“Pea hybrids form germinal and pollen cells that in their composition correspond in equal numbers to all the constant forms resulting from the combination of traits united through fertilization.”
Gregor Johann Mendel was born on July 22, 1822 to peasant parents in a small agrarian town in Czechoslovakia. During his childhood he worked as a gardener, and as a young man attended the Olmutz Philosophical Institute. In 1843 he entered an Augustinian monastery in Brunn, Czechoslovakia. Soon afterward, his natural interest in science and specifically hereditary science led him to start experiments with the pea plant. Mendel's attraction for scientific research was based on his love of nature in general. He was not only interested in plants, but also in meteorology and theories of evolution. However, it is his work with the pea plant that changed the world of science forever.
His beautifully designed experiments with pea plants were the first to focus on the numerical relationships among traits appearing in the progeny of hybrids. His interpretation for this phenomenon was that material and unchanging hereditary “elements” undergo segregation and independent assortment. These elements are then passed on unchanged (except in arrangement) to offspring thus yielding a very large, but finite number of possible variations.
Mendel often wondered how plants obtained atypical characteristics. On one of his frequent walks around the monastery, he found an atypical variety of an ornamental plant. He took it and planted it next to the typical variety. He grew their progeny side by side to see if there would be any approximation of the traits passed on to the next generation. This experiment was “designed to support or to illustrate Lamarck's views concerning the influence of environment upon plants.” He found that the plants' respective offspring retained the essential traits of the parents, and therefore were not influenced by the environment. This simple test gave birth to the idea of heredity.
Overshadowing the creative brilliance of Mendel's work is the fact that it was virtually ignored for 34 years. Only after the dramatic rediscovery of Mendel’s work in 1900 (16 years after Mendel's death) was he rightfully recognized as the founder of genetics.1
Mendel was well aware that there were certain preconditions that had to be carefully established before commencing investigations into the inheritance of characteristics. The parental plants must be known to possess constant and differentiating characteristics. To establish this condition, Mendel took an entire year to test “true breeding” (non-hybrid) family lines, each having constant characteristics. The experimental plants also needed to produce flowers that would be easy to protect against foreign pollen. The special shape of the flower of the Leguminosae family, with their enclosed styles, drew his attention. On trying several from this family, he finally selected the garden pea plant (Pisum sativum) as being most ideal for his needs. Mendel also picked the common garden pea plant because it can be grown in large numbers and its reproduction can be manipulated. As with many other flowering plants, pea plants have both male and female reproductive organs. As a result, they can either self-pollinate themselves or cross-pollinate with other plants. 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.3
Mendel observed seven pea plant traits that are easily recognized in one of two forms:
1. Flower color: purple or white
2. Flower position: axial or terminal
3. Stem length: long or short
4. Seed shape: round or wrinkled
5. Seen color: yellow or green
6. Pod shape: inflated or constricted
7. Pod color: yellow or green
Mendel's Law of Segregation
Mendel's hypothesis essentially has four parts. The first part or “law” states that, “Alternative versions of genes account for variations in inherited characters.” In a nutshell, this is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. For example, each pea plant has two genes that control pea texture. There are also two possible textures (smooth and wrinkled) and thus two different genes for texture.
The second law states that, “For each character trait (ie: height, color, texture etc.) an organism inherits two genes, one from each parent.” This statement alludes to the fact that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same (true-breeding organisms), or different (hybrids).
The third law, in relation to the second, declares that, “If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance.”
The fourth law states that, “The two genes for each character segregate during gamete production.” This is the last part of Mendel's generalization. This references meiosis when the chromosome count is changed from the diploid number to the haploid number. The genes are sorted into separate gametes, ensuring variation. This sorting process depends on genetic “recombination.” During this time, genes mix and match in a random and yet very specific way. Genes for each trait only trade with genes of the same trait on the opposing strand of DNA so that all the traits are covered in the resulting offspring. For example, color genes do not trade off with genes for texture. Color genes only trade off with color genes from the opposing allelic sight as do texture genes and all other genes. The result is that each gamete that is produced by the parent is uniquely different as far as the traits that it codes for from every other gamete that is produced. For many creatures, this available statistical variation is so huge that in all probability, no two identical offspring will ever be produced even given trillions of years of time.
So, since a pea plant carries two genes, it can have both of its genes be the same. Both genes could be “smooth” genes or they could both be “wrinkled” genes. If both genes are the same, the resulting pea will of course be consistent. However, what if the genes are different or “hybrid”? One gene will then have “dominance” over the other “recessive” gene. The dominant trait will then be expressed. For example, if the smooth gene (A) is the dominant gene and the wrinkle gene (a) is the recessive gene, a plant with the “Aa” genotype will produce smooth peas. Only an “aa” plant will produce wrinkled peas. For instance, the pea flowers are either purple or white. Intermediate colors do not appear in the offspring of these cross-pollinated plants.
