Concept

Over the centuries, people observed that some traits, such as brown eyes, are more common than others. Less common traits, such as blue eyes, sometimes skip a generation. Mendel saw the same patterns in peas. In a field, yellow peas are more common than green. Cross-pollination of pure-bred yellow and pure-bred green peas produces only yellow offspring. Although these hybrid offspring must have received a different form of color gene from each parent, there is no blending of color. Rather, the gene alternate for yellow is "dominant" over the gene alternate for green — the dominant trait is shown whenever a single copy of its gene is inherited. When he crossed the hybrid offspring, some green seeds reappeared in the following generation. Mendel reasoned that the "recessive" green trait is shown only when a copy of the recessive gene form is inherited from each parent.

Animation

I crossed pure-bred green with pure-bred yellow peas. Surprisingly, all of the offspring were yellow! The green phenotype had disappeared. I allowed the yellow pea plants to self fertilize to see what the second generation would look like. Some of the second generation offspring were green! After performing many experiments, I figured out the basic rules of inheritance and why the green color reappeared. Remember I found that pea color is controlled by one gene, which has a "green" form and a "yellow" form. Each form is called an allele. When a pure-bred green plant is crossed with a pure-bred yellow plant, the offspring inherit one allele of the color gene from each parent. Plants with two different alleles of a gene are heterozygous. Since all the heterozygous offspring are yellow, then the yellow allele must be dominant over the recessive green allele. The green allele didn't disappear; its effect is not seen in heterozygotes. Peas are yellow when they have either two yellow alleles . . . . . . or one yellow allele and one green allele. Green peas must have two copies of the recessive green allele. Now, let's see how the genes sort out when we cross two heterozygous plants. When two heterozygous plants are crossed, the resulting progeny mostly show the dominant phenotype. But, the recessive phenotype reappears.

Gallery

In 1910, this statue of Mendel was erected "by the friends of science." It orginally stood in Mendel Square in Brno. The statue now stands in the garden at the Mendelianum, the Mendel Museum, in Brno.

Medal struck to honor Mendel's centennial--front.

Audio/Video

Audio Glossary

Allele, Dominant, Genotype, Heterozygous, Homozygous, Phenotype, Recessive

Video Interviews

Robert Olby

Professor of History Robert Olby teaches at the University of Pittsburgh, and has written several books on the history of genetics, including Origins of Mendelism and The Path to the Double Helix.

Clip 1 (0:45)
To whom was Mendel's paper distributed?

Clip 2 (0:40)
Comparing Mendel to Doppler: the need to widely publicize findings.

Clip 3 (0:30)
Was Mendel disappointed that his paper was not widely read?

Biography

JOHANN GREGOR MENDEL (1822-1884)

The Unappreciated

Gregor Mendel wasn't the only one with an interest in heredity, and he wasn't the first to work with plants. So why were his results almost unknown until 1900 and the rediscovery of the laws of inheritance?

The common assumption is that Mendel was a monk working alone in a scientifically isolated atmosphere. His work was ignored because it was not widely distributed, and he didn't make an effort to promote himself. In actual fact, the reasons are more complex.

Mendel was part of the social and scientific circle of the time. He attended the University of Vienna, and came into contact with many prominent scientists. He had opportunities to travel to and attend scientific conferences. His paper, when published in 1865 in The Proceedings of the Brünn Natural Science Society, was exchanged with the publications of at least 120 other associations and societies, and was available in many libraries and scientific institutes. In addition, Mendel sent out 40 reprints to some of the most famous botanists at the time.

Through correspondence with another leading botanist, Carl Nägeli, Mendel did spark an interest but Nägeli was working on hawkweed, a plant that unbeknownst to him reproduced sexually and asexually. Nägeli convinced Mendel to do further hybridization experiments with this plant. Mendel was unable to prove his laws of inheritance using hawkweed as a model, and so abandoned the effort. From his letters, Mendel appeared to be a humble person. The fact that he failed with hawkweed would have cast doubt into his mind as to whether his Laws were truly "fundamental" to all living things. Nägeli, of course, would see that Mendel's results didn't apply to hawkweed, and so they must be incorrect.

