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Gregor Mendel and the discovery of genetics

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Where did the modern science of genetics come from? How were the basic rules developed?

The studies of Gregor Mendel

This section is from Basic Principles of Genetics: An Introduction to Mendelian Genetics, Dennis O’Neil, Behavioral Sciences Department, Palomar College, San Marcos, CA

For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids.

It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown.

Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

By the 1890’s, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction.

The focus of genetics research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children. A number of hypotheses were suggested to explain heredity, but Gregor Mendel, a little known Central European monk, was the only one who got it more or less right.

His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death. His early adult life was spent in relative obscurity doing basic genetics research and teaching high school Greek, mathematics, physics, in Brno (today, this town is in the Czech Republic.)

Historical aside: Today Brno is in the Czech Republic – but that nation didn’t exist at that time. Back then it was part of “The Margraviate of Moravia” – hey, I never heard of that either! Not a full-size nation, it was a marcher state existing from 1182 to 1918. – RK

Brno (now in the Czech Republic)

In his later years, he became the abbot of his monastery and put aside his scientific work.

While Mendel’s research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals.

Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.


Earliest scientific genetic experiment

How did Mendel cross-breed peas?

Plants with flowers have gendered sex-organs – male and female.

Mendel used this fact to breed plants as desired.

To keep things as simple as possible, Mendel decided to study traits that are easily recognized and easy to track.

These were traits that only occur in one of two forms:

1. flower color is purple or white
2. flower position is axil or terminal
3. stem length is long or short
4. seed shape is round or wrinkled
5. seed color is yellow or green
6. pod shape is inflated or constricted
7. pod color is yellow or green

Obviously he wasn’t studying the genetic of anything else in these flowers – and there’s a lot more going on inside plants. But this at least was a start.

This observation that these traits do not show up in offspring plants with intermediate forms was critically important

His results would soon disprove the then-leading idea:

Some scientists in the 19th century, including Charles Darwin, were considering a model called “pangenesis.” The idea was that hereditary molecules in our bodies are affected by what we do during our lifetime. These modified hereditary particles were thought to migrate via blood to the reproductive cells (sperm or eggs.) Subsequently these changed traits would be inherited by the next generation.

The hypothesis allowed for the possibility that a change to you could be inherited by your children.

In contrast, the “germ-plasm” hypothesis means that changes to you would not be inherited by your children.

It turned out that the germ-plasm concept is right. Today we don’t use the word “germ” – we call these particles “chromosomes”

The experiments

Pea plants can either self-pollinate (sex with one parent)

or cross-pollinate with another plant. (sex with two parents)

This illustrations shows how Mendel was able to selectively cross-pollinate purebred plants with particular traits

He then could observe the outcome over many generations.


Studying first and second generation results

From Ask-A-Biologist: Mendel’s Garden

Mendel controlled breeding by separating the male and female parts of the flowers so they couldn’t reproduce on their own.

Next, he used a small brush to move pollen between plants.

Pea plants had a number of visible traits, called phenotypes.

The inner pea color, for example, could be either green or yellow.

At first glance, pea plants might seem to have very little in common with animals or humans.

The way that genes and chromosomes work is similar in all living things. The same rules that determine how traits like pea color are passed down from parent to offspring, also determine how traits like freckles or dimples are passed down in humans.

Parent Generation

Mendel began his experiments with true breeding strains, meaning groups of plants that pass down only one phenotype to their offspring. These true breeding strains were created by mating plants with the same traits for many generations.

First Generation

Mendel mated two different true breeding strains together, a green pea strain and a yellow pea strain, to see what phenotype the first generation of offspring would have.

When Mendel looked at the offspring, called the F1 (or first) generation, he saw that every single one of the plants had yellow seeds.

Second Generation

Next, Mendel took the first generation plants and mated them with each other.

What color seeds would you expect the next generation to have?

To Mendel’s surprise, 25% of the offspring, called the F2 (or second) generation, actually had green seeds, even though all of the F1 parent plants had yellow seeds!

This result led Mendel to believe that it was possible for a trait to be present, but not visible, in an individual.

Something from the original green parent plants was skipping a generation and being passed to the grandchildren.

How is this possible? There must be something in cells that stores information – yet somehow this info isn’t always expressed. It was a great mystery at the time. We’ll be learning about these information storage molecules (genes) soon.


1. Where and when in history did Gregor Mendel live? What was his job?

2. Describe the sexual anatomy of plants with flowers.

3. There were 7 biological traits that Mendel studied in peas. Why did he choose these 7 traits?

4. Consider this sentence “This observation that these traits do not show up in offspring plants with intermediate forms was critically important” – Please explain – what does this sentence mean?

5. In complete sentences briefly explain the 2 different ideas about how traits could be inherited: pangenesis vs the germ-cell theory.

6. Looking at the step-by-step illustration, explain how Mendel cross-bred flowering plants.

7. What are phenotypes?

8a. Mendel mated a green pea strain and a yellow pea strain, to see what phenotype the first generation of offspring would have. When he looked at the offspring (F1 generation), what did he see?

8b. What might one logically conclude from this one experiment?

9a. In his next experiment Mendel mated 2 of these yellow pea pod plants together. What were his results?

9b. This result led Mendel to believe that  _____________________


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