The observation that there are inheritable traits that do not show up in intermediate forms was critically important because the leading theory in biology at the time was that inherited traits blend from generation to generation (Charles Darwin and most other cutting-edge scientists in the 19th century accepted this “blending theory.”). Of course there are exceptions to this general rule. Some genes are now known to be “incompletely dominant.” In this situation, the “dominant gene has incomplete expression in the resulting phenotype causing a “mixed” phenotype. For example, some plants have “incomplete dominant” color genes such as white and red flower genes. A hybrid of this type of plant will produce pink flowers. Other genes are known to be “co-dominant” were both alleles are equally expressed in the phenotype. An example of co-dominant alleles is human blood typing. If a person has both “A” and “B” genes, they will have an “AB” blood type. Some traits are inherited through the combination of many genes acting together to produce a certain effect. This type of inheritance is called “polygenetic.” Examples of polygenetic inheritance are human height, skin color, and body form. In all of these cases however, the genes (alleles) themselves remain unchanged. They are transmitted from parent to offspring through a process of random genetic recombination that can be calculated statistically. For example, the odds of a dominant trait being expressed over a recessive trait in a two-gene allelic system where both parents are hybrids are 3:1. If only one parent is a hybrid and the other parent has both dominant alleles, then 100% of the offspring will express the dominant trait. If one parent has both recessive alleles and the other parent is a hybrid, then the offspring will have a phenotypic ratio of 1:1.
Mendel's Law of Independent Assortment
The most important principle of Mendel's Law of Independent Assortment is that the emergence of one trait will not affect the emergence of another. For example, a pea plant's inheritance of the ability to produce purple flowers instead of white ones does not make it more likely that it would also inherit the ability to produce yellow peas in contrast to green ones. Mendel's findings allowed other scientists to simplify the emergence of traits to mathematical probability (While mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with two traits showed 9:3:3:1 ratios).
Mendel was so successful largely thanks to his careful and nonpassionate use of the scientific method. Also, his choice of peas as a subject for his experiments was quite fortunate. Peas have a relatively simple genetic structure and Mendel could always be in control of the plants' breeding. When Mendel wanted to cross-pollinate a pea plant he needed only to remove the immature stamens of the plant. In this way he was always sure of each plants' parents. Mendel made certain to start his experiments only with true breeding plants. He also only measured absolute characteristics such as color, shape, and texture of the offspring. His data was expressed numerically and subjected to statistical analysis. This method of data reporting and the large sampling size he used gave credibility to his data. He also had the foresight to look through several successive generations of his pea plants and record their variations. Without his careful attention to procedure and detail, Mendel's work could not have had the same impact that is has made on the world of genetics.
Mendel and Darwin
Mendel's ideas on heredity and evolution were diametrically opposed to those of Darwin and his followers (although neither Mendel nor Darwin knew of the other’s work).2 Darwin believed in the inheritance of acquired characters. This led him to his famous theory of continuous evolution. Mendel, in contrast, rejected both the idea of inheritance of acquired characters (mutations) as well as the concept of continuous evolution. The laws discovered by him were understood to be the laws of constant elements for a great but finite variation, not only for cultured varieties but also for species in the wild.3 In his short treatise, Experiments in Plant Hybridization, Mendel incessantly speaks of "constant characters", "constant offspring", "constant combinations", "constant forms", "constant law", "a constant species" etc. (in such combinations the adjective "constant" occurs 67 times in his original paper). He was convinced that the laws of heredity he had discovered corroborated Gärtner's conclusion "that species are fixed with limits beyond which they cannot change". And as Dobzhansky aptly put it, "It is...not a paradox to say that if someone should succeed in inventing a universally applicable, static definition of species, he would cast serious doubts on the validity of the theory of evolution".
As the Darwinians won the battle for the minds in the 19th century, no space was left in the next decades for the acceptance of the true scientific laws of heredity discovered by Mendel. Further work in genetics was continued mainly by Darwin's critics. In agreement with de Vries, Tschermak-Seysenegg, Johannsen, Nilsson, et al., Bateson stated:
“With the triumph of the evolutionary idea, curiosity as to the significance of specific differences was satisfied. The Origin was published in 1859. During the following decade, while the new views were on trial, the experimental breeders continued their work, but before 1870 the field was practically abandoned. In all that concerns the species the next thirty years are marked by the apathy characteristic of an age of faith. Evolution became the exercising-ground of essayists. The number indeed of naturalists increased tenfold, but their activities were directed elsewhere. Darwin's achievement so far exceeded anything that was thought possible before, that what should have been hailed as a long-expected beginning was taken for the completed work. I well remember receiving from one of the most earnest of my seniors the friendly warning that it was waste of time to study variation, for "Darwin had swept the field.”” 4
The general acceptance of Darwin's theory of evolution and his ideas regarding variation and the inheritance of acquired characters are, in fact, the main reasons for the neglect of Mendel's work, which (in clear opposition to Darwin) pointed to an entirely different understanding of the questions involved.1
1. Genetics 131: 245-253, 1992.
2. Callender, L. A., Gregor Mendel: An opponent of descent with modification. History of Science 26: 41-75. 1988.
3. Mendel, Gregor. Experiments in Plant