Gregor Mendel and the discovery of genetics
Text from BASIC PRINCIPLES OF GENETICS: An Introduction to Mendelian Genetics
Dr. Dennis O’Neil, Behavioral Sciences Department, Palomar College, San Marcos, California
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 mathematics, physics, and Greek in 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.
-> the mechanisms of heredity are essentially the same for all complex life forms.
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.
How did Mendel selectively cross-breed peas?
Plants with flowers have gendered sex-organs (male and female parts)
He mated one type of plant with another.
For instance, the pea flowers are either purple or white – intermediate colors do not appear in the offspring of cross-pollinated pea plants.
Mendel observed seven traits that are easily recognized and apparently 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
This observation that these traits do not show up in offspring plants with intermediate forms was critically important
-> because the leading theory in biology at the time was that inherited traits blend from generation to generation.
Most of the leading scientists in the 19th century accepted this “blending theory.”
Charles Darwin proposed another equally wrong theory known as “pangenesis”
This held that hereditary “particles” in our bodies are affected by the things we do during our lifetime. These modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation.
This was essentially a variation of Lamarck’s incorrect idea of the “inheritance of acquired characteristics.”
2 Hypotheses about inheritance – 1800s to early 1900s
The pangenesis hypothesis allowed for the possibility that a change to you, could be inherited by your children.
The germ-plasm hypothesis means that changes to you, could 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”
Mendel picked common garden pea plants for the focus of his research:
they can be grown in large numbers. Their reproduction can be manipulated.
Pea plants have both male and female reproductive organs.
As a result, they can either self-pollinate themselves or cross-pollinate with another plant.
In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits, and observe the outcome over many generations.
This was the basis for his conclusions about the nature of genetic inheritance.
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.
But the way that genes and chromosomes work is extremely 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.
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.
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.
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.
Mendel repeated this experiment with many different characteristics. He tested inner pea color, outer pea color, pea shape, flower position, stem length, unripe pod color, and pod shape.
He had similar results every single time.
How is this possible?
Let’s take a closer look at what’s happening on a genetic level with the help of a Punnett Square.
Idea: There must be some chemical in the cells that stores this information
Mendel and other scientists of his time had no idea what this molecules was
In the mid 20th century, scientists first hypothesized that this information was stored in proteins.
That turned out to be incorrect.
Then they hypothesized that this information was stored in nucleic acids, in the chromosomes.
That turned out to be correct.