Sex-Linked Chromosomal Disorders


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Sex-Linked Chromosomal Disorders

by April Jones

Rabun County High School, Tiger, Georgia

Primary Learning Outcomes

Students will learn the names, characteristics, and causes of several different sex linked disorders, including muscular dystrophy. Students will create, describe and predict genotypes according to genetic pedigrees.

Additional Learning Outcomes

Students will be introduced to the ideas of genetic engineering and genetic counseling.


  1. Paper

  2. Colored pencils

  3. Display board

  4. Printer

  5. Glue

Technology Connection

Students will need access to the Internet to complete research on a sex-linked chromosomal disorder.

Total Duration

2 hours, 50 minutes


Step 1 Duration: Varies

Teacher Preparation

Review the lecture notes in Step 2 to become familiar with the concepts and assure that it is at the right level for the students. Students need prior knowledge of genetics terminology like dominant, recessive, carrier, sex-linked, as well as familiarity with pedigree symbols. A complete list is included in the “Genetics Terminology Word List”. If they are not familiar with genetics terms, please use the “Definitions for Sex-linked Disorder Vocabulary” to help acquaint them. Additionally, a key to pedigree symbols is included in the “Pedigree Symbols” document for the students to review if needed. Both the vocabulary sheet and the pedigree symbols document can be passed out before the lecture. Refer to the Web resources below for additional information about genetics terminology and pedigree symbols.

Web Resources

Title: Basic Principles of Genetics
Description: This is a tutorial Web site on the basics of genetics. This site includes vocabulary with definitions as well as links to other related information.

Title: Pedigree symbols and Genetic Terms
Description: This Web site contains some additional pedigree symbols and genetic terms with definitions. It includes symbols for twins and still births.
Title: Pedigree symbols, rules, guidelines and examples
Description: This Web site contains pedigree symbols, rules, guidelines and examples. In addition, this site provides directions on how to create a family pedigree.

Supplemental Documents

Title: Genetics Terminology Word List

File Name: Sex linked disorders word list.doc
Description: This document contains a list of genetic terminology that students should be familiar with before continuing this lesson.
Title: Definitions for Sex-linked Disorder Vocabulary

File Name: Sex-linked word definitions.doc
Description: This documents provides brief and easy to understand definitions to the vocabulary for sex-linked disorders.
Title: Pedigree Symbols

File Name: Pedigree Symbols.doc
Description: This Word document contains some commonly used pedigree symbols. Additional symbols can be found in the Web resources.

Step 2  Duration: 55 minutes

Notes on Sex-Linked Genes and Inheritance

In this step, students will learn about sex-linked genes, sex chromosomes and inheritance using the “Lecture Notes for Sex-Linked Disorders” provided as a supplemental document. This lecture will begin with a general discussion about genes, chromosomes, and inheritance and then will move on to sex-linked genes, followed by sex chromosomes. Please go through the lecture notes carefully and conclude by answering any questions about the material covered. The Web resources provide additional information on genetic counseling.

Web Resources

Title: Genetic counseling
Description: This National Health Museum Web site contains information on genetic counseling.
Title: Frequently Asked Questions about Genetic Counselors and the National Society of Genetic Counselors
Description: This National Society of Genetic Counselors Web site contains frequently asked questions and answers about genetic counseling.

Supplemental Document

Title: Lecture Notes for Sex-Linked Disorders

File Name: Lecture Notes.doc
Description: These lecture notes discuss the genetics of sex-linked genes and sex chromosomes.
Step 3  Duration: 15 minutes

Pedigree Practice

Using what the students have just learned from the lecture notes, they will answer questions in the “Pedigree Practice Sheet” included as a supplemental document. The students should work in groups or individually, but should be brought back together for discussion of the pedigree charts when they have completed them. Use the “Pedigree Practice Answer Key” to check for accuracy and to facilitate this discussion.

