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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.
Materials
- Paper
- Colored pencils
- Display board
- Printer
- 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
Procedures
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 URL: http://anthro.palomar.edu/mendel/Default.htm 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 URL: http://www.people.virginia.edu/~rjh9u/pedsymb.html
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 URL: http://wwwrhjh.lkwash.wednet.edu/homehelp/familytree/ 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 URL: http://www.accessexcellence.org/AE/AEC/CC/counseling_background.html 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 URL: http://www.nsgc.org/consumer/faq_consumers.asp 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
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
- 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 URL: http://www.mdausa.org/ Description: This Muscular Dystrophy Association (MDA) Web site contains links to the different types/forms of the muscular dystrophy.
Title: Becker Muscular Dystrophy URL: http://www.mdausa.org/disease/bmd.cfm 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 URL: http://www.mdausa.org/disease/dmd.cfm 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 URL: http://www.mdausa.org/disease/edmd.cfm 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 URL: http://www.mdausa.org/disease/index.html 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 URL: http://www.mdausa.org/publications/fa-dmdbmd-what.html 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 URL: http://www.ninds.nih.gov/disorders/md/md.htm 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.
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.
Modifications
Extension 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 URL: http://www.msu.edu/~langley6/chs/Bio/Genetics/Peddirect.htm 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 LIFE SCIENCE, CONTENT STANDARD C: 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
Sex-Linked Disorders Word List
Sex-Linked Chromosomal Disorders April Jones, CDC’s 2004 Science Ambassador Program
- Sex linked
- Chromosomes
- Autosome/autosomal
- Pedigree
- Recessive
- Dominant
- Gene
- X chromosome
- Y chromosome
- Sex chromosomes
- Genotype
- Phenotype
- Heredity
- Alleles
- Traits
- Centromere
- Homozygous
- Heterozygous
- Genetic disorder
- Mutation
Word list came in part from: 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
Symbol Description
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
Source: Coriell Institute for Medical Research. Pedigree Symbol Definitions [online]. 1998. [cited 2004 July 16]. Available from URL:http://locus.umdnj.edu/ada/gennid/phase1/pedigrees/pedhelp.html.
Lecture Notes about Sex-Linked Genes
Sex-Linked Chromosomal Disorders April Jones, CDC’s 2004 Science Ambassador Program
General Genetic Information
Autosome 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. o Most organisms: XX=female XY=male
- The sex of some animals is determined in different ways. o Grasshoppers XX=girl XO=male o Ants and bees have NO sex chromosomes—sex determined by total chromosome number. Males-unfertilized egg, haploid Females-fertilized egg, diploid o 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 a. Affects people of all ages b. Different forms have different onset ages, ranges from infancy to middle age or later c. No specific treatment for any of the forms d. Life expectancy of individuals varies by form e. Examples of types of muscular dystrophy include: i. Duchenne Muscular Dystrophy, with an onset of 2 to 4 years, (X-linked)
Pedigree for an X-Linked Gene
Genetic Counseling
What is genetic counseling? Genetic counseling is effort of genetic counselors, doctors and healthcare providers to help people with genetic disorders. Genetic counseling can include the following: o an assessment of the patient’s heritable risk factors o information concerning the consequences of the disorder o information on the probability of developing or transmitting the disorder, o a discussion of ways in which the genetic disorder can be prevented, treated, or managed o assistance in dealing with psychosocial issues (e.g. guilt, blame, shame)
Who is involved in genetic counseling? Genetic counseling can be provided to those directly affected and their family members. Genetic counseling can be performed by many different healthcare professionals including doctors, nurses, therapists, scientists or psychologists. The title “genetic counselor” specifically refers to someone with a master’s degree in the field. Others might do genetic counseling, but they are not referred to as genetic counselors.
Who receives genetic counseling? The whole family of a patient with a genetic disorder receives genetic counseling; however, a person may receive genetic counseling if he/she knows a disorder runs in their family before they decide to have children. It can also be very rewarding to finally understand inheritance patterns.
