Download 129

CHAPTER 13: DNA: Chromosomes, Mapping, and the Meiosis–
Inheritance Connection
WHERE DOES IT ALL FIT IN?
Chapter 13 revisits and builds upon the principles of meiosis and sexual reproduction. It
introduces students to the value of understanding how chromosomes and epigenetic factors relate to
cell function. The information covered in Chapter 13 is important for understanding the principles of
evolution covered later in the textbook. The concepts mentioned in this Chapter should be reviewed
when covering the molecular biology of DNA information introduced in Chapter 14.
SYNOPSIS
Mendel was fortunate that he chose straight forward traits. The inheritable characteristics he
studied made it simple to calculate the predictable probabilities of gene expression in offspring.
However, there are more complex genetic patterns associated with continuous variation,
pleiotropic genes, lack of complete dominance, environmental modifications of genes, and
epistasis. Many human genetics disorders follow Mendelian principles. Most are recessive like
Tay-Sachs disease. Hunington’s disease is an example of a dominant allele that remains in
populations because its effect is not expressed until after children are born. Human blood groups
are an example of traits stemming from multiple alleles. In the ABO system, four phenotypes
arise from the combination of three alleles coding for red cell surface antigens. The transmission
of a genetic disorder can often be tracked through pedigree analysis, shown in example by Royal
hemophilia in the lineages of the British monarchy. Disorders like sickle-cell anemia, are a result
of nucleotide changes that alter the linear and three-dimensional structure of critical proteins.
Current genetic research uses molecular techniques to try to cure disorders like muscular
dystrophy by inserting new genes into disabled cells.
Modern geneticists have modified Mendel’s laws to be consistent with discovery of meiosis and
crossing over, identification of chromosomes as hereditary material, and the structure of genes
and DNA. Genetic crosses in which recombination is evident can be used to construct gene
maps, identifying the location of alleles on chromosomes and specific positions within
chromosomes. The Human Genome Project has produced vast amounts of data elucidating the
genetic sequence of our own genome. A normal human cell possesses twenty-two pairs of
autosomal and one pair of sex chromosomes for a total of forty-six chromosomes. Any variance
from that number is detrimental and often lethal. Down syndrome, one of the few non-lethal
trisomies, results from primary nondisjunction during meiosis. Abnormal separation of the sex
chromosomes can result in individuals with extra or absent X or Y chromosomes. The minimal
amount of sex chromatin needed for survival is a single X chromosome. A YO zygote fails to
develop as the Y lacks the necessary information present on the X. Genetic counseling attempts
to prevent the production of children with genetic disorders by identifying parents at risk.
Prenatal diagnosis is valuable and uses amniocentesis, ultrasound, and/or chorionic villi
sampling.
Mendel did not have an understanding of epigenetic factors that influence an organism’s
characteristics. Eukaryotic cells are now known to be influenced by the genetic information
106
carried in chloroplasts and mitochondria. These organelles can contribute to or modify gene
expression of the cell’s genomic DNA. They are also subject to genetic variation that produces
genetic disorders inherited by transfer of the organelle during gamete formation.
LEARNING OUTCOMES






Evaluate the effects of continuous variation, pleiotropic genes, lack of complete dominance,
environmental modifications of genes, and epistasis on disease
Understand the importance of crossing over in terms of gene assortment and construction of
genetic maps.
Describe the many genetic disorders discussed in the text, their symptoms, relative frequency in
specialized populations, and their genetic basis.
Understand the consequences of nondisjunction at various stages of gametogenesis and its affect
on the sex chromosomes.
Understand the value and purpose of genetic counseling and describe two techniques of prenatal
genetic screening.
Explain how chloroplasts and mitochondria contribute to the characteristics of eukaryotic cells.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 13 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.












