Inheritance Patterns (#4 of 6): Sex-linked Traits

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Objective

1. Students will apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population. (HS-LS-3) 2. Students will understand that cells store and use genetic information to guide their functions. An organism’s genotype determines its phenotype. These traits can be dominant or recessive depending on the alleles found on their genes.

Big Idea

Nearly all human traits, even many diseases, are inherited in predictable ways. Using the tools of mathematics and modeling, these inheritance patterns can be properly deduced.

Learner Goals

Note: I recommend that you first check out this resource in order to get the most out of this lesson!

In high school I took several drafting classes and, for a while, I had hoped to become an architect. With respect to planning instruction and teaching, I feel that I can still live out the detailed approach to building something intricate and complex even though the product is a lesson rather than a certain "built environment".

The lesson-planning document that I uploaded to this section is a comprehensive overview of how I approach lesson planning. This template includes the "Big Three" aspects of the NGSS standards: Disciplinary Core Ideas, Crosscutting Concepts, and Science Practices. Of course, there are many other worthy learning goals, skills, instructional strategies, and assessments that can be integrated into a class session. I don't feel compelled to check every box but, rather, use it as a guide to consider various options and tailor the lesson in light of these. Furthermore, for the bigger picture of this unit of study please refer to this document.

With regard to this particular lesson (as part of the series) students will be able to...

1. Explain how are traits passed from one generation to another and why do members of the same family have different traits.

2. Explain why some traits are hidden in one generation and expressed in the next.

3. Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population. (HS-LS-3)

4. Solve monohybrid (one trait) and dihybrid (two trait) cross problems (AKA Punnett Squares).

a)    Complete dominance (click here to link to lesson)

b)    Incomplete dominance (click here to link to lesson)

c)    Co-dominance (e.g. ABO blood types) (click here to link to lesson)

d)    Sex-linked (e.g. colorblindness and hemophilia) (focus of this lesson)

5. Interpret a pedigree and the symbols used to represent males, females, affected and unaffected individuals. (click here to link to lesson)

6. Make and defend a claim for the inheritance pattern(s) found in a given pedigree. (click here to link to lesson)

I hope you get some value from my work! 

Anticipatory Set ("Hook")

10 minutes

Teaching Challenge: How do I support my students in analyzing data in order to address a question of interest?

In this case the question is "How are traits passed from one generation to another and why do members of the same family have different traits?"

According to the American Academy of Opthalmology,  "Researchers from the Multi-Ethnic Pediatric Eye Disease Study Group tested 4,005 California preschool children age 3 to 6 in Los Angeles and Riverside counties for color blindness. They found the following prevalence by ethnicity for boys:

5.6 percent of Caucasian boys
3.1 percent of Asian boys
2.6 percent for Hispanic boys
1.4 percent of African-American boys

The prevalence of color blindness in girls measured 0 percent to 0.5 percent for all ethnicities, confirming findings in prior studies. However, the numbers were so low overall for girls that researchers say they cannot statistically compare rates between females among the four ethnicities studied." (Source)

Sex Linked Traits

I direct students to complete the Ishihara Test  which is a 24 plate colorblindness assessment. I'll ask students to note which, if any, of the plates they could not properly see. In a classroom of 30 students no more than 2-4 total boys should report some form of colorblindness. By taking a quick straw poll, we can determine if the expected ratios hold for the class. There should be no females reporting colorblindness. This could lead to a statistics discussion about sample size and normal distributions. You can decide how in-depth to go with it but it is an interesting place to launch into the lesson.

I will then discuss the basics of sex-linked traits (disparity among men due to X-chromosome dosages, women and sample traits among humans: pattern baldness, hemophilia, and color blindness). I then have students compare and contrast the genotype system for sex-linked traits with the prior three patterns.

They should note that there are two X (sex) chromosomes in females but only one in males. This provides for “insurance” among females who have one recessive allele for a given sex-linked trait.

Brief overview of the mechanics of eyesight...

Instructional Input/Student Activities

35 minutes

Punnett Squares Practice Packet (Sex-Linked Traits pp. 6-7) & (Dihybrid Cross Problems pp. 8-10)

Work #1-4 together with students. Then students will complete #5-10 on their own.
As they do so, I roam the class checking that students are faithfully following the full process and being on hand to help students as needed.

Topic shift back to complete dominance...

Punnett Squares Practice packet (Dihybrid Cross Problems pp. 8-10)

As we turn our attention to pp. 8-10, I remind students of the rules for complete dominance (three genotypes, two phenotypes with AA and Aa both “presenting” as dominant). I then model the process for completing dihybrid cross problems and the “FOIL” method for distribution of all possible allele combinations among the gametes of the parents. This is another challenging layer to the Punnett Squares experience! Work #1 & 2 together with students. Then students will complete #3-4 on their own.

As they do so, I roam the class checking that students are faithfully following the full process and being on hand to help students as needed.
I offer extra credit for solving the testcross problem #1a, b on p. 10.

Closure: What did we learn? Where do we go from here?

10 minutes

Formative assessment is crucial for getting that "dipstick" measurement of student learning. There are a great many ways in which this can be done. One of the more recent and very transformational strategies I have used is called the Peer Instruction Protocol first devised by Dr. Eric Mazur.

Peer Instruction Protocol (PIP) 

As a wrap-up for today, I conduct the review using the PIP assessment (only questions #7 & 8).

Lesson Extension & Follow-Up Activities

Assign only the viewing and study of two pedigree videos as homework. This way, some of the basic instruction can be addressed at home and when class resumes a bit of momentum toward solving pedigrees can be gained. Of course, feel free to substitute any other video that you prefer!
Video Title: Pedigree Basics


Video Title: Sample Pedigree Analysis
*It is relevant to note here that in my teaching context, all students are issued district laptops and therefore have no barrier of access to technology. As a result, my general practice is to assign the podcast or online resource for homework and then we will discuss the concepts and (occasionally quiz student understanding) the next day in class.