Mendel also did present his data in a way that was atypical for a botanist. The natural sciences were traditionally more of a descriptive one where details were noted for their use in classification purposes. Botanists at the time certainly counted hybrid offspring, but they did so more to note yields than to connect the numbers with trait inheritance. Mendel's mathematical ratio of inheritance was probably not understood by many botanists. Subsequent mentions of Mendel's results usually focused on the hybrids that Mendel made, ignoring the mathematics altogether.

As is often the case in science and in other areas, a topic captures the imagination and overshadows everything else. In the 1860's the hot topic was Charles Darwin's theory of evolution. The controversy generated over that theory made it easy to overlook a pea plant study. Ironically, it was the examination of how variations are inherited during the course of evolution that led to the rediscovery of Mendel's laws in 1900.

Links

Blazing a Genetic Trail

This is the Howard Hughes Medical Institute site on genetics and molecular biology. It has a section on dominant and recessive inheritance in humans.

Bibliography

Bowler, Peter J., 1989, The Mendelian Revolution, The Athlone Press, London.
Dunn, L. C., 1965, A Short History of Genetics, McGraw-Hill, Inc., New York.
Iltis, Hugo, 1932, Life of Mendel, W. W. Norton & Company, Inc., London.
Moore, John A., 1985, Science as a Way of Knowing, American Society of Zoologists.
Olby, Robert C., 1966, Origins of Mendelism, Constable and Company Ltd., London.
Orel, Vitezslav, 1996, Gregor Mendel, the First Geneticist, Oxford University Press, Oxford.
Portugal, Franklin H., and Cohen, Jack S., 1977, A Century of DNA: A History of the Structure and Function of the Genetic Substance, The Massachusetts Institute of Technology, Cambridge, Massachusetts.
Stern, Curt, and Sherwood, Eva R., ed., 1966, The Origin of Genetics: A Mendel Source Book, W. H. Freeman and Company, San Francisco.
Stubbe, Hans, 1972 (English Translation), History of Genetics, The Massachusetts Institute of Technology, Cambridge, Massachusetts.
Sturtevant, A. H., 1965, A History of Genetics, Harper & Row, Publishers, New York.

Glossary

Allele - One of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variation in inherited characteristics such as hair color or blood type. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one).
Dominant - A gene that almost always results in a specific physical characteristic, for example, a disease, even though the patient's genome possesses only one copy. With a dominant gene, the chance of passing on the gene (and therefore the disease) to children is 50-50 in each pregnancy.
Genotype - The genetic identity of an individual, which may not show as outward characteristics.
Heterozygous - Possessing two different forms of a particular gene, one inherited from each parent.
Homozygous - Possessing two identical forms of a particular gene, one inherited from each parent.
Phenotype - The observable traits or characteristics of an organism, for example hair color, weight, or the presence or absence of a disease. Phenotypic traits are not necessarily genetic.
Recessive - A genetic disorder that appears only in patients who have received two copies of a mutant gene, one from each parent.

Children resemble their parents.
Genes come in pairs.
Genes don't blend.
Genetic inheritance follows rules.
Genes are real things.
All cells arise from pre-existing cells.
Sex cells have one set of chromosomes; body cells have two.
Specialized chromosomes determine gender.
Chromosomes carry genes.
Genes get shuffled when chromosomes exchange pieces.
Evolution begins with the inheritance of gene variation.
Mendelian laws apply to human beings.
Mendelian genetics cannot fully explain human health and behavior.
DNA and proteins are the molecules of the cell nucleus.
One gene makes one protein.
A gene is made of DNA.
Bacteria and viruses have DNA too.
The DNA molecule is shaped like a twisted ladder.
A half DNA ladder is a template for copying the whole.
RNA is an intermediary between DNA and protein.
DNA words are three letters long.
A gene is a discrete sequence of DNA nucleotides.
The RNA message is sometimes edited.
Some viruses store genetic information in RNA.
RNA was the first genetic molecule.
Mutations are changes in genetic information.
Some types of mutations are automatically repaired.
A chromosome is a package for DNA.
Higher cells incorporate an ancient chromosome.
Some DNA does not encode protein.
Some DNA can jump.
Genes can be turned on and off.
Genes can be moved between species.
DNA responds to signals from outside the cell.
Different genes are active in different kinds of cells.
Master genes control basic body plans.
Development balances cell growth and death.
A genome is an entire set of genes.
Living things share common genes.
DNA is only the starting point for understanding human biology.
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