrooms. 

Many of these strategies are discussed in the Teacher Guides in sidebar callout boxes titled “Attending to Equity” and subheadings such as “Supporting Emerging Multilingual Learners” or “Supporting Universal Design for Learning.” Other callout boxes with strategies are found as “Additional Guidance”, “Alternate Activity,” and “Key Ideas” and various discussion callouts. Finally, each unit includes the development of a Word Wall as part of students’ routines to “earning” or “encountering” scientific language. 

For more information about each of these different strategies with example artifacts, please see the OpenSciEd Teacher Handbook.

What mathematics is required to fully access the unit’s learning experiences?

In Lesson 2, students analyze muscle cell data presented in box plots. Prerequisite math concepts that may be helpful include the following:

  • CCSS.MATH.CONTENT.6.SP.B.4: Display numerical data in plots on a number line, including dot plots, histograms, and box plots.

In Lesson 3, students analyze and interpret data from graphs and charts about the effects of dietary protein and exercise on muscle growth. This work draws upon multiple Common Core Mathematics Standards from elementary grades 1-5 within the domain of Measurement and Data, under the category of Represent and Interpret Data. For example:

  • CCSS.MATH.CONTENT.3.MD.B.3: Draw a scaled picture graph and a scaled bar graph to represent a data set with several categories. Solve one- and two-step “how many more” and “how many less” problems using information presented in scaled bar graphs. 


In Lesson 8, students examine several examples of pedigrees to discover patterns in the proportion of genotypes in offspring that result from specific crosses. They also use simple mathematical models to help predict the probability that a known cross will result in a particular genotype. Prerequisite math concepts that may be helpful include the following:

  • CCSS.MATH.CONTENT.7.SP.C.5: Understand that the probability of a chance event is a number between 0 and 1 that expresses the likelihood of the event occurring. Larger numbers indicate greater likelihood. A probability near 0 indicates an unlikely event, a probability around 1/2 indicates an event that is neither unlikely nor likely, and a probability near 1 indicates a likely event.
  • CCSS.MATH.CONTENT.7.SP.C.6: Approximate the probability of a chance event by collecting data on the chance process that produces it and observing its long-run relative frequency, and predict the approximate relative frequency given the probability. 
  • CCSS.MATH.CONTENT.7.SP.C.7: Develop a probability model and use it to find probabilities of events. Compare probabilities from a model to observed frequencies; if the agreement is not good, explain possible sources of the discrepancy.
  • CCSS.MATH.CONTENT.7.RP.A.2: Recognize and represent proportional relationships between quantities.
  • CCSS.MATH.CONTENT.5.NF.B.4: Apply and extend previous understandings of multiplication to multiply a fraction or whole number by a fraction.

In Lesson 16, students co-construct and analyze a histogram from arm-span length data. Prerequisite math concepts that may be helpful include the following:

  • CCSS.MATH.CONTENT.6.SP.B.4: Display numerical data in plots on a number line, including dot plots, histograms, and box plots.
How do I shorten or condense the unit if needed? How can I extend the unit if needed?

The following are example options to shorten or condense parts of the unit without completely eliminating important sensemaking for students:

