genetics case study answer key

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5.1 Case Study: Genes and Inheritance

Created by: CK-12/Adapted by Christine Miller

Case Study: Cancer in the Family

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t  see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2 , located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Chapter Overview: Genetics

At the end of this chapter, you will find out Rebecca’s test results. By then, you will have learned how traits are inherited from parents to offspring through genes, and how mutations in genes such as BRCA1 and BRCA2 can be passed down and cause disease. Specifically, you will learn about:

• The structure of DNA.
• How DNA replication occurs.
• How DNA was found to be the inherited genetic material.
• How genes and their different alleles are located on chromosomes.
• The 23 pairs of human chromosomes, which include autosomal and sex chromosomes.
• How genes code for proteins using codons made of the sequence of nitrogen bases within RNA and DNA.
• The central dogma of molecular biology, which describes how DNA is transcribed into RNA, and then translated into proteins.
• The structure, functions, and possible evolutionary history of RNA.
• How proteins are synthesized through the transcription of RNA from DNA and the translation of protein from RNA, including how RNA and proteins can be modified, and the roles of the different types of RNA.
• What mutations are, what causes them, different specific types of mutations, and the importance of mutations in evolution and to human health.
• How the expression of genes into proteins is regulated and why problems in this process can cause diseases, such as cancer.
• How Gregor Mendel discovered the laws of inheritance for certain types of traits.
• The science of heredity, known as genetics, and the relationship between genes and traits.
• How gametes, such as eggs and sperm, are produced through meiosis.
• How sexual reproduction works on the cellular level and how it increases genetic variation.
• Simple Mendelian and more complex non-Mendelian inheritance of some human traits.
• Human genetic disorders, such as Down syndrome, hemophilia A, and disorders involving sex chromosomes.
• How biotechnology — which is the use of technology to alter the genetic makeup of organisms — is used in medicine and agriculture, how it works, and some of the ethical issues it may raise.
• The human genome, how it was sequenced, and how it is contributing to discoveries in science and medicine.

As you read this chapter, keep Rebecca’s situation in mind and think about the following questions:

• BCRA1 and BCRA2 are also called Breast cancer type 1 and 2 susceptibility proteins.  What do the BRCA1 and BRCA2 genes normally do? How can they cause cancer?
• Are BRCA1 and BRCA2 linked genes? Are they on autosomal or sex chromosomes?
• After learning more about pedigrees, draw the pedigree for cancer in Rebecca’s family. Use the pedigree to help you think about why it is possible that her mother does not have one of the BRCA gene mutations, even if her grandmother, aunt, and uncle did have it.
• Why do you think certain gene mutations are prevalent in certain ethnic groups?

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In  Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia . https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In  Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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A Family in Need: In-Class Case Study on Cancer Genetics

By Janet A. De Souza-Hart

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This case is designed as an in-class, problem-based learning activity for students to learn about several innovative medical applications of molecular biology. Students assume the role of a second-year medical student assigned to work with a pediatric oncologist who has just biopsied a tumor-like growth in the adrenal gland of her 17-year-old patient, Lee F. After taking Lee’s family history and performing a pedigree analysis, students review clinical and genetic characteristics of several syndromes associated with adrenal cancer. Students then explore various diagnostic and biomedical research techniques such as PCR, DNA sequencing, and pre-implantation genetic diagnosis. The case concludes with a consideration of how to treat Lee’s condition with the help of gene cloning and the potential of gene therapy. Although originally written for an upper-level college genetics course, the case could also be adapted for an introductory molecular/cellular biology course, a non-majors biology course, or a professional school medical genetics course. The case has two versions: an "in-class version" and an "Internet version." This version is the in-class version.  The other, Internet-enhanced version requires that students work more independently as they use various websites and databases to discover key pieces of information for the case and is completed outside of class.

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• Draw a pedigree for a genetic disorder with appropriate symbols and notations using a family history.
• Analyze a pedigree to determine whether it is consistent with specific human inheritance patterns.
• Understand the connection between specific genetic mutations, gene expression, and phenotype as it relates to an inherited form of cancer (Li Fraumeni syndrome).
• Describe in detail the molecular genetics and phenotype of Li Fraumeni syndrome.
• Describe the current state of prevention (both pre- and post-natal) for Li Fraumeni syndrome.
• Describe several types of genetic tests, as well as examine ethical issues related to genetic testing.
• Define and apply PCR, DNA sequencing, pre-implantation genetic diagnosis, and microarray techniques to the diagnosis of a genetic disorder.
• Evaluate a microarray for differences in gene expression as they relate to cancer. In addition, define different categories of cancer genes (tumor-suppressor, genome maintenance, etc.).
• Describe diff erent types of point mutations and predict how they might impact gene expression.
• Identify the goal of and current techniques related to gene therapy.
• Apply knowledge of gene therapy to a specific type of cancer and assess/evaluate risks and benefits.
• Apply scientific/medical knowledge to a written assignment to reflect on the potential of gene therapy.

rDNA technology; Li Fraumeni syndrome; cancer; PCR; DNA microarrays; gene cloning; Southern blotting; DNA sequencing; genetic testing; gene therapy; pedigree analysis; P53; virotherapy

Subject Headings

EDUCATIONAL LEVEL

Undergraduate lower division, Undergraduate upper division

TOPICAL AREAS

TYPE/METHODS

Teaching Notes & Answer Key

Teaching notes.

