Download LEARNING OBJECTIVE 1: Explain how gene discoveries are

CHAPTER 24
GENETICS
LEARNING OBJECTIVE 1: Explain how gene discoveries are relevant to the study of anatomy and
physiology and to health care.
Lecture Suggestions and Guidelines
1.
2.
3.
4.
Introduce the term gene.
Discuss gene mapping and markers.
Describe recent medical genetics research and its impact on society.
Discuss ways in which genetic discoveries explain physiological and pathological processes.
Application Question(s)
1. Ask students to collect information from family members to illustrate the pattern by which some trait is
inherited within that family. Is it a dominant or recessive trait? What percentage of family members exhibit the
trait?
Answer: Responses will vary.
Critical Thinking Issue(s)
1. Provide students with an example of a karyotype form and chromosome analysis report form from a
cytogenetics laboratory. (Take care to insure confidentiality and anonymity). Discuss the findings with the
class.
Answer: A karyotype form will illustrate the actual chromosomal pairings. The chromosome analysis report
form summarizes the clinical findings. Introducing students to the importance of understanding medical
genetics and birth defects by discussing true life examples of chromosome testing is very beneficial.
LEARNING OBJECTIVE 2: Distinguish between genes and chromosomes.
and
LEARNING OBJECTIVE 3: Define genome.
Lecture Suggestions and Guidelines
1. Discuss the roles of DNA.
2. Define genome.
3. Describe the magnitude of the numbers of genes, which have been identified. Describe the twenty-three pairs of
human chromosomes.
4. Introduce the location of known genes on specific chromosomes.
Application Question(s)
1. If one parent is heterozygous for a defective autosomal dominant gene and the other parent is normal for that
trait, what percentage of their children have a chance of being affected? Give examples of autosomal dominant
disorders.
Answer: Fifty percent of their children have the chance of being affected. Achondroplasia and polydactyly are
examples.
Critical Thinking Issue(s)
1. To reinforce an understanding of the differences between a gene and a chromosome, illustrate the concept of
alleles with examples of Punnett squares for eye color, blood type, sickle cell anemia, and hair texture.
LEARNING OBJECTIVE 4: Define the two types of chromosomes.
Lecture Suggestions and Guidelines
1. Define the term autologous.
2. Give examples of diseases, which are determined on the first twenty-two pairs of chromosomes.
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3. Give examples of sex-linked diseases.
4. Describe the mechanism by which the X and Y chromosomes determine sex.
Application Question(s)
1. Provide students with karyotype forms (which illustrate chromosome pairings) for a variety of cases. Which are
normal? Abnormal? What condition do they indicate?
Answer: Examples might include a normal male karyotype, normal female karyotype, Down’s Syndrome,
Klinefelter’s Syndrome, Turner’s Syndrome, and X-trisomy.
Critical Thinking Issue(s)
1. Discuss the following chromosomal diseases: Turner’s Syndrome, Down’s Syndrome, Klinefelter’s syndrome,
achondroplasia, color-blindness, X-trisomy, and polydactyly. Which are sex-linked disorders and which are
autosomal?
Answer: Turner’s Syndrome, Klinefelter’s Syndrome, color-blindness, and X-trisomy are sex-linked. Down’s
Syndrome, achondroplasia, and polydactyly are autosomal.
LEARNING OBJECTIVE 5: Explain how genes can have many alleles (variants), but a person can
have only two alleles of a particular gene.
Lecture Suggestions and Guidelines
2. Describe the terms homozygous, heterozygous, dominant, and recessive and give examples of traits in each
category.
3. Define allele.
4. Introduce the use of a Punnett square.
5. Discuss the concept of crossing over.
Application Question(s)
1. Provide students with a Punnett square which features a heterozygous male and heterozygous female that wish
to determine, in a simplified manner, the eye color of their future children. Assume the parents’ genotypes
consist of a brown allele and a blue allele, and their phenotype is brown. What is the probability that their first
child will be blue-eyed? Brown-eyed? What is the probability that their first two children will be blue-eyed?
Answer: The probability that their first child has blue eyes is 1:4. The probability that their first child has
brown eyes is 3:4. The probability that both the first and second child will have blue eyes is 1:16.
