RAVEN 9/e

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 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

  • Describe sex-linked inheritance in fruit flies.
  • Explain the evidence for genes being on chromosomes.
  • Describe the relationship between sex chromosomes and sex determination.
  • Explain dosage compensation in mammals and its genetic consequences.
  • Explain why the presence of DNA in organelles leads to non-Mendelian inheritance.
  • Describe the inheritance pattern of this organellar DNA.
  • Recognize that genes on the same chromosome may not assort independently.
  • Explain how recombination frequency is related to genetic distance.
  • Review how data from testcrosses is used to construct genetic maps.
  • Explain how mutations can cause disease.
  • Describe the consequences of nondisjunction in humans.
  • Recognize how genomic imprinting can lead to non-Mendelian inheritance.

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

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 /
  • 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.

Analysis /
  • 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 that have chloroplasts with valuable genetic characteristics.

Synthesis /
  • 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.

Evaluation /
  • 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:

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
  • Four noodles of one color (Color A)
  • Two are labeled chromosome 1
  • Two are labeled chromosome 2
  • 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:
  3. One students holds Color A chromosome 1
  4. One students holds Color B chromosome 1
  5. One students holds Color A chromosome 2
  6. One students holds Color B chromosome 1
  7. Ask the class to explain what the students need to do to represent the genetic conditions of the DNA after the interphase of meiosis.
  8. Then have the students take the duplicate chromosome and holding one chromosome in each hand.
  9. 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.
  10. Then ask the students holding the noodles to represent how nondisjunction would occur during anaphase of meiosis I.
  11. You or the class can redirect the students to demonstrate the concepts more accurately if necessary.

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.

  1. The following materials should be provided to a small group of students:
  2. Six pennies per group of students
  3. A piece of paper to tally the results
  4. 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.
  5. Then tell them that these traits can be calculated by evaluating the number of dominant genes compared to the number of recessive.
  6. 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.
  7. Ask each group of students to flip all six coins on the lab table at once.
  8. 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
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

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  1. 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.
  2. 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.

ETYMOLOGY OF KEY TERMS

di- two; twice (from the Greek di- two)

bi- two (from the Latin bi- two)

hetero- different (from the Greek heteros- the other of two)

homeo- likeness; resemblance; similarity (from the Greek homoios- like)

extra- outside; beyond (from the Latin exter- being on the outside)

mono- one; single; alone (from the Greek monos- alone)

phenotype observable traits (from the Greek phainein- to show and typostype