In the above pedigree, the mother is a carrier who passed the disorder to two of her sons. She has also passed on the gene to her one daughter, making that daughter a carrier. This daughter then passed the gene on to one of her own daughters; therefore, she also became a carrier.
What is the process of genetic counseling like for the recipients? The process of genetic counseling can be very difficult for the recipients. They are often asked very personal questions and often required to have medical testing or medical records available to determine if a particular gene is present.
What are the techniques used in genetic counseling? Genetic counselors use pedigrees to determine family history and inheritance patterns, medical tests like karotyping or DNA analysis, X-rays, ultrasounds, urine analysis, skin biopsy, or physical exams.
What are the biggest challenge for genetic counselors? The biggest challenge of genetic counseling is helping families cope with the emotional, psychological, medical, social and economic consequences of genetic disease.
How do patients react during genetic counseling? Patients react in different ways when they learn their genetic risk status. Some take the information matter-of-factly. Others react with anger, shock, denial, grief, depression, confusion, and guilt.
What are some positive results from genetic counseling? Genetic counseling may help patients decide if having children is an option and if a prenatal diagnosis is available. It may also help patients by letting them know that they are not alone and that help is available.
What are some of the adverse effects from genetic counseling? Someone diagnosed with a genetic disease may be avoided by other relatives because the relatives do not know what to say or because they do not want to face up to the possibility that they too may develop the same genetic disease.
References Resta, Robert G. Genetic Counseling: Coping with the Human Impact of Genetic Disease [online]. [Date Cited 10 October 2004] Available at URL: http://www.accessexcellence.org/AE/AEC/CC/counseling_background.html.
National Society of Genetic Counselors. FAQs about Genetic Counselors and the NSGC [online]. 1995-2004. [Date Cited 19 November 2004] Available at URL: http://www.nsgc.org/consumer/faq_consumers.asp.
Pedigree Practice Answer Key
Sex-Linked Chromosomal Disorders April Jones, CDC’s 2004 Science Ambassador Program
- There is a 50% chance that each daughter will be a carrier. This is because the mother will contribute one X-chromosome to her daughter and there is a 50% chance that she will contribute the X chromosome that carries the gene as well as a 50% that she will contribute the X chromosome that does not carry the gene.
- The probability that the boy’s son will be colorblind is 0. This is because the gene is X-linked and he will contribute only his Y chromosome, not his X chromosome, to his son.
What Disorder Am I?
Sex-Linked Chromosomal Disorders April Jones, CDC’s 2004 Science Ambassador Program
Preparation On different pieces of colored construction paper list the name of a disorder or concept from the lecture on one side of the paper and the definition any descriptive characteristics on the other side. Sample cards are provided as an attachment in step 4.
Directions This activity can be done as a class or in small groups. Divide the class or small groups into two teams. Have a member from one team read the definition and any descriptive characteristics of a disorder or concept from one of the colored cards. If the opposing team is able to name the correct concept or disorder that matches the definition and descriptive characteristics, that team earns one point. If the team is unable to answer, then the team receives no points. The teams should take turns reading the cards and answering.
After the students have played for a while, have the asking team read the name of the disorder or concept and the answering team identify the definition or descriptive characteristics.
A Twist Have the students pretend they are a patient in a doctor’s office or that they are a genetic counselor. Then the patient from one team can ask about a disorder or concept and the genetic counselor from the other team can discuss the definition or descriptive characteristics keeping in mind the role of a genetic counselor.
Cards for “What Disorder Am I?” Game
Sex-Linked Chromosomal Disorders April Jones, CDC’s 2004 Science Ambassador Program
Please print the cards below. Cut out the cards along the solid line. Once cut the cards can be folded and glued or taped together so that the concept or disorder appears on one side of the card and the definition and characteristics appear on the other side.
Sex Chromosomes
I am your X and Y
chromosomes.
What am I?
Autosomes
I am all of your chromosomes
except
your sex chromosomes.
What am I?
Emery-Dreifess Muscular
Dystrophy
I degenerate skeletal and
voluntary muscles. I am
caused by an X-linked or
autosomal gene and affect
people between their
childhood and
adulthood. What am I?
