Mendel's Genetics

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Students will know how Mendel’s laws of heredity can be used to analyze how biological traits are passed on to successive generations.

Big Idea

It’s a matter of chance that siblings look alike.


5 minutes

Warm-Up: What traits would you desire in a pet?  What could you do to ensure that your pet would possess these traits?

Start by displaying a few pictures of different breeds of dogs to engage students before moving into the warm-up discussion. The warm-up question allows students to activate prior knowledge about the inheritance of traits using the real-life scenario of dog breeding.  Instruct students to turn and talk with a seatmate for 1 minute before allowing students to share their thoughts with the class.  Make sure that discussion quickly moves to what can be done to ensure that a pet inherits desired traits. Listen for students to identify that pets are bred with same trait pets to ensure desired traits are inherited.

Point out that breeders use genetic probability to ensure that pets inherit desired traits.  Ask students if they know the term used to describe pets that are bred for specific traits.  Look for students to identify the term, pure-bred.  Ask how many students have a pure-bred pet.  Ask 1-3 students to explain what the term, pure-bred means.  Listen for students to identify that a pure-bred pet means that you can be sure of the traits that the dog has and that you know the genetic background of the pet.  

 These questions and discussion set the stage for the Mendel’s genetics lesson by activating prior knowledge about the inheritance of traits.  It’s a good warm-up question because it allows students to relate a major concept to a real-world topic that many students know something about.

Introduce New Material

20 minutes

Inform students of the learning targets:

  • I can explain how selective breeding is used to produce organisms with desired traits.
  • I can compare inbreeding and hybridization.
  • I can describe Mendel’s Law of Segregation and Mendel’s Law of Independent Assortment.

Introduce the vocabulary associated with the lesson: Gregor Mendel, quantitative, inheritance, heredity, traits, characteristics, pure(bred), hybrid, parental generation, F1 generation, gene, allele, dominant, recessive, probability, homozygous, heterozygous, Law of Segregation, Law of Independent Assortment.  Instruct students to add bolded terms to their vocabulary maps.  Remind them that the bolded terms have Greek or Latin root words, prefixes or suffixes that will be identified during the lesson. 

Display information and provide instruction on Mendel’s genetics. Distribute guided notes or use a note-taking format that has been taught.  Emphasize the significance of Mendel’s work in quantifying the inheritance of traits.

Explicitly teach the term, heredity.  Explain that the root word is “ heir”, meaning the passing from one generation to another.  Use the word in a sentence:  If my last name is Hilton, I would be heir to the Hilton family fortune, which means I would inherit a fortune from my parents.  This type of explicit vocabulary instruction will help reinforce student understanding.

Use visual aids to explain Mendel’s Pea’ experiments. Introduce the term, allele.  Teach students that alleles are expressed as letters. Make an association between the letter “l” in letters and the 3 letter “l’s” in the word alleles to help students remember that versions of alleles (genes) are written as letters.

Memory Cue: think alleles = think letters.

Once it is established that students understand the term, allele, introduce the term genotype. Ask students to look within the term and identify what word is within the term.  Students should note that gene is found within the term, genotype.  Explain that the genotype is expressed as letters, now known as alleles. Emphasize that the genotype is “worn” but not seen on the outside.

Introduce the term, phenotype.  Explain that phenotype refers to the physical appearance.

Contrast the genotype with the term, phenotype.  Tell students to think of physical appearance when they see the term, phenotype.  Make the connection between ph- of the word, phenotype and the ph- of the word, physical appearance.  Give students a memory cue to remember the relationship between genotypes and phenotypes:

Memory Cue: The letters (genotype) determine the look (phenotype).

Briefly introduce the terms heterozygous and homozygous, referencing the root words in both terms to help students see that each word’s meaning is easily identifiable when the root words are known.   It’s not necessary to explicitly teach these two terms at this time as they will be taught explicitly in the Genetic Crosses lesson.  But, it’s often helpful to introduce upcoming terms as a way to chunk the teaching of new terms over a series of lessons. 

