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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 11. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. Who was George Huntington and what was his contribution to the medical field?
  2. How many different genes comprise the human genome?
  3. On which chromosome is the SRY gene located and what role does it play in determining sex in humans?
  4. How does the process of crossing-over work to generate nonparental genotypes?
  5. Why are genetic disorders caused by recessive alleles much more common than those caused by dominant alleles?
  6. What are the processes amniocentesis, chorionic villus sampling, and preimplantation genetic diagnosis used for?
  7. What do the genetic disorders Down syndrome, Klinefelter syndrome, and Turner syndrome all have in common?

A Guide to the Reading

When exploring the content in Chapter 11 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Homologous Chromosomes

As discussed in the text, the work of Gregor Mendel and August Weismann suggested that genes were located on physical structures (later determined to be chromosomes) present in the cell.  This led to the development of the “chromosome theory of inheritance”.  The process of inheritance involves the contribution of chromosomes from both parents.  Specifically, each individual possesses one complete set of chromosomes from the father and one complete set from the mother.  This results in the presence of two copies of each chromosome (and therefore two copies of each gene) in each and every cell.  Each pair of chromosomes containing the same genes are referred to as homologous chromosomes.  During the process of meiosis (Chapter 9), these homologous chromosomes pair at the metaphase plate for proper independent assortment.  

For more information on this concept, be sure to focus on:

  • Section 11.1, The Role of Chromosomes in Inheritance
  • Section 11.4, Origins of Genetic Differences among Individuals

Genetic Linkage and Crossing Over

Despite Gregor Mendel’s elaborate work, scientists often observed that certain genes seemed to be inherited together, violating the law of independent assortment.  This contradictory observation remained a mystery until the early 1900s when the studies of Thomas Hunt Morgan helped solve the puzzle.  Morgan, working with inherited traits in fruit flies, was able to determine that two genes physically located near one another on a chromosome were more likely to be inherited together, violating the law of independent assortment.  Of course, this makes sense since we now know that chromosomes are inherited as intact units.  Therefore, any genes located on the same chromosome should be inherited together in a linked fashion.  However, Morgan also observed that genes which should have been linked would, on occasion, assort independently, producing unexpected phenotypes.  This could be explained by the process of crossing-over, where genetic material is exchanged between paired homologous chromosomes during the process of meiosis.  The key to understanding crossing-over is to realize that the further apart two genes are physically located on a chromosome, the more likely it is that crossing-over will occur between these genes.  Conversely, the closer two genes are located along a chromosome, the less likely crossing-over will occur between them, resulting in a “tighter” genetic linkage.

For more information on this concept, be sure to focus on:

  • Section 11.3, Linkage and Crossing Over
  • Figure 11.4, Some Alleles Do Not Assort Independently
  • Figure 11.5, Linkage is not Complete

Sex Linkage

Humans possess 23 pairs of homologous chromosomes.  22 of these pairs provide both males and females with 2 copies of each gene.  The remaining pair of chromosomes constitute the sex chromosomes (X and Y).  Genes located on these chromosomes are considered to be sex-linked since males (XY) would possess only a single copy of each of these genes.  As a result, males are particularly susceptible to sex-linked recessive disorders since the inheritance of only a single copy of the defective allele from the mother would result in the presence of the disease.  This is in contrast to females, where two copies of the defective allele (one located on each X chromosome contributed by each parent) would be required.  Keep in mind that 15 of the 1,200 genes found on the sex chromosomes are, in fact, shared between the X and Y chromosomes.  As a result, since 2 copies of these 15 genes are present in both males and females, they would not be considered sex-linked.   

For more information on this concept, be sure to focus on:

  • Section 11.7, Sex-Linked Inheritance of Single-Gene Mutations
  • Figure 11.11, Inheritance of X-linked Recessive Disorders

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Non-inherited Genetic Disorders

  • Interlude B, An Interplay of Factors Can Cause Cancer

Sickle-Cell Anemia

  • Chapter 13 – Section 13.7, Putting It All Together: From Gene to Phenotype

Gene Expression

  • Chapter 14 - Section 14.3, Patterns of Gene Expression

DNA Technology

  • Chapter 15 – Section 15.3, Applications of DNA Technology
  • Chapter 15 – Section 15.4, Ethical Issues and Risks of DNA Technology

Inheritance of Alleles and Evolution

  • Chapter 17 – Section 17.4, Mutation: The Random Production of New Alleles
  • Chapter 17 – Section 17.7, Natural Selection: The Effects of Advantageous Alleles

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