Gregor Mendel: The Pea Plant Experiment

Although the influence of heredity has been recognized since prehistoric times, scientific understanding of inheritance is a fairly recent event. Modern genetics begins with the work of Gregor Mendel, an Austrian monk whose breeding experiments with garden peas led him to formulate the basic laws of heredity.

Mendel published his findings in 1866, but his discoveries were ignored till 1900 when a number of researchers independently rediscovered Mendel's work and grasped its significance.

Mendel’s Pea Experiment

The Pea Plant Experiment
Dominant and recessive phenotypes. (1) Parental generation. (2) F1 generation. (3) F2 generation.

In one experiment, Mendel cross-pollinated smooth yellow pea plants with wrinkly green peas. (The organisms that are used as the original mating in an experiment are called the parental generation and are marked by P in science textbooks). Every single pea in the first generation crop (marked as f1) was as yellow and as round as was the yellow, round parent. Somehow, yellow completely dominated green and round dominated wrinkly.

Mendel learned from this that there are two kinds of traits - dominant and recessive. In this case, the dominant traits are the yellow color and the round shape since they show up at the expense of the green color and the wrinkly shape. He also learned that the inheritance of each trait is determined by "units" or "factors" - now called genes.

Now he went on with his experiment and planted seeds from the all-yellow, all-round crop, achieved from the parent generation, and self-pollinated the grown up plants. The results led to some surprises. Most of the second generation (marked as f2) of peas were yellow and smooth, but some were green or wrinkly. Mendel repeated his experiment many times and the f2 generation consistently had a 3:1 ratio of yellow to green and round to wrinkly. How did Mendel explain his results?

As mentioned above, Mendel postulated that there are dominant and recessive traits in heredity.
In his experiment Mendel marked with capital letters dominant traits and with small letters recessive traits.

When the f1 plants breed, each has an equal chance of passing on either Y or g units to each offspring (f2 generation), and the same is with R and w.

In the pollination process the offsprings get one trait unit (gene) from every parent; in the case of color the possible combinations are gY Yg YY gg. Three combinations from the four - gY Yg YY - will give yellow offsprings since all of them possess at least one Y dominant unit. Only one combination from the four - gg - will give green offsprings since it has two recessive units.

Because, in order to show-up, a dominant trait needs only one trait unit from one of the parents, and the recessive one needs two, from both parents, in order to prevail, that is the reason why the ratio between occurrences of dominant traits and recessive traits is 3:1. The same explanation applies to the shape traits.

Try this interactive experiment where you can breed your own hybrid pea plants.

Mendel came to three important conclusions from these experimental results:

That the inheritance of each trait is determined by "units" or "factors" (now called genes) that are passed on to descendents unchanged
That an individual inherits one such unit from each parent for each trait
That a trait may not show up in an individual but can still be passed on to the next generation.

Repeat Mendel’s Pea Plant Experiment

For his Experiments, Mendel chose pea plants because they had some distinctive measurable traits and being easy to breed with a short breeding period - some cultivars reach maturity about 60 days after planting.

He measured seven pea characteristics:

Color and smoothness of the seeds - grey and round or white and wrinkled
Color of the cotyledons (part of the embryo within the seed) - yellow or green
Color of the flowers - white or violet
Shape of the pods - full or constricted
Color of unripe pods - yellow or green
Position of flowers and pods on the stems
Height of the plants - short or tall

Mendel’s actual experiments are more complicated than described above. For example, in 1936, the statistician R.A. Fisher used a chi-square test to analyze Mendel's data and concluded that Mendel's results with the predicted ratios were far too perfect, indicating that adjustments (intentional or unconscious) had been made to the data to make the observations fit the hypothesis. Later authors have claimed Fisher's analysis was flawed, proposing various statistical and botanical explanations for Mendel's numbers. It is also possible that Mendel's results are "too good" merely because he reported the best subset of his data — Mendel mentioned in his paper that the data was from a subset of his experiments (Wikipedia).
Fairbanks, D. J.; Rytting, B. (01 May 2001). "Mendelian Controversies: A Botanical and Historical Review". American Journal of Botany 88 (5): 737.

Before you begin, consult the link section of this page and the further resources provided. The list begins with basics and ends with more advanced resources including Mendel's writings himself. Ensure that you understand the basic principals. Surf the web further and consult your local library, your teachers and other knowledgeable adults and experts.

Links

Mendel's Pea Plant Experiments
Mendel's Genetics - Dennis O'Neil
Law Of Segregation - The Mendel Pea Plant Experiment - experiment-resources.com
Gregor Mendel: Planting the Seeds of Genetics - Field Museum
Mendel: Experiment 1 - Access Excellence
Mendelian Inheritance - Wikipedia
Mendelian Genetics-Introduction - Tawnya S. Brown, Angela W. Tung
Intro to Genetics - M.J. Farabee
Modeling Mendel's Pea Experiment - Access Excellence
Experiments in Plants Hybridization (1865) by Gregor Mendel - ESP
Experiments in Plant Hybridization (1865) by Gregor Mendel - MendelWeb
Mendel's experiments - National Center for Biotechnology Information (NCBI)
The MendelWeb Reference Page
Revision of Fisher's Analysis of Mendel's Garden Pea Experiments - Charles E. Novitski
Mendel Experiments - Indiana University

Mendel's Biographies
Gregor Mendel - Wikipedia
Gregor Mendel - Access Excellence
Mendel in Darwin's Shadow - David Allen
Gregor Mendel - Strange Science

Genetics Science Fair Projects & Experiments
Genetics and Genomics Science Fair Projects
Plant Genetics Science Fair Projects
Genetics Experiments

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Mendel’s Pea Experiment

Repeat Mendel’s Pea Plant Experiment

Links

Gregor Mendel Books