Chapter 4

Learning Objectives:

  1. Identify the molecular or developmental interactions between genotypes at two loci that produce different patterns of phenotypic ratios in an F2 population from a Mendelian dihybrid cross.
  2. Explain why more than one locus is necessary to form a phenotype, and give the term used to describe this fact.
  3. Explain why "partial genotypes" are necessary for practical studies of Mendelian genetics even though they never functionally exist.
  4. Explain how any genotype will be phenotypically expressed in more than one trait, and give the term used to describe this fact.
  5. Explain how a 2:1 phenotypic ratio for a given locus with two alleles (3 possible genotypes) is produced for one trait, yet the same locus simultaneously reflects another trait (lethality) with a phenotypic ratio of 1 dead:3 alive.
  6. Explain why "pleiotropy" is the rule, rather than the exception, for phenotypic expressions of genotypes.
  7. Explain why dominance and recessivity may differ for separate traits "determined" by a set of genotypes at a single locus.
  8. Explain how selection of gametes may affect the zygotic ratios of the following generation.
  9. Describe the phenomenon of multiple alleles, and show how they are experimentally differentiated from multiple loci with two or more alleles.
  10. Describe how dominance and recessive properties are expressed when multiple alleles are present, using examples from corn seed color.
  11. Relate the terms "epistasis," "penetrance," "pleiotropy," and "expressivity" to biochemical, cellular and developmental processes. (This objective is universally included in all parts of the course. Your answers will develop more detail and concreteness.)

Understanding terms:

  1. dominance, incomplete dominance, codominance, overdominance: "Dominance" and "recessivity" refer to the results of interactions between alleles (homologus loci). All of the terms involving "dominance" refer to the phenotype of the heterozygote relative to one or both homozygotes. When the heterozygote has:
    1. a phenotype the same as a homozygote, there is dominance;
    2. the heterozygote is intermediate between the homozygotes, there is lack of dominance ("incomplete dominance");
    3. the heterozygote is the aggregate (not a blend) of the phenotypes of the homozygotes, there is codominance;
    4. the heterozygote exceeds the phenotypes of the homozygotes, there is overdominance.

    Figure 1-22c shows that different culture temperatures can change the relationship to range from dominance where curves for Infrabar (Bb) and Ultrabar (BB) intersect to overdominance where Infrabar is more extreme (left of the intersection) to lack of dominance ("incomplete dominance") to the right of the intersection (see p. 531 for more details).

  2. multiple alleles: With the paradigm that "particulate" genes exist, one gene occupies a certain region of DNA in a chromosome (gene = locus). We recognize the locus and the alleles since the different genotypes are associated with different phenotypes. Therefore, there must be a minimum of two alleles at a locus to use Mendelian analysis of modes of inheritance. So, if there is a minimum of two alleles present, even more kinds of alleles in these regions are expected to exist. When there exist more than two alleles at a locus, then we use the term "multiple alleles," or the locus has a "multiple allelic series." Of course, a diploid cell can have at most two alleles at a locus, even though there may be many more alleles present in the species.
  3. genes for traits of reproductive incompatibility and lethality: These traits either prevent reproduction in certain situations, or kill the individual. In either case genes cannot be transmitted so that these genes affect the patterns of phenotypic expression in pedigrees. Note, however, since the lethal or incompatible phenotypes may not develop before reproduction is possible, this incomplete penetrance would allow limited transmission of the alleles.
  4. epistatic and hypostatic genotypes: A genotype at one locus that prevents the phenotypic expression of a genotype at another locus is analogous to a dominant allele on one chromosome that prevents the expression of a homologus recessive allele on another chromosome. The genotype that prevents the expression of another genotype is "epistatic" and the genotype that cannot express its phenotype is "hypostatic." This is an example of interactions between genotypes at different (non-homologus) loci.
    (hook: Interaction between non-homologus genes and genotypes is the rule in living organisms. We will expand on the implications of this model throughout the course.)
  5. penetrance of genotypes: Penetrance is a term used for traits with discrete classes of phenotypes. Although a genotype has a characteristic phenotype, it may not always express this phenotype. The times that a genotype succeeds in expressing its characteristic phenotype represents it's "penetrance," or (with a bit of literal distortion) it's "potential has penetrated to it's expression." Penetrance is measured by the percentage of individuals of a given genotype that manifest the characteristic phenotype.
  6. expressivity of genotypes: Genotypes may express a range of phenotypes within a characteristic class. The variation within a class is called "variable expressivity." If the phenotypic variation ranges of two genotypes overlap so that some individuals may show a phenotype more characteristic of the other genotype, then there is also a reduction in penetrance.
  7. pleiotropy: Genes code for proteins such as enzymes, whose activity produces certain biochemical results. These results may change the developmental expressions, such as flower color or growth form. Therefore, different consequences may arise from a common biochemical step. A single genotype thus may be expressed in several traits that share common underlying steps. The traits with common genetic causes exhibit a pattern of variation called "pleiotropy." The genes for which we have noticed multiple traits we call "pleiotropic genes," although we surmise that all genes are actually pleiotropic. (hook: look for expressions of pleiotropy as the course progresses)
  8. modified Mendelian ratios: the ratios studied are modified ratios from the dihybrid cross, and assume both Mendelian Laws hold. Note, however, that the modified ratios pertain to a single trait, rather than one trait per locus illustrated in elementary examples of multilocus Mendelian genetics. The usual effect for modified ratios is to reduce the amount of phenotypic variation (fewer classes) while leaving the underlying genotypic differences unchanged.
  9. complementation and allelism: The epistatic 9:7 ratio is associated with a new concept that becomes the second test for allelism, mutants that compliment, or fail to compliment. If two mutants compliment, they are not alleles, but reside at different loci; if they fail to compliment, they are different mutants at the same locus.
    (hook: Notice the epistatic and hypostatic genotypes, how more than one locus may affect a single trait, and how the underlying biochemical or developmental processes may lead to the phenotypes from some more basic trait such as an enzyme or hormone variation caused by the genotypes.)

EXERCISES: Chap. Integration Problem, Solved Prob. 1, SP 2, Prob.'s 1,3,6, 7,8,9,10,15,20,36 & 37

(hook: note that genes do not code directly for traits, such as flower color, but for traits such as certain enzymes. Relate this idea back to earlier chapters, and refer back to it later when we are studying chapters on gene function.)

Also try these "thought provokers."

  1. If reproduction can have genes that promote it, or that interfere with it, why not genes for stages in meiosis as well? Do genes code for traits/phenotypes or functions in a process? Are phenotypes reflections of what we study in genetics, or actually WHAT we study?
  2. hook: these questions may seem esoteric or philosophical, but it is important to know the differences they address to understand genetics -- think in terms of controlling processes in the cell which then have a cascading series of effects.
  3. What kinds of phenotypic expression would be observed in loci with genotypes affecting the meiotic process?
  4. If there can be genes for abnormal death, can there be genes for normal death?
  5. Can there be genes for life? ... for non-death? What might they be "named"?

Have you read any science fiction written by Robert A. Heinlein, such as Friday, or Time Enough for Love? The theme of these books is about genetic engineering and social effects.

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Last updated  09/15/00