  • Lesson 2: If you feel your students already have a solid understanding of muscle structures, skip the video and reading at the beginning of Lesson 2 and start at the gallery walk comparing the muscles from organisms with typical muscles to those with extra-big muscles. Your students will miss an opportunity to practice integrating qualitative scientific information in written text with that contained in media and visual displays to clarify claims and findings, If you make this adjustment, be sure to still put “protein” on the Word Wall and discuss examples such as myosin and actin to support students’ understanding of proteins before Lessons 5 and 6, specifically.
  • Lesson 3: If you feel students can do without evidence that dietary protein and exercise both do influence muscle growth, skip readings 1 and 2 and instead begin with readings 3 and 4 where the class splits into two groups to determine the specific role of each of these environmental factors.  
  • Lesson 9: Rather than having all students read all three articles and compare them, do the readings as a jigsaw activity where students each read only one article. Continue with the comparison discussion as written in the Teacher Guide. This adjustment will reduce the time the lesson requires, but it will also limit students’ experience with a new element of SEP 8: “Evaluate data, hypotheses, and/or conclusions in scientific and technical texts in light of competing information or accounts”. You could also choose to skip the step where students look up some of the sources cited in a reading to develop the habit of checking for credibility, again at the expense of the practice with obtaining and evaluating information.   
  • Lesson 12: Replace the classroom lab extraction of genetic material from strawberries in favor of watching the provided video. Students could also view the provided video of the negative control investigation instead of seeing it as an in-person classroom demonstration. 
  • Lesson 13: If students are already secure in the knowledge that flowers are the part of the plant involved in making seeds and that seeds are plant offspring, you can skip ahead to observing seeds in the fruits themselves. Also, skip the flower dissection and have students go straight to using the plant structures diagram to label plant parts as reproductive structures, if you need to save time. Finally, choose only one opportunity for students to explain what they’ve figured out about plant reproduction–peer feedback with a partner or completing the exit ticket–rather than doing both.


To extend or enhance the unit, consider the following:

  • Lesson 14: Students could spend more time researching details about organisms that reproduce asexually and/or students could communicate the information they’ve learned in a different format than the suggested slide or to a different audience than just their peers.
  • Lesson 16: Let your students explore the Arm Span Data Set and online data tool by themselves before discussing it as a class.
  • Lesson 17: Provide time for students to work on the alternate activity described in the Teacher Guide where students obtain and evaluate information about how climate change is affecting redwood trees and then communicate that information to an audience of their choice in a format appropriate for that audience.
  • After Lesson 16 or 17: Students can return to the specific examples of trait variations they brought in during Lesson 1 and apply the model they’ve developed in this unit to explain those. As additional practice with ”obtaining, evaluating, and communicating information”, students could do additional research to find details about those trait variations, such as the function of specific protein(s), the number of genes, and/or the specific environmental factors that influence that trait variation.
Unit Acknowledgements
Unit Development Team
  • Gail Housman, Unit Lead, Northwestern University
  • Tara McGill, Field Test Unit Lead & Reviewer, Northwestern University
  • Holly Hereau, Writer, BSCS Science Learning
  • Natalie Keigher, Writer, Lisle Junior High School
  • Karin Klein, Writer, Independent Contractor, Northwestern University
  • Wayne Wright, Writer, BSCS Science Learning
  • Katie Van Horne, Assessment Specialist, Concolor Research
  • Ravit Golan Duncan, Advisory Team Chair, Rutgers University
  • Brian Donovan, Advisor, BSCS Science Learning
  • Carrie Tzou, Advisor, University of Washington, Bothell
  • Barbara Hug, Advisor, University of Illinois at Urbana-Champaign
  • Andrea Poppiti, Field Test Teacher and Advisor, Virginia Beach City Public Schools
Production Team

BSCS Science Learning

  • Stacey Luce, Copyeditor and Editorial Production Lead
  • Renée DeVaul, Project Coordinator and Copyeditor
  • Valerie Maltese, Marketing Specialist & Project Coordinator
  • Chris Moraine, Multimedia Graphic Designer
  • Kate Chambers, Multimedia Graphic Designer
Additional Acknowledgements

Special thanks to the following people for their significant contributions to this unit:

  • Kyle Kehrli and Wilbur Kehrli, American Blue Cattle Breeders, Winthrop, IA
  • Connie Brooks, MNP Farm, Fair Grove, MO
  • Jud Parker, Liberty Cattle Company, Fairfield, IA 
  • Penny Heidenreich, Stockton, IL
Unit External Evaluation
NextGenScience’s Science Peer Review Panel

An integral component of OpenSciEd’s development process is external validation of alignment to the Next Generation Science Standards by NextGenScience’s Science Peer Review Panel using the EQuIP Rubric for Science. We are proud that this unit has earned the highest score available and has been awarded the NGSS Design Badge. You can find additional information about the EQuIP rubric and the peer review process at the nextgenscience.org website.