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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 10. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What was the “Legend of Princess Anastasia” and how did the science of genetics play a part in solving this mystery?
  2. What organism did the “Father of Genetics,” Gregor Mendel, work with to devise his theory of inheritance?
  3. What is the ultimate source of all new genetically inherited traits?
  4. Why do true-breeding purple-flowered pea plants, when crossed with true-breeding white-flowered pea plants only produce purple-flowered offspring?
  5. How did the experiments of Gregor Mendel disprove the “blending” theory of inheritance?
  6. What is a Punnett square and how can it be used to illustrate Mendel’s “law of segregation”?
  7. What is “incomplete dominance”?
  8. Why might a mouse with two copies of the dominant black fur gene still have white fur?
  9. What are polygenic traits and how do they determine characteristics such as skin color in humans?

A Guide to the Reading

When exploring the content in Chapter 10 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.

Dominant vs. Recessive

The concept of gene dominance is vital to the understanding of inheritance.  Recall that the phenotype of an organism refers to the physical, observable trait resulting from the individual’s genotype, or genetic makeup.  A perfect example of this is flower color in pea plants as studied by Mendel.  In this case, flower color is determined by two different alleles, or versions, of a single gene.  The P allele contains the genetic code for producing purple flowers (the phenotype).  The p allele contains the genetic code for producing white flowers.  When a single copy of each allele is present in the same plant, the resulting genotype of the organism is Pp.  Since the two copies of the gene are dissimilar, we refer to this organism as a heterozygote.  In heterozygous organisms, the influence of one gene may take precedence over the other in the production of the phenotype.  In this example, the heterozygous plant would produce purple flowers.  Therefore, the P allele is considered to be dominant to the recessive p allele since the purple flower phenotype is displayed over the white flower phenotype.  Keep in mind that the terms dominant and recessive are comparative; an allele, if considered on its own, cannot be determined to be dominant or recessive unless one is comparing it to an alternate form of the same gene.

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

  • Section 10.1, Essential Terms in Genetics
  • Section 10.3, Basic Patterns of Inheritance

Mendel's Law of Segregation

As stated in the chapter, the law of segregation states “that the two copies of a gene separate during meiosis and end up in different gametes.”  To understand this statement, be sure to review the discussion of meiosis in Chapter 9 (in particular Section 9.4, Meiosis: Halving the Chromosome Number).  If you understand that it is physically impossible for more than one copy of a single gene to end up in a single sperm or egg cell (under normal circumstances), then understanding the law of segregation becomes easy.  Keep in mind that it is this law of segregation that makes it possible to utilize the Punnett square to predict the probability of obtaining offspring with a given genotype and phenotype. 

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

  • In Section 10.4, Mendel’s first law: Segregation
  • Figure 10.6, The Punnet Square Method

Mendel's Law of Independent Assortment

As stated in the text, the law of independent assortment states “that when gametes form, the separation of the two copies of one gene during meiosis is independent of the separation of the copies of other genes.”  This law basically states that each gene present in an organism is inherited independently of all of the other genes.  Mendel showed this with his experiments on pea plants while observing the inheritance of multiple traits (such as pea color and shape) simultaneously.  Recall that while the pea color trait (yellow versus green) and the pea shape trait (wrinkled versus smooth) both obey the law of segregation as expected, these two traits behave independently of each other during the process of meiosis.  Therefore, the inheritance of a particular pea color has no influence whatsoever on the inheritance of pea shape.   

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

  • In Section 10.4, Mendel’s second law: Independent assortment
  • Figure 10.7, Independent Assortment of Alleles
  • Figure 10.8, Are Genes Inherited Independently?

Incomplete Dominance and Codominance

Not all traits follow a simple dominant/recessive pattern.  For these traits, an allele may not produce its maximum phenotype or may contribute to multiple phenotypes.  Consider a gene with two alleles which produce distinct phenotypes.  When heterozygotes display a phenotype that is intermediate between the two phenotypes displayed by homozygotes, this is referred to as incomplete dominance.  The examples used in the text include flower color in snapdragons and coat color in horses.  The key to identifying incomplete dominance is the fact that heterozygous individuals display a different phenotype than either of the two homozygous forms.  This is in contrast to codominance where “the phenotype of the heterozygote is determined equally by each allele.”  In this case, different alleles of the same gene may produce their maximum phenotype simultaneously.  The example provided in the text is that of blood types in humans where two alleles (A and B) will be expressed regardless of the combination of alleles inherited.  The key to identifying codominance is understanding that heterozygous individuals essentially display two phenotypes (one for each different allele) simultaneously.  In the case of blood types, an individual who is heterozygous (AB) will display the type A phenotype and the type B phenotype simultaneously.   Blood types are complicated slightly by the presence of a third allele (O) which is recessive to both the A and B alleles. 

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

  • In Section 10.5, Many alleles do not show complete dominance
  • Figure 10.9, Incomplete Dominance in Horses
  • Figure 10.10, Codominance in the Blood Typing System


Genes are complex entities.  Rarely does a gene exert its phenotypic effect without interaction from the presence or absence of other genes.  When the effect of a particular allele depends on the presence of other alleles from a separate, distinct gene, this is called epistasis.  The example provided in the text for epistasis is coat color in mice.  Separate genes for the pigment melanin and a biochemical precursor to the pigment are inherited independently.  In this case, the alleles for melanin production result in either black fur (B, dominant) or brown fur (b, recessive).  Both alleles for pigment production rely on the second gene for the precursor which has two alleles as well (C, dominant, pigment produced; c, recessive, no pigment produced).  Therefore, regardless of the genotype for fur color, if the mouse is homozygous recessive for the precursor (cc), the mouse will be incapable of producing pigment, and will be white.  In this case, the melanin gene is dependent upon the independently inherited precursor gene.  When considering epistasis, it is important to remember that even though multiple genes may interact to produce a single phenotype, these genes still must obey the law of independent assortment. 

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

  • In Section 10.5, Alleles for one gene can alter the effects of another gene
  • Figure 10.11, Gene interactions

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:

Sex-linked Inheritance

  • Chapter 11 - Section 11.7, Sex-Linked Inheritance of Single-Gene Mutations

Exceptions to Mendelian Genetics

  • Chapter 11 – Section 11.3, Linkage and Crossing-Over


  • Chapter 13 – Section 13.6, The Effect of Mutations on Protein Synthesis

DNA Technology and Human Disease

  • Chapter 15 – Section 15.3, Applications of DNA Technology

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