Introduce the terms, dominant and recessive. Discuss the misconceptions about dominance:

  • —  A dominant trait is “stronger or better”.
  • —  A dominant trait is the more common or desirable trait.

Use video clips or pictures to provide information about dominant traits like Huntington's disease or polydactyly to help dispel the misconception that dominant traits are “better”. 

As a check for understanding, display an image and ask students to identify which color is dominant and which color is recessive.  More importantly, ask how they know which color is dominant. Look for students to identify that green is dominant because it is displayed in a 3:1 ratio in the F1 generation.  Ideally, students should use academic language in their responses.

Introduce Mendel’s Law of Segregation.  Ask students to think about what context they have seen the word used before. Look for students to recall that segregation refers to the separation of racial groups that occurred in the U.S. prior to the civil rights movement.  Emphasize that segregate means separate.  Explain how the term segregation is a good descriptor of how the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. As a check for understanding, display a Law of segregation image and instruct students to interpret the image, then respond to each of the following prompts in a “turn and talk” with their seatmate:

  • What do the pink and blue colors represent in both parents?
  • Describe each of the 4 offspring and explain the color representation for each offspring.

Finish the discussion on the Law of Segregation with a video animation.

Introduce Mendel’s Law of Independent Assortment. Display a law of independent assortment image and interpret the visual aloud to increase students’ understanding.  Finish the discussion with a video animation



Guided Practice

10 minutes

Quickly summarize each of Mendel's Laws and ask students to consider how we might show understanding of each of the laws using pictures, shapes or symbols.   Using the think aloud strategy, model how to create a picture, set of shapes or symbols to show your understanding for one of Mendel's Laws.

Use two different color markers to show how the law of segregation can be illustrated with two colors.  Explain your reasoning for selecting two colors to represent the two parent traits. As you perform the "think aloud", be sure to use the language of the standards.  Talk about how the alleles separate during meiosis and lead to genetic variation. The example shows how easily students can use two color markers to depict this law.

Then, use a simple Punnett square example to show how it serves to model how alleles segregate in the law of independent assortment.   After modeling each example, ask students to think about Mendel's Laws and use images, shapes or symbols to demonstrate their understanding of either one. 

Independent Practice

20 minutes

The purpose of this activity is to allow students to illustrate the laws  in order to increase their understanding.

Distribute markers and paper to students. Encourage them to use their notes or images from the text or PCs to help them create accurate depictions of Mendel’s laws.  Offering students the opportunity to use their textbooks or PCs serves as a resource for students who need the support.  

For those who utilize these resources, the benefit will be that they will see a correct depiction of the laws, which will help build their understanding.  

If students are able to use their notes or visual images to draw meaning from them and then transfer that understanding to their own depiction of Mendel’s laws, then reinforcement of concepts and learning has occurred, which meets the learning target for the lesson.

Expect to see varied responses to the assignment.  Some students will stick closely to the task and provide work very similar to what they have seen in a text.  Other students will be creative in how they respond to the task.  Expect to see the more artistic students create artistic work that allow them to show their skills, and more importantly, their understanding.  


5 minutes

To ensure that students leave class possessing a solild understanding of the terms, heterozygous and homozygous, display a Lesson Close. Instuct students to complete each blank:

  1. A person with one dominant trait and one recessive trait is ______.
  2. A hybrid is __________.
  3. Homozygous means________.
  4. Plants that always produce offspring with that trait are said to be____________________.

After giving students 1-3 minutes to complete each, as a whole group, allow students to chorally respond to each prompt.  Listen for differing responses in the group.  Stop, as needed to correct errors in thinking.

Students should be able to identify that:

A person with one dominant trait and one recessive trait is heterozygous.

A hybrid is heterozygous.

Homozygous means having two alleles that are the same.

Plants that always produce offspring with that trait are said to be homozygous.