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Materials & Media

Supplemental materials.

The supplemental material below may be used in conjunction with this case study.

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Interactive Case Study for The Search for a Mutated Gene

• Biotechnology
• Genetic Disease
• Experimental Design

Resource Type

• Interactive Videos

Description

This video case study explores the approaches scientists used to identify a mutation that causes retinitis pigmentosa (RP), a progressive disease that leads to blindness.

RP results in the deterioration of the retina and loss of vision. Some cases of RP are inherited and caused by mutations in one of several different genes. Many mutations that cause RP have been identified. But when scientists tested the DNA of the patient featured in this video, Sam, they did not find any of these known mutations. The video follows physician-scientist Edward Stone as he tried to uncover the mutation that causes Sam’s RP.

This video incorporates embedded questions at automatic pause points, where students are asked to make predictions, construct explanations, and analyze data. After answering all the questions, students can view their answers in a “Report” that can be printed. They can also add further explanation to each answer in the Report if their thinking has changed. The video can also be shown without embedded questions using "Presentation Mode." 

The “Resource Google Folder” link directs to a Google Drive folder of resource documents in the Google Docs format. Not all downloadable documents for the resource may be available in this format. The Google Drive folder is set as “View Only”; to save a copy of a document in this folder to your Google Drive, open that document, then select File → “Make a copy.” These documents can be copied, modified, and distributed online following the Terms of Use listed in the “Details” section below, including crediting BioInteractive.

Student Learning Targets

• Formulate a hypothesis to explain how a mutation in a gene would affect the function of a cell and an organism.
• Describe the possible steps involved in identifying a disease-causing gene mutation in a patient.
• Predict how replacing a mutated gene with a functioning copy of that gene will affect the phenotype of a cell and/or organism.
• Explain how the identification of disease-causing mutations can be used to develop medical treatments.  

Estimated Time

genetic disease, genomics, model organism, mutation, retinitis pigmentosa (RP), tRNA

Primary Literature

DeLuca, Adam P., S. Scott Whitmore, Jenna Barnes, Tasneem P. Sharma, Trudi A. Westfall, C. Anthony Scott, Matthew C. Weed, et al. “Hypomorphic mutations in TRNT1 cause retinitis pigmentosa with erythrocytic microcytosis.” Human Molecular Genetics 25, 1 (2016): 44–56. https://doi.org/10.1093/hmg/ddv446 .

Terms of Use

Please see the Terms of Use for information on how this resource can be used.

Accessibility Level (WCAG compliance)

Version history, curriculum connections, ngss (2013).

HS-LS1-1, HS-LS3-1, HS-LS3-3; SEP6

AP Biology (2019)

IST-1.K, IST-1.P, IST-2.E, IST-4.A; SP1, SP3

IB Biology (2016)

Vision and change (2009).

CC3; DP1, DP6

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Activity Transcript

Abbreviations, webmd network.

Case Study: Why Are There No Male Calico Cats?

This presentation case study asks two important questions regarding cat coloration*. The instructor uses the slides as an interactive lecture to discuss the topic and develop an understanding of genetics. Students can work in small groups to answer questions as the instructor facilitates a discussion. Questions included in the presentation can be answered as part of this discussion and turned in at the end of class.

Objectives:

• Understand how genes located on sex chromosomes are inherited
• Make predictions about the genotypes of male and female calico cats
• Examine a karyotype and identify abnormalities
• Understand how errors in meiosis can produce these abnormalities
• Discuss how the inactivation of X chromosomes can affect phenotypes

Target Audience: Advanced or AP Biology, could be modified for other courses

Time Frame: Can be completed in about 50-60 minutes.

Question 1: Why are there no male calico cats? Why are there a few cases of male calicos?

Question 2: Why do cloned Calico cats look different?

Questions 3: What is Spaz's genotype?

Queston 4: Describe Nondisjunction

Question 5: How many chromosomes are in a human zygote?

...... Question 11: Why do CC and Rainbow look differerent even though they are clones?

As students expore these questions, they will be asked to analzye data and make predictions about the inheritance patterns of cat coloration. The case is intended to showcase the relationship between chromosomes, genes, and inheritance.

Concepts Included:

Karyotypes Autosomes vs Sex Chromosomes Sex-Linked Traits Cloning Meiosis Nondisjuntion Trisomy & Monosomy Barr Body

*Cat coloration has been simplified in this exercise to only examine the orange and black alleles located on the X chromosome.

There are many genes that affect cat colors, if students are interested in learning more, the Feline Genetics Primer explains how other cat colors are inherited.

Other Resources on Sex-Linkage

SRY not SRY - A case study in sex determination Practice Genetics - Sex-linked Genes The Genetics of Calico and Tortoiseshell Cats Practice Problems - Gene Linkage Sexy Chickens - exploring ZW inheritance