"X" Marks The Spot

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SWBAT demonstrate their understanding of genetics by applying the concept of "X" chromosome inactivation to explain the variation of traits that are observed in a population.

Big Idea

An X-citing lesson examining "X" chromosome inactivation in female offspring. Students will create a unique calico cat that displays mosiacism to model this phenomenon.

Hook - "X" Marks The Spot

5 minutes

Students will read X Marks the Spot Handout as an introduction to the phenomenon of "x" inactivation.  This activity is to hook the students' interest and will be explained in more detail in the next section of the lesson.

Direct Instruction - "X" Inactivation Explained

6 minutes

Students will watch the video clip which will serve as the direct instruction for today's lesson.  Students will record notes during the 6 minute video and the class will conduct a quick whole-group review to discuss the most interesting highlights about the concept of "x"inactivation.  Students will be encouraged to ask questions and seek clarification during our class discussion.

X-inactivation occurs because females are XX and have two copies of the X chromosomes.  One of the X chromosomes must be turned off so the genes on the other X chromosome can be expressed. 


Independent Practice -"X" Inactivation Kitties

25 minutes


Students will incorporate their understanding of the process of x-inactivation from the previous section by complete the following hands-on X-Inactivation Activity.

Students will use the flip of a coin to represent the turning off of one of the female x-chromosomes which occurs in the human body at the 32-cell stage.  

Modeling Genetics:

  • If the students flip a heads up on their coin - the XB (dominant trait) will be shut off leaving the Xb  allele to be expressed at a patch of orange fur.
  • If the students flip a tails up on their coin - the Xb(recessive trait) will be shut off leaving the Xallele to be expressed at a patch of brown fur.

Sample of Student Work: Creating The "X" Cats #1 - All student work samples will represent a different color combination pattern to demonstrate the x-inactivation randomness during the embryonic stage of development.

Sample of Student Work:Creating the "X" Cats #2 - This activity enabled students to experience the independent nature of each trial because many students flipped multiple heads in a row and a multiple tails in a row.  When students were asked the probability of flipping a heads next . . . they contemplated and responded 50%.  Students began to understand that no matter how many times you flip the coin, they had a 50/50 chance to get heads.

Sample of Student Work:Creating the "X" Cats #3 - Students enjoyed creating a model of their cats that represented the x-inactivation to provide a visual of one X chromosome being turned off while the other was being expressed.


Guided Practice - "X"ploring Genetics with BINGO

10 minutes

As a kinesthetic review for students to work with traits associated with human genetics, the students will participate in a class competition of Human Genetics Bingo.  

Activity Preparation:

    1. Students will fold their paper into 1/3's and then into 1/3's again so then when it is unfolded the paper will contain 9 boxes.
    2. Students will brainstorm physical traits that they can observe in their classmates.  Some student-generated examples may include:
  • Freckles - No freckles
  • Hitchhiker's thumb - No hitchhiker's thumb
  • Widow's peak - No widow's peak
  • Dimples - No dimples
  • Cleft chin - No cleft chin
  • Attached earlobes - Unattached (dangling) earlobes
  • PTC paper taster - Non-PTC paper taster
  • Brown hair - Blonde hair - Black hair - Red hair
  • Brown eyes - Blue eyes - Green eyes - Hazel eyes


3. Students will select 9 of the traits and write one trait in each of the 9 boxes of their folded paper.  

4. Once students have prepared their Genetics BINGO card, the fun begins!  Students will ask their peers to sign their BINGO card if that peer expresses the phenotype that is named on the selected square.  Students are not allowed to just give their paper to their peers and expect them to sign it.  Students must work on their communication skills as well as their understanding of Biology.

5. Make the game as exciting as you want . . . think of prizes!!!

Student Work: Genetics BINGO Cards - Students each made their own unique BINGO cards based on the list of single gene traits that were brainstormed in the list above.  Students worked to fill out their cards until a student in the class achieved "blackout" and filled their card.  

Close - "X" Linked Traits

10 minutes

Students will complete the Sex-Linked Traits Practice Problems as homework to reinforce their understanding of this genetics concept.