Mendel’s Theory of Heredity

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Table of Contents

  1. Introduction
  2. Scientific Theories
  3. Big Picture Graphic
  4. Experiments with Pea Plants
  5. Puzzling Results
  6. Postulates of Mendel’s Theory
  7. Remember, in Mendel’s Day
  8. Predictions using Punnett Squares and Probability Trees
  9. Exercise: What’s the probability?
  10. A Different kind of Prediction
  11. Postscript


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 Graphic

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:

Postulates of Mendel’s Theory

  • 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

  • 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.

Predictions using Punnett Squares and Probability Trees

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

Exercise: 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.