Critical Thinking Issue(s)
1. What is PKU? Sketch a diagram, which illustrates the underlying cause of this disease.
Answer: PKU is phenylketonuria, an autosomal recessive disease. It occurs in the absence of an enzyme, which
converts phenylalanine to tyrosine, which is needed for normal melanin synthesis. The buildup of unconverted
phenylalanine becomes toxic to normal brain development.
LEARNING OBJECTIVE 6: Distinguish among the modes of inheritance.
and
LEARNING OBJECTIVE 7: Explain how gene expression varies among individuals.
Lecture Suggestions and Guidelines
1.
2.
3.
4.
5.
6.
Reiterate the concept of dominant and recessive inheritance.
Give examples of autosomal recessive conditions; autosomal dominant conditions.
Compare the processes of codominance and incomplete dominance.
Introduce penetrance and expressivity.
Describe pleiotropy.
Discuss genetic heterogeneity.
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Application Question(s)
1. Reiterate the concepts of dominant and recessive inheritance by illustrating with a Punnett square how one
might inherit sickle cell trait or sickle cell anemia.
Answer: Assume both parents are heterozygous for sickle cell trait. The probability of their first offspring
inheriting sickle cell anemia is 1:4. The probability of their first offspring inheriting sickle cell trait is 2:4, and
the probability of their first offspring inheriting neither sickle cell anemia nor sickle cell trait is 1:4.
Critical Thinking Issue(s)
1. Ask students to prepare a short report on cystic fibrosis, an autosomal recessive disorder.
Answer: The report should include a definition of the disease, cause, manifestations, course, outcome,
prognosis, and survival rate.
LEARNING OBJECTIVE 8: Describe how genes and the environment interact to produce complex
traits.
Lecture Suggestions and Guidelines
1. Discuss traits that are monogenic.
2. Introduce examples of polygenicity.
3. Describe how traits are considered to be multifactorial or complex, and give examples of traits, which are
influenced by the environment, nutrition, and other factors, as well as genetic inheritance.
Application Question(s)
1. Why might a hereditary trait, such as height, be considered a complex trait?
Answer: Height is influenced by a number of factors, including one or more “height genes”, nutrition, and
environmental factors. A multifactorial trait, such as height, appears to have a familial incidence.
Critical Thinking Issue(s)
1. How would the study of twins help scientists to understand the inheritance of complex traits?
Answer: Studies have been conducted using identical twins that were separated at birth and raised in different
environments. Distinctive traits were shown to be extremely similar, beyond the probability of coincidence.
Refer to the textbook chapter for specific examples.
LEARNING OBJECTIVE 9: Describe how traits are transmitted on the sex chromosomes, and how
gender affects gene expression.
Lecture Suggestions and Guidelines
1.
2.
3.
4.
Describe the process, which determines the gender of a zygote.
Introduce the importance of the SRY gene.
Discuss the connection between sex chromosomes and sex-linked traits.
Describe ways in which a male inherits a sex-linked trait from a carrier mother and a female inherits a sexlinked mutant gene from her carrier mother.
5. Discuss ways in which gender can affect gene expression, including sex-limited traits and sex-influenced traits.
Application Question(s)
1. What is the significance of the SRY gene?
Answer: SRY is an acronym for sex-determining region of the Y. It encodes a transcription factor, a special
type of protein, which influences the action of other genes that determine male embryonic structures.
Critical Thinking Issue(s)
1. Ask students to prepare a short paper on hemophilia, a sex-linked disease whose mutant gene is located on the
X chromosome.
Answer: Responses will vary.
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LEARNING OBJECTIVE 10: Explain how deviations in chromosome number or arrangement can harm
health, and how these abnormalities are detected.
and
LEARNING OBJECTIVE 11: Explain how conditions caused by extra or missing chromosomes reflect a
meiotic error.
Lecture Suggestions and Guidelines
1.
2.
3.
4.
Discuss ways in which extra, missing, or rearranged chromosomes or parts of them can cause syndromes.
Define polyploidy and give examples of this phenomenon.
Introduce nondisjunction and subsequent aneuploidy.
Give examples of monosomic and trisomic conditions.
Application Question(s)
1. Ask students to discuss an example of a disease caused by chromosomal translocation.
Answer: One example is Rett Syndrome, which research has revealed may be the result of chromosomal
translocation between the third autosomal chromosome and the X chromosome. Signs and symptoms include
mental retardation, uncoordinated walking, staring, and loss of communication skills.