Supplemental Documents

Title: Pedigree Practice Sheet

File Name: Pedigree Practice Sheet.doc
Description: Word problems that allow the students to create three different pedigrees and answer a question about each of them.
Title: Pedigree Practice Answer Key

File Name: Pedigree Practice Answer Key.doc
Description: Answer key with pedigrees drawn out and answers to the questions posed on the pedigree practice sheet.

Step 4  Duration: 20 minutes

Discussion and Summarization

After the students have completed their pedigrees, have them review additional concepts through the “What Disorder Am I?” game. In this game the students will divide into teams and quiz each other on important concepts and disorders discussed in the lecture notes. The directions are included in the “What Disorder Am I?” document and the cards for the game are included in the “Cards for ‘What Disorder Am I?’ Game” document. After the students have finished the game, bring the students back together as a group and ask them to describe some types and characteristics of muscular dystrophy. This discussion will lead them into the research they will be conducting on muscular dystrophy in the following step.

Supplemental Documents

Title: What Disorder Am I?

File Name: Lecture Review Game - Directions.doc

Description: This document includes directions for a game to review the information presented in the “Lecture Notes for Sex-Linked Disorders” in Step 2. In addition, the game serves as a lead in to the muscular dystrophy research students will conduct in Step 5.

Title: “Cards for ‘What Disorder Am I?’ Game”

File Name: Cards for Lecture Notes Game.doc
Description: This document contains game cards for the “What Disorder Am I?” game.
Step 5  Duration: 1 hour, 20 minutes

Research and Presentation

Group the students into groups of two. Assign the group a research topic, one of the types of muscular dystrophy from the “Lecture Notes for Sex-Linked Disorders” in Step 2. Ask the students to break up the questions listed in the “Muscular Dystrophy Research Guidelines” included as a supplemental document in this step. An example of answers to these questions is provided in the “Research Answer Key”. Once all of the questions have been researched and answered, create a story board. The storyboards need to be on either a tri-fold board or a piece of poster board. See the “Display Board Rubric” for the information the storyboard should contain. After all students have finished their boards, have the students present their information to the class.

Web Resources

Title: Muscular Dystrophy Association Web site
Description: This Muscular Dystrophy Association (MDA) Web site contains links to the different types/forms of the muscular dystrophy.
Title: Becker Muscular Dystrophy
Description: This MDA Web site provides information on Becker muscular dystrophy. In addition, this site contains links that students should be encouraged to explore.
Title: Ducheene Muscular Dystroyphy Web Site

Description: This MDA Web site provides information on Duchenne muscular dystrophy. In addition, this site contains links that students should be encouraged to explore.

Title: Emery-Dreifuss Muscular Dystrophy Web Site
Description: This MDA Web site provides information on Emery-Dreifuss muscular dystrophy. In addition, this site contains links that students should be encouraged to explore.
Title: Muscular Dystrophy Association Diseases Page
Description: This Web site is the start of MDA’s diseases page. It contains links to many of the types of muscular dystrophy with information that could be included on the poster boards.
Title: Facts About Duchenne and Becker Muscular Dystrophies
Description: This MDA Web site contains information on Duchenne and Becker muscular dystrophies, two common types.
Title: National Instutute of Neurolocial Disorder and Stroke’s Muscular Dystrophy Information Page
Description: This National Institutes of Health Web site contains basic information on muscular dystrophy. Also, it includes information on several of the different types of muscular dystrophy.

Supplemental Documents

Title: Muscular Dystrophy Research Guidelines

File Name: Muscular Dystrophy Research Guidelines.doc
Description: These research guidelines provide a set of questions for students to answer while doing their research on their type of muscular dystrophy.
Title: Research Answer Key

File Name: Muscular Dystrophy Research Guidelines Answer Key.doc

Description: This document includes possible answers to the “Muscular Dystrophy Research Guidelines” questions for Duchenne muscular dystrophy.

