Chapter 2 Genes, Chromosomes, and DNA

Overview

How do offspring come to resemble their parents physically? This is the major question posed by the field of biology called genetics, the study of inherited traits. Genetics begins with the unifying assumption that biological inheritance is transmitted by structures called genes. The discovery of what genes are and how they work has been the subject of many years of research. Among the earliest findings was the fact that the same basic patterns of inheritance apply to most organisms.  The inheritance of many human traits can be explained by the same hypotheses first formulated from the study of pea plants. These basic principles are discussed in this chapter. The inheritance of other human traits, such as sex determination and susceptibility for many diseases, involves further complications that are addressed in Chapter 3.

            We now know that genes are made of DNA, and we will discuss how we know this later in the chapter. We begin, however, with experimental evidence for the existence of genes and their role in inheritance.

Chapter Outline

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Review Questions

THE PURPOSE of these review quizzes is to guide students in where their knowledge and understanding is strong, where it is weak, and where time should best be spent in studying.

CHAPTER 2:

  1. Can you explain the difference between dominant and recessive traits, and explain how this distinction led Mendel to reject the earlier concept of blending inheritance?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.1.)
  2. Can you explain Mendel’s hypothesis of independent assortment, and the evidence that led him to this concept?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.1.)
  3. Can you explain the various events (involving the chromosomes, the nuclear membrane, and the cytoskeletal filaments that make up the spindle) that must occur for a cell to divide? Can you identify the key differences between cell division by mitosis vs. meiosis and the purposes of these two types of cell division? (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.2.)
  4. Can you explain the chromosomal theory of inheritance, and the reasoning that led scientists to support this theory?  Can you explain how this theory was experimentally confirmed?    (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.2.)
  5. Can you explain the concept of bacterial transformation that was discovered by Griffiths and how Avery, MacLeod and McCarty used this phenomenon to identify DNA as the molecule of heredity? Can you explain how Hershey and Chase used a different strategy (involving bacteriophage) to further confirm this finding? (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.3.)
  6. Can you describe the structure of the building blocks of DNA and explain how they are linked together in a DNA molecule?  Can you explain how the Watson-Crick model of DNA structure explains Chargaff’s rules and also suggests a mechanism for how DNA is copied?    (If you cannot explain these things, or if you are not sure, then you need to reread Section 2.3.)

Open Response Study Questions

These questions are designed to assess your understanding of the topics explored in this chapter. You can use these questions in three ways:

Before you start …

Read through the questions before you read the chapter to help prime you to read the text more carefully and strategically. Remember that you are just starting out on your learning journey, so don’t feel disheartened if you don’t know how to answer them yet!

Whilst you read …

As you work through the chapter, have another go at answering the questions to see how you are progressing. You can also answer the questions with the textbook open in front of you, in order to create model answers that can be used to refer back to later.

At the end …

Answer the questions once you have finished reading to see what you have learned. Check your responses against your model answers and use these to identify any gaps in your understanding.

DEFINITIONS:

In your own words, define ANY TWO of the following terms:

cytokinesis      recessive         Chargaff’s rules           homologous chromosomes  metaphase

linkage (in genetics)               pure line

ESSAYS:

Answer any two of the following questions.  Make sure to answer all parts of any question you choose.

1. Blue eyes are an autosomal recessive trait in humans; the dominant condition is brown eyes (and we are simplifying by ignoring several less common types of eye color).  Answer all parts of this question:

        (A). If a blue-eyed individual has two brown-eyed parents, what are the genotypes of the parents? 

        (B). In the same family as (A), what are the chances that the next child of this couple would be heterozygous?  

        (C). In another family, each child has a 50% chance of being blue-eyed.  What are the possible genotypes of the two parents?

2. Explain how experiments by Griffith AND at least one follow-up study showed that DNA is the molecule that contains hereditary information.

3. With the aid of a diagram, explain how Harriet Creighton and Barbara McClintock were able to confirm the chromosomal theory of inheritance.

4. When Watson and Crick proposed the double-helix model of DNA structure, explain what they meant when they said that this model immediately suggested a hypothesis to explain how replication takes place.

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Bibliography

Avery, O.T., C.M. Macleod, and M. McCarty. 1944. Studies on the chemical nature of the substance inducing transformation of pneumococcal types: induction of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type III. J. Exper. Med. 79(2): 137-158.

Brooker, R.G. 2021. Genetics: Analysis and Principles, 7th ed.  New York: McGraw-Hill.

Brooker, R.G. 2022. Concepts of Genetics, 4th ed.  New York: McGraw-Hill.

Chargaff, E. 1950. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experientia 6: 201-209.

Creighton, H.B., and B. McClintock. 1931. A correlation of cytological and genetical crossing-over in Zea mays.  Proc. Nat. Acad. Sci. U.S.A. 17(8): 492-497.

Edlin, G. 1990. Human Genetics. A Modem Synthesis. Boston: Jones and Bartlett.

Fausto-Sterling, A. 2000. Sexing the Body: Gender Politics and the Construction of Sexuality. New York: Basic Books/Perseus.

Goldberg, M., et al.  2021. Genetics: from Genes to Genomes, 7th ed.  New York: McGraw-Hill.

Griffith, F. 1928. The significance of pneumococcal types. J. Hygiene 27(2): 113-159.

Hartl, D.L.  2019. Genetics: Analysis of Genes and Genomes, 9th ed.  Sudbury, MA: Jones and Bartlett.

Hartwell, L.H., et al. 2011. Genetics: From Genes to Genomes, 4th ed.  New York: McGraw-Hill.

Hershey, A.D., and M. Chase. 1952. Independent functions of viral protein and nucleic acid in growth of bacteriophage. J. General Physiol. 36: 39-56.

Klug, W.S. et al. 2020.  Concepts of Genetics, 12th ed. London: Pearson.

Koopman, P. 2001. The genetics and biology of vertebrate sex determination. Cell 105: 843-847.

Levitan M. 1988. Textbook of Human Genetics. 3rd ed. New York: Oxford Univ. Press.

Mendel, G.  1965. Experiments in Plant Hybridisation. [Translation of Mendel’s original paper, with forward by P.C. Mangelsdorf.] Cambridge, MA: Harvard University Press.

Mukherjee S.  2016.  The Gene:  An Intimate History.  New York:  Scribner’s.

Pierce, B.A. 2020. Genetics. A Conceptual Approach, 7th ed. London: Macmillan.

Sayre, A. 1975. Rosalind Franklin and DNA. New York: W. W. Norton.

Singer, M.. and P. Berg. 1991. Genes and Chromosomes. A Changing Perspective. Mill Valley CA: University Science Books.

Snustad, P.J., and M.J. Simmons. 2015. Principles of Genetics, 7th ed.  New York: Wiley.

Stern, C., and E.R. Sherwood, editors. 1966. The Origin of Genetics: a Mendel Source Book. San Francisco: W.H. Freeman.

Stratham. P. 1999. Crick, Watson, and DNA. New York: Anchor Books.

Sutton, W.S. 1903. The chromosomes in heredity. Biol. Bulletin 4(5):231-251.

Watson, J.D., and F.H.C. Crick. 1953. A structure for deoxyribose nucleic acid. Nature, 171: 737-738.

Watson, J.D. 1968. The Double Helix. New York: Atheneum.

Chapter 3
Human Genetics and Gene Expression