Investigation Links

Figure 8.1 Mendel’s Monohybrid Experiments

In his early experiments, Gregor Mendel set out to test the hypothesis that the inheritance of traits was a result of the irreversible blending of different versions of a trait. Mendel crossed a true-breeding spherical seed pea plant with a true-breeding wrinkled seed pea plant. He observed that the F₁ generation for all of the pea plants had spherical seeds. Mendel then allowed the F₁ generation plants to self-pollinate, and the seed shapes of the resulting F₂ generation were analyzed. Results showed that three-quarters of the F₂ seeds were spherical, while one-quarter of the seeds were wrinkled. Importantly, Mendel observed this same pattern of inheritance regardless of which seed type contributed the pollen in the parental generation. If the theory of blending inheritance had been correct, all of the F₁ progeny would be slightly wrinkled. Further, the wrinkled trait would have been lost forever and would not have reappeared in the F₂ generation. Clearly, the results of Mendel’s monohybrid cross were inconsistent with his theory, forcing him to reject the hypothesis. The significance of Mendel’s work was not realized until more than a decade after his death in 1884. From these simple but cleverly designed monohybrid experiments and his later dihybrid crosses, Mendel observed that traits were inherited in certain numerical ratios. These results led Mendel to derive three basic principles of inheritance: the law of segregation, the law of independent assortment, and the law of uniformity. Mendel’s work eventually became the foundation for modern genetics.

Original Paper

The original German version of Mendel’s paper, Versuche über Pflanzen-Hybriden, with an English translation and extensive explanatory notes, is available online:
http://www.mendelweb.org/Mendel.plain.html

Links

Miko, I. 2008. Gregor Mendel and the Principles of Inheritance. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Gregor-Mendel-and-the-Principles-of-Inheritance-593

McGuire, T. 2008. Gene Inheritance and Transmission Topic Room. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Gene-Inheritance-and-Transmission-Topic-Room-56822

Mendel Museum: Mendel’s Experiment
http://www.mendel-museum.com/eng/1online/experiment.htm

John Kimball’s Home Page: Mendel’s Monohybrid Cross
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mendel.html

Estrella Mountain Community College: Introduction to Genetics
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookgenintro.html

The City University of New York Brooklyn College: Do Traits Blend?
http://www.brooklyn.cuny.edu/bc/ahp/MGInv/MGI.Q2.html

University of Arizona: The Biology Project: Problem 1: The Monohybrid Cross
http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/01t.html

Figure 8.4 Homozygous or Heterozygous?

Mendel realized that one problem that must be solved is how to tell the difference between homozygous and heterozygous individuals with dominant phenotypes. He developed the idea of a test cross, based on the reasoning that the genotype of an individual may be determined by crossing it with a homozygous recessive individual and observing the phenotypes of the progeny produced. Specifically, Mendel took spherical seed pea plants of an unknown genotype and crossed them with wrinkled seed pea plants, which were known to be homozygous recessive. Results showed that if the plant being tested was homozygous, all of the progeny would display the dominant spherical phenotype. By contrast, if the plant being tested was heterozygous, then 50 percent of the progeny would have spherical seeds while the other 50 percent would have wrinkled seeds. Mendel obtained the expected results, consistent with the hypothesis that a test cross can reveal the genotype of an individual. It is important to realize that in order for a test cross to accurately identify the genotype of an individual, it is essential that the “tester” be homozygous recessive for the allele that is to be determined. This rule can be examined by considering the possibility of what would happen if the tester plant in Mendel’s test cross experiment was homozygous for spherical instead of wrinkled seeds. If spherical seed pea plants of an unknown genotype were crossed with plants homozygous for spherical, all of the resulting progeny would display the dominant spherical phenotype regardless of their genotype. Thus, results from such a cross would be misleading and fail to reveal the genotype of the individual in question.

