Mendel’s Theory of Heredity

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Gregor Mendel conducted experiments breeding 28,000 pea plants between 1856 and 1863. He then developed a simple, ingenious theory predicting his results.

Scientific Theories

  • scientific theory is an axiom system
    • designed to explain certain kinds of phenomena
    • defined by its postulates
    • supported or disproved by its predictions

Big Picture

Experiments with Pea Plants

  • A Puzzle about Hybrids
    • The progeny of hybrids appear to become progressively like the original parents.
    • By crossbreeding, however, plant and animal breeders are able to produce new breeds.
  • Bear in Mind
    • In Mendel’s day, the mid 1800s, no one knew about genes, chromosomes, or DNA
  • Traits of pea plants Mendel experimented with:

Puzzling Results

  • First Experiment
    • Mendel developed pure, true-breeding plants of each pea color: a pure line of green-pea plants that, bred among themselves, produced only green-pea offspring; and a pure line of yellow-pea plants that produced only yellow-pea offspring. He then crossed the two lines.
    • The puzzling result was that the offspring’s peas were all yellow.
  • Second Experiment
    • Mendel self-pollinated the all-yellow-pea offspring of the first experiment
    • The result, even more puzzling, was a second generation of plants that was approximately 75% yellow-pea and 25% green-pea.
  • Observable Situation after the Experiments:


  • The Postulates use the German word Merkmale (murk-mall-uh)
    • In the paper announcing his results, Versuche über Pflanzen-Hybriden (1865), Mendel postulated the existence of what he called Merkmale, German for characteristics.
  • Determination Postulate
    • Inside every pea plant are two Merkmale, each either green-pea-producing or yellow-pea-producing. The Merkmale determine pea color as follows: 
      • if both Merkmale are green-pea-producing, the peas are green;
      • if both Merkmale are yellow-pea-producing, the peas are yellow;
      • if one Merkmale is green-pea-producing and the other yellow-pea-producing, the peas are yellow.
  • Inheritance Postulate
    • The Merkmale of a pea plant are inherited from the parent plants, one Merkmale from each parent; for each Merkmale of a parent plant the probability it’s inherited is ½.

Remember ….

  • In Mendel’s day no one knew about genes, chromosomes, or DNA
  • Mendel did not discover Merkmale or characteristics or factors or thingies or whatever you want to call them. He postulated their existence. He said, in effect:
    • Suppose each plant has two of these thingies and they work like the postulates describe.  Then we can derive the results of all the experiments.

Prediction using a Punnett Square and a Probability Tree

If each parent plant has Yg-Merkmale, Mendel’s postulates predict that the probability a child plant has yellow peas is ¾.

Punnett Square where each parent has Yg-Merkmale

Probability Tree where each parent has Yg-Merkmale

What’s the probability?

If one parent plant has gg-Merkmale and the other Yg-Merkmale, what’s the probability a child plant has yellow peas?

A Different kind of Prediction

  • Most of Mendel’s predictions predict the probability a child plant has an observable property (e.g. yellow peas) given the parent plants have certain Merkmale.
  • Here’s a different kind of prediction:
    • All offspring of plants with green peas likewise have green peas.
  • That’s because a pea plant with green peas has gg-Merkmale, per the Determination Postulate.  And given that each parent has gg-Merkmale, the probability a child plant has green peas = 1.


  • Mendel’s paper Experiments on Plant Hybrids was presented in 1865 and ignored until 1900.
    • Darwin’s Origin of Species was published in 1859.
  • In 1900 three scientists independently “rediscovered” Mendel’s experiments.
  • Mendel’s postulated Merkmale were discovered and called genes.