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Samford University -- Department of Biological and Environmental Sciences
Genetics -- Biol 333

Mendel (Chapter 3 through the first two paragraphs of page 52)

Pre-Mendelian Concepts of Heredity: In order to understand the true significance of Mendel's contribution to the understanding of heredity, we need to have an idea of the mistaken ideas that were common in his day. (Note: the concept of heredity is not new; James used a genetic example). Pre-mendelian mistaken ideas include:
  • Spontaneous Generation: This widely held idea said that new life continually arises from non-living material. This idea is a denial of the concept of heredity. It was disproved by Redi and Pasteur.
  • Preformation: A tiny preformed human, a homunculus, is present in one of the gametes: if so heredity would come from one parent only.
  • The Inheritance of Acquired Characteristics: The idea that traits acquired during an organisms lifetime can be passed along to the offspring. One example is Lamarck's theory--Lamarckism.
  • The Blood as the Hereditary Material: The genetic material is in the blood (or sap) and crossing mixes the blood of two organisms.


Mendel: Gregor Mendel was an Augustinian Monk who became known as the "Father of Genetics." In the monastery garden, his experimental organism was the garden pea, which was available to him in many varieties, produced many offspring, and could be selfed (self-crossed) as well as crossed.
  • Monohybrid Cross: Mendel first crossed strains of peas differing for one trait (e.g.: round seeds x wrinkled cotyledons).
    • Results: When tall plants were crossed with dwarf plants (P generation), the next generation (F1 generation) were all tall like the P generation. When the F1 generation plants were selfed the result F2 generation consisted of 787 tall and 277 dwarf.
    • Conclusions: Mendel drew at least three conclusion from his monohybrid crosses.
      • Pairs of Factors: Each individual has a pair of factors for each trait.
      • Dominance: If an individual has two different factors, one will be expressed (the dominant one) and one will not (recessive one).
      • Segregation: When the individual produces gametes, the pair separates with one going to each gamete (Law of Segregation).
  • Terminology: Today, we used the following terms.
    • Gene, allele: The factors Mendel described are called gene. A pair of factors are alleles.
    • Dominant, recessive: When two different alleles are present, the expressed on is the dominant allele. The one that is not expressed is the recessive allele.
    • Genotype, phenotype: The genotype is the genetic make up, usually expressed by designating symbols for the genes (DD, Dd, and dd are examples of genotypes). The phenotype is the expression of the genotype ("what you see")(yellow seeded and green seeded are examples of phenotypes).
    • Homozygous, heterozygous: An individual with two identical alleles is homozygous. An individual with two different alleles is heterozygous. (AA is homozygous dominant, aa is homozygous recessive.)
    • Reciprocal cross, test cross, back cross: A cross in which the sex of two parents is reversed (not a sex-change operation) is a reciprocal cross. (Tall female x dwarf male is the reciprocal cross when compared to dwarf female x tall male.)(Mendel did reciprocal crosses for all his crosses.) A test cross is a cross to a homozygous individual (e.g., Aa x aa), usually done to determine the genotype of an unknown individual. A back cross is a cross to an individual with the same genotype as one of the parents.
QUIZ: What are these?

  • Dihybrid Cross: Mendel next crossed pea plants that differed with respect to two traits (e.g. yellow-, round-seeded plants x green-, wrinkled-seeded plants).
    • Results: When yellow-seeded, round-seeded plants were crossed with green-seeded, wrinkled-seeded plants, the F1 were all yellow seeded and round seeded. When the F1 were selfed, the F2 were: yellow, round = 315; yellow, wrinkled = 108; green, round = 101; green, wrinkled = 32.

  • Conclusions: Mendel drew one conclusion from his dihybrid crosses.
    • Independent Assortment: The segregation of one pair of alleles is independent of the segregation of another pair of alleles (Law of Independent Assortment).
Correns, deVries, Tschermak: In 1900, these three discovered Mendel's article and made his work known to the scientific world. (Revision of the early history of genetics: Two Tschermaks)

Mendel's Data:
When Mendel's data is carefully analyzed, it looks too good. Did he fudge his data?
Answer to Quiz.
Thought of the Day: How is it one careless match can start a forest fire, but it takes a whole box to start a campfire?