Original Paper

The original German version of Mendel’s paper, Versuche &UUML;ber Pflanzen-Hybriden, with an English translation and extensive explanatory notes, is available online:
http://www.mendelweb.org/Mendel.plain.html

Links

Koning, Ross E. 1994. Plant Physiology Information Website: Mendel’s Seed Shape Trials
http://plantphys.info/plants_human/seedshape.shtml

University of Arizona: The Biology Project: Mendelian Genetics: Problem 7: The Test Cross
http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/07t.html

Miko, I. 2008. Test crosses. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Test-Crosses-585

Miko, I. 2008. Gregor Mendel and the Principles of Inheritance. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Gregor-Mendel-and-the-Principles-of-Inheritance-593

McGuire, T. 2008. Gene Inheritance and Transmission Topic Room. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Gene-Inheritance-and-Transmission-Topic-Room-56822

Chial, H. 2008. Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Mendelian-Genetics-Patterns-of-Inheritance-and-Single-966

Miko, I. 2008. Genetic Dominance: Genotype–Phenotype Relationships. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Genetic-Dominance-Genotype-Phenotype-Relationships-489

Figure 8.13 Some Alleles Do Not Assort Independently

Morgan and colleagues at Columbia University used Drosophila as a model organism to study genetics. The researchers soon realized that some of the crosses did not follow the rule of independent assortment. Morgan crossed BbVgvg (phenotypically wild-type) flies with bbvgvg (black body, vestigial wings) flies and expected to get a phenotypic ratio of 1:1:1:1. Instead, however, he observed a phenotypic ratio of approximately 4:1:1:4. This departure from the ratios predicted by Mendel’s law can be attributed to linkage. Today, we know that the genes black and vestigial are on the same chromosome. The traits with which Mendel chose to work and publish were either on separate chromosomes, resulting in independent assortment, or very far apart on the same chromosome, essentially appearing to assort independently. However, if two of these traits were linked, he would not have obtained the 9:3:3:1 phenotypic ratio in the F₂ generation from a dihybrid cross. For example, if the genes for seed shape and seed color were linked, and he crossed SSYY plants to ssyy plants as in Figure 8.5, he would have still obtained SsYy plants in the F₁ generation. However, the only gametes produced by the plants in the F₁ generation would be either SY or sy. Assuming complete linkage of these alleles, all of the plants in the F₂ generation would be SsYy and, thus, phenotypically identical.

Original Papers

Morgan, T. H., and C. J. Lynch. 1912. The linkage of two factors in Drosophila that are not sex-linked. Biol. Bulletin 23: 174–182.
http://www.biolbull.org/cgi/reprint/23/3/174.pdf

Morgan, T. H. 1912. Complete Linkage in the Second Chromosome of the Male of Drosophila. Science 36: 719–720.
http://www.jstor.org/stable/1638106

Morgan, T. H. 1914. No crossing over in the male of Drosophila of genes in the second and third pairs of chromosomes. Biol. Bulletin 26: 195–204.
http://www.biolbull.org/cgi/reprint/26/4/195.pdf

Morgan, T. H., A. H. Sturtevant, H. J. Muller, and C. B. Bridges. 1915. The Mechanism of Mendelian Heredity. Henry Holt and Company: New York.
http://www.esp.org/books/morgan/mechanism/facsimile/title3.html

Links

Kimball’s Biology Pages: Genetic Linkage and Genetic Maps
http://home.comcast.net/~john.kimball1/BiologyPages/L/Linkage.html

Dr. Thomas H. Morgan: The Nobel Prize in Physiology or Medicine 1933
http://nobelprize.org/nobel_prizes/medicine/laureates/1933/morgan-bio.html

Thomas Hunt Morgan: The Fruit Fly Scientist
http://www.nature.com/scitable/topicpage/Thomas-Hunt-Morgan-The-Fruit-Fly-Scientist-6579789

Miko, I. 2008. Thomas Hunt Morgan and Sex Linkage. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Thomas-Hunt-Morgan-and-Sex-Linkage-452

Lobo, I. & Shaw, K. 2008. Discovery and Types of Genetic Linkage. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Discovery-and-Types-of-Genetic-Linkage-500

Lobo, I. & Shaw, K. 2008 Thomas Hunt Morgan, Genetic Recombination, and Gene Mapping. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Thomas-Hunt-Morgan-Genetic-Recombination-and-Gene-496

Shaw, K. & Miko, I. 2008. Chromosome Theory and the Castle and Morgan Debate. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/Chromosome-Theory-and-the-Castle-and-Morgan-456

Nature Publishing Group: Why didn't Gregor Mendel find linkage?
http://www.nature.com/nature/journal/v256/n5514/abs/256206a0.html

Columbia University: Thomas Hunt Morgan
http://www.columbia.edu/cu/alumni/Magazine/Morgan/morgan.html