Students have trouble distinguishing chromatin from chromosomes
Students do not fully understand the role of genetics and environment on determining
observable variation in organisms
Students believe acquired characteristics can be inherited
Students think that all genetic disorders are homozygous recessive
Students believe that inbreeding causes genetic defects
Students do not take into account the role of crossing over in classical inheritance
variation
Students believe that gender in all organisms is determined by X and Y chromosomes
Students confuse the roles of autosomes and sex chromosomes
Students do not associate gene expression with inherited characteristics
Students believe sexual reproduction always involves mating
Students do not understand other mechanisms of sexual reproduction besides mammalian
reproduction
Students are unaware of the impacts of chloroplast and mitochondrial DNA on eukaryotic
traits
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
107
This chapter covers the principles and applications of chromosome theory and epigenetics. It is
important to review cell structure reinforce the locations of chromosomes and organelles
involved in trait expression. Examination of sex chromosome abnormalities is an excellent
chance to review meiosis, in terms of determining at what point of gametogenesis each
nondisjunction occurs. One may want to discuss the sex chromosome tests associated with
Olympic sports competition.
There are recent developments concerning the identification of a genetic marker associated with
Huntington’s disease. It may be worth while to discuss the moral and ethical implications of
genetic therapy. Would you want to know whether or not you were going to develop the disease?
Or perhaps worse, your children? Recent psychiatric studies show that those tested as possessing
the gene for Huntington’s disease do not become significantly depressed when faced with the
news. Rather, they are less depressed than those who have not been tested or whose tests are
inconclusive. (Southern blot/probe tests are 95% to 98% accurate in identifying this gene.)
Genetics have been implicated in autoimmune diseases like multiple sclerosis and lupus as well.
There’s a very interesting article on “genomic imprinting” in the December 1997 issue of Equus.
The authors present the phenomenon of paternal imprinting as the reason that certain
Thoroughbred sires are quality racehorses themselves, sire barely better-than-average progeny,
but whose daughters produce again superb quality racehorses. They cite Secretariat as a most
evident example.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 13.
Application
Analysis

Have students predict the genetic probabilities of color blindness or other
sex-linked disorders.

Have students explain the impacts of inbreeding on contributing to the
presence of genetic disorders in a population.

Ask students explain produce a pedigree of family produced by a couple
who both exhibit a mitochondrial disorder.

Have students explain the factors that contribute to a large degree of
nondisjunction in the ovaries of older females.

Ask students to identify the most likely stage of meiosis that would
produce disorders in which a zygote has fewer or extra chromosomes.

Ask students to explain a strategy for breeding pure populations of plants
108
that have chloroplasts with valuable genetic characteristics.
Synthesis
Evaluation

Ask students come up with a way that a physician could use determine if
a disease is caused by a sex-linked gene or by mitochondria.

Have students explain how a person appearing female could develop from
an XY zygote.

Ask students to predict the outcomes of accidental X-chromosome
inactivation in a male.

Ask students evaluate the pros and cons of testing all people for
Huntington’s disease.

Ask students to evaluate the ethical implications of testing a fetus for
nondisjunction disorders.

Ask to explain the pros and cons of a drug blocks the function of the gene
responsible for Huntington’s disease.
VISUAL RESOURCES
It is important to use large visual models of chromosomes to demonstrate chromosomes changes
that produce the genetic disorders covered in this chapter. Diagrams or models of chloroplasts
and mitochondria are also important to refresh the class’s knowledge of these organelles.
Projected images or photographs of animals and plants expressing genetic mosaics and
epigenetic factors are very useful. It is also helpful to provide students with images of the human
genetic disorders described in this chapter.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Virtual DNA Extraction
Introduction
This fun and fast demonstration engages students in developing a human karyotype. The
click and drag animation allows the instructor to interact with students while selecting
chromosomes to build a karyotype diagram.
Materials



Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark to Learn Genetics DNA Extraction animation at:
http://learn.genetics.utah.edu/units/biotech/extraction/
109
Procedure & Inquiry
1. Introduce the idea of knowing how to extract DNA as a means of identifying DNA
sequences.
2. Pull up Learn Genetics DNA Extraction website.
3. Start the sequence by clicking on the start button.
4. It may be necessary to read the pop-up reading material to the class.
5. Ask the students to answer the questions appearing in the pop-up reading.
B. Dance of Nondisjunction
Introduction
This visual activity is a fun way to demonstrate nondisjunction. It particularly focuses on
the DNA separation errors of meiosis that lead to nondisjunction.
Materials