Title: Display Board Rubric

File Name: SexLinked ChromDisorder- Rubric.doc
Description: This rubric provides guidelines for scoring storyboard project. It can be used for student self-assessment as well as teacher assessment.
Students will be assessed on their creation of pedigrees in Step 3 using the “Pedigree Practice Answer Key” and on their generation of a sex-linked disorder display board using the “Display Board Rubric” in Step 5.


Further Internet research on other types of sex-linked disorders with interested students could be facilitated. Additionally, a guest speaker from a local support group or a special education teacher could speak to the class about disability and health.

Web Resources

Title: Genetic Pedigrees
Description: This is a Web site that students can use to generate family pedigree. It provides a great for extra credit.

Education Standards

National Science Education Standards


As a result of their activities in grades 9-12, all students should develop understanding of

  • The cell

  • Molecular basis of heredity

  • Biological evolution

  • Interdependence of organisms

  • Matter, energy, and organization in living systems

  • Behavior of organisms


As a result of activities in grades 9-12, all students should develop understanding of

  • Personal and community health

  • Population growth

  • Natural resources

  • Environmental quality

  • Natural and human-induced hazards

  • Science and technology in local, national, and global challenges

Georgia State Science Standards

Grade: 9-12, Science, Applied Biology/Chemistry II 8

Topic: Continuity of Life

Standard: Explains what animal breeders need to know about genetic inheritance in animals to produce more economically valuable breeds.

Grade: 9-12, Science, Biology 10

Topic: Genetics (Mendelian Genetics)

Standard: Explains and uses the basic Mendelian genetic principles.

Sex-Linked Disorders Word List
Sex-Linked Chromosomal Disorders

April Jones, CDC’s 2004 Science Ambassador Program

  1. Sex linked

  2. Chromosomes

  3. Autosome/autosomal

  4. Pedigree

  5. Recessive

  6. Dominant

  7. Gene

  8. X chromosome

  9. Y chromosome

  10. Sex chromosomes

  11. Genotype

  12. Phenotype

  13. Heredity

  14. Alleles

  15. Traits
  16. Centromere

  17. Homozygous

  18. Heterozygous

  19. Genetic disorder

  20. Mutation

Word list came in part from:

Biology: The Dynamics of Life, McGraw-Hill Company, Inc., 2002, Columbus, OH.

Sex-Linked Disorder Definitions
Sex-Linked Chromosomal Disorders

April Jones, CDC’s 2004 Science Ambassador Program

  1. Sex linked traits -- traits that are controlled by genes located on the sex chromosomes.

  2. Chromosomes – cell structure that carries the genetic material that is copied and passed from one generation to the next.

  3. Autosome/autosomal – pairs of matching homologous chromosomes in somatic cells.

  4. Pedigree – graphic representation of genetic inheritance used to map genetic traits.

  5. Recessive – trait of an organism that can be masked by the dominant form of a trait.

  6. Dominant – observed trait of an organism that masks the recessive form of a trait.

  7. Gene – segment of DNA that controls the protein production and the cell cycle.

  8. X chromosome – one of the sex chromosomes

  9. Y chromosome – one of the sex chromosomes, responsible for determining the sex of a gamete.

  10. Sex chromosomes – in humans, the 23rd pair of chromosomes; determine the sex of an individual and carry sex-linked characteristics. Containing the X and Y chromosomes.

  11. Genotype -- combination of genes in an organism.

  12. Phenotype – outward appearance of an organism, regardless of it genes.

  13. Heredity – passing on of characteristics from parents to offspring.
  14. Alleles – alternative forms of a gene for each variation of a trait of an organism.

  15. Traits – characteristic that is inherited; can be either dominant or recessive.

  16. Centromere – cell structure that joins two sister chromatids of a chromosome.

  17. Homozygous – paired chromosomes with genes for the same traits arranged in the same order.

  18. Heterozygous – two different alleles for a trait

  19. Genetic disorder – disorder related to the genes or chromosomes.

  20. Mutation – any change or random error in a DNA sequence.


Biology: The Dynamics of Life, McGraw-Hill Company, Inc., 2002, Columbus, OH.