Critical Thinking Issue(s)
1. Discuss Down’s Syndrome.
Answer: Reinforce the significant effects that an extra chromosome in the twenty-first position has during the
discussion of the disease’s manifestations, course, prognosis, and survival rate.
LEARNING OBJECTIVE 12: Explain how gene therapy works.
Lecture Suggestions and Guidelines
1.
2.
3.
4.
5.
Discuss challenges researchers face in attempting to diagnose inherited diseases.
Describe the value of genetic counseling.
Introduce the concept of gene therapy.
Distinguish between heritable and nonheritable gene therapy.
Discuss various current uses of gene therapy and its impact on the future.
Application Question(s)
1. What challenges do researchers face in attempting to implement effective gene therapy?
Answer: Researchers must 1) find more effective ways to isolate genes; 2) find better ways to introduce genes
into the body; 3) find an accurate way to introduce genes to a precise location; and 4) find an efficient means by
which to regulate the gene’s effects.
Critical Thinking Issue(s)
1. Discuss the major social and ethical issues surrounding gene therapy.
Answer: Responses should include a discussion of 1) the ability to genetically alter human life; 2) the positive
and negative viewpoints of cloning; 3) the effect upon future gene pools; 4) technological errors; and 5) the
possibility of abuse of learned technologies.
RELATED DISEASES OF HOMEOSTATIC INSTABILITY
1. Trisomy 21—Commonly referred to as Down’s Syndrome, an extra chromosome appears in the twenty-first
position resulting from nondisjunction, the failure of two chromosomes to separate as the gametes are being
formed. Manifestations include mental retardation. The life expectancy of a child can be short. Cardiovascular
system, intestine, and kidney problems are common.
2. Klinefelter’s Syndrome—A sex-linked chromosomal disorder in exhibiting an xxy configuration in the twentythird position. This condition results in a male with female traits. The breasts may be enlarged. The testes are
small and infertile. There is decreased mental capacity in some males.
3. Sickle Cell Anemia—A hereditary disease that results from the sickling of red blood cells due to the presence
of an abnormal form of hemoglobin. Typical symptoms include severe pain in the arms, legs, and abdomen,
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jaundice of the sclera, and recurrent fever. There is no cure for sickle cell anemia. Blood transfusions,
administration of fluids, and analgesics are usually indicated.
SUGGESTIONS FOR ADDITIONAL READING
Bouchard, T.J., Jr. June 17, 1994. Genes, environment, and personality. Science, vol. 264. Identical twins separated
at birth offer clues to hereditary and environmental influences on behavior.
Carmelli, Dorit, et al. September 17, 1992. Genetic influence on smoking-a study of male twins. New England
Journal of Medicine, vol. 327. Smoking behavior among thousands of twin pairs suggests a hereditary influence.
Chakravarti, Aravinda. September 1994. The behavior of meiosis in sperm. American Journal of Human Genetics,
vol. 55. In the past, organisms with short life cycles were used to trace meiotic events. New technology enables
researchers to do this with human sperm.
Haseltine, William A. March, 1997. Discovering genes for new medicines. Scientific American.
Hoffman, Eric P. January 1994. The evolving genome project: current and future impact. American Journal of
Human Genetics, vol. 54. Physicians must learn a new field of genetics.
Housman, David. February 2, 1995. Human DNA polymorphism. New England Journal of Medicine, vol. 332. DNA
differences form the basis of many gene-based technologies.
Lewis, Ricki. January 1991. Genetic imprecision. BioScience. Chromosome checks can reveal abnormalities, but
they are not necessarily linked to symptoms.
Lewis, Ricki. March 1993. Choosing a perfect child. The World and I. The diagnosis of a genetic disease is possible
before conception by probing the genes of polar bodies.
Lewis, Ricki. December 1994. The evolution of a classic genetic tool. BioScience, vol. 44. Computers have greatly
increased the information content of pedigrees.
Motulsky, Arno G. October 1994. Predictive genetic diagnosis. American Journal of Human Genetics, vol. 55.
Should tests that can predict one’s medical future be given, if the identified impending illness has no cure?
Mulligan, R.C. May 14, 1993. The basic science of gene therapy. Science, vol. 260. A review of the essentials of this
new type of therapy.
Wertz, Dorothy, Joanna H. Fanos, and Philip R. Reilly. September 21, 1994. Genetic testing for children and
adolescents. Who decides? Journal of the American Medical Association, vol. 272. Should people under the age
of 18 be permitted to undergo genetic testing?
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