4 student volunteers
8 large swimming “pool noodles” representing homologous chromosomes
o Four noodles of one color (Color A)
 Two are labeled chromosome 1
 Two are labeled chromosome 2
o Four noodles of another color (Color B)
 Two are labeled chromosome 1
 Two are labeled chromosome 2
Procedure & Inquiry
1. Review the basic principle of nondisjunction.
2. Call 4 students to the front instruct the following:
a. One students holds Color A chromosome 1
b. One students holds Color B chromosome 1
c. One students holds Color A chromosome 2
d. One students holds Color B chromosome 1
3. Ask the class to explain what the students need to do to represent the genetic conditions
of the DNA after the interphase of meiosis.
4. Then have the students take the duplicate chromosome and holding one chromosome in
each hand.
5. Then ask the class to explain what the students need to do to represent the genetic
conditions of the DNA during metaphase of meiosis I.
6. Then ask the students holding the noodles to represent how nondisjunction would occur
during anaphase of meiosis I.
7. You or the class can redirect the students to demonstrate the concepts more accurately if
necessary.
110
USEFUL INTERNET RESOURCES
1. Gathering information about genetic disorders is an interesting way to stress the
importance of predicting the probabilities of offspring characteristics. The Contact a
Family Website provides valuable insight to share with the class about the impacts of
genetic disabilities on families. It also provides a Mendelian genetics primer. This
website can be found at http://www.cafamily.org.uk/index.html.
2. Cold Spring Harbor Laboratory provides a valuable website for teaching the basic
genetics needed to understand variations in patterns of inheritance that are not explained
by Mendelian theory. It provides many animated discussions and videostreams. The
website can be found at http://www.dnaftb.org/dnaftb/.
3. Mitochondrial diseases are rare disorders that are unknown to most students. The United
Mitochondrial Disease Foundation has an up-to-date and informative website with many
tidbits of information that can be used in class. This website can be found at
http://www.umdf.org/
4. Science Magazine On-line published by the American Association for the Advancement
of Science has an informative website of epigenetics. It provides links to information that
can be used to reinforce the concepts covered in this chapter. The website can be found
at http://www.sciencemag.org/feature/plus/sfg/resources/res_epigenetics.dtl
LABORATORY IDEAS
This activity provides a model for demonstrating polygenic traits. It uses the random
tossing of pennies to show students that polygenic traits are controlled by more than one gene.
The demonstration can be adapted to discussions on the genetics of hair color, height, skin color,
and weight.
a. The following materials should be provided to a small group of students:
a. Six pennies per group of students
b. A piece of paper to tally the results
b. Explain to the students that polygenic traits such as weight are due to the percentage of
dominant and recessive alleles in several sets of genes.
c. Then tell them that these traits can be calculated by evaluating the number of dominant
genes compared to the number of recessive.
d. Then instruct the students to model the polygenetic inheritance of height using coins to
represent the alleles of six sets of genes. Heads represents the dominant characteristic,
whereas tails is the recessive allele.
e. Ask each group of students to flip all six coins on the lab table at once.
f. Have the students record the number of heads and tails and calculate the phenotype using
the rubric below:
Penny Toss
Approximate Height
0 Tails and 6 Heads
6 feet 1 inch
1 Tail and 5 Heads
5 feet 11 inches
111
2 Tails and 4 Heads
5 feet 9 inches
3 Tails and 3 Heads
5 feet 7 inches
4 Tails and 2 Heads
5 feet 5 inches
5 Tails and 1 Head
5 feet 3 inches
6 Tails and 0 Heads
5 feet 1 inch
.
g. Ask the student to conduct the tossing twenty times to calculate the percentage of each
phenotype after the twenty mating trials. Inform them that the tosses represent parents
heterozygous for height.
h. Have the students compare their data to other students. They should be asked to make
conclusions about the diversity of characteristics for hair color, height, skin color, and
weight that would be available in populations of people heterozygous for those
characteristics.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students present a public forum on the benefits and risks of genetic testing for
inherited disorders.
2. Have students design an educational animated PowerPoint presentation on genetic
disorders for middle school teachers.
3. Have students tutor middle school or high school biology students studying genetics.
4. Have students present literature on the biology of genetic disorders for a booth at a health
fair.
112