Pedigree Symbols
Sex-Linked Chromosomal Disorders

April Jones, CDC’s 2004 Science Ambassador Program



Unaffected male

Unaffected female

Affected male

Affected female

Deceased male

Deceased female

Possibly affected male or unknown

Possibly affected female or unknown

Carrier male of autosomal recessive disorder

Carrier female of autosomal recessive disorder

Carrier female with an X-linked disorder

Sometimes these symbols will be used in families with 2 different disorders – each half will tell whether the person is affected for that disease.

Connected Symbols

Twins Married couple

Divorced Couple Siblings

Coriell Institute for Medical Research. Pedigree Symbol Definitions [online]. 1998. [cited 2004 July 16]. Available from URL:

Lecture Notes about Sex-Linked Genes
Sex-Linked Chromosomal Disorders

April Jones, CDC’s 2004 Science Ambassador Program

General Genetic Information

All chromosomes (1-22) except the sex chromosomes

Sex chromosome

The Y and X chromosomes

Sex determination

  • autosomes and sex chromosomes segregate during meiosis (review meiosis if necessary)
  • sex of offspring is determined by the combination of sex chromosomes in the fertilized egg. The mother can only contribute an X chromosome because her genotype is XX, however the father can contribute either an X or a Y, because his genotype is XY. Therefore, the genotype of the off-spring is either male or female depending on whether the gamete receives an X or Y from the father.

    • Most organisms: XX=female XY=male

- The sex of some animals is determined in different ways.

    • Grasshoppers XX=girl XO=male

    • Ants and bees have NO sex chromosomes—sex determined by total chromosome number.

Males-unfertilized egg, haploid Females-fertilized egg, diploid

    • Fish and reptiles-sex determined by factors in the environment—like the temperature

Sex-Linked Genes

Sex-linked gene

A gene that is located on the X or Y chromosomes

Sex-linked gene examples:

  • White-eyes in fruit flies: If a fruit fly is white-eyed, then it must be male because the gene that makes the color of the eyes white is located on the Y chromosome. White eyes in fruit flies is a Y-linked trait..

  • Muscular dystrophy: Characterized by progressive weakness and degeneration of the skeletal and voluntary muscles with control movement

    1. Affects people of all ages

    2. Different forms have different onset ages, ranges from infancy to middle age or later

    3. No specific treatment for any of the forms

    4. Life expectancy of individuals varies by form

    5. Examples of types of muscular dystrophy include:

      1. Duchenne Muscular Dystrophy, with an onset of 2 to 4 years, (X-linked)

      2. Becker Muscular Dystrophy, with an onset in the teen and young adult years, (X-linked)

      3. Emery-Dreifuss Muscular Dystrophy, with an onset of childhood or adulthood, (X-linked or autosomal)
      4. Note: there are other forms of muscular dystrophy that are autosomal instead of X-linked

  • Hairy ears in humans: The gene for hairy ears is found only on the Y chromosome (Y-linked). Therefore, males are the only ones to have this sex-linked gene.

Below is an example of a pedigree for a Y-linked gene.

  • Color-blindness in humans: The gene for color blindness is located on the X chromosomes (X-linked) and is recessive. This means that color blindness affects more males than females because males only get one X chromosome while females get two X chromosomes. If a female has the gene for color blindness on one of her X chromosomes, it is likely that she will have the dominant gene on her other X chromosome and therefore will not express the trait. On the other, if a male has the gene for color blindness he does not have a second gene that could be dominant, so he will express the trait.

  • Hemophilia in humans: Like color-blindness, the gene for hemophilia is recessive and located on the X chromosome (X-linked). This means that males are more likely to be affected than females as explained above.

Historically, hemophilia has had an interesting effect on Queen Victoria and her family. She possessed the X-linked gene for hemophilia and passed this gene on to her children. This gene was passed down the royal family line, eventually affecting the youngest son of Tsar Nicolas the II, who received the gene from his mother, the granddaughter of Queen Victoria.

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