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CHAPTER 24
GENETICS AND GENOMICS
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. Introduce the term gene.
2. Discuss gene mapping and markers.
3. Describe recent medical genetics research and its impact on society.
4. 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.
4. Describe the twenty-three pairs of human chromosomes.
5. 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.
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 Xtrisomy 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
1. Describe the terms homozygous, heterozygous, dominant, and recessive and give
examples of traits in each category.
2. Define allele.
3. Introduce the use of a Punnett square.
4. 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. Reiterate the concept of dominant and recessive inheritance.
2. Give examples of autosomal recessive conditions; autosomal dominant
conditions.
3. Compare the processes of codominance and incomplete dominance.
4. Introduce penetrance and expressivity.
5. Describe pleiotropy.
6. Discuss genetic heterogeneity.
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. Describe the process, which determines the gender of a zygote.
2. Introduce the importance of the SRY gene.
3. Discuss the connection between sex chromosomes and sex-linked traits.
4. Describe ways in which a male inherits a sex-linked trait from a carrier mother
and a female inherits a sex-linked 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.
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. Discuss ways in which extra, missing, or rearranged chromosomes or parts of
them can cause syndromes.
2. Define polyploidy and give examples of this phenomenon.
3. Introduce nondisjunction and subsequent aneuploidy.
4. 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 Discuss challenges researchers face in attempting to diagnose inherited diseases.
2. Describe the value of genetic counseling.
3. Introduce the concept of gene therapy.
4. Distinguish between heritable and nonheritable gene therapy.
5. 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 twenty-third 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, 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
Balter, Michael. July 8, 2005. Are humans still evolving? Science, 2005:234-237.
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.
Seppa, Nathan. November 30, 2002. Predisposed to trouble–Gene variants implicated in
stomach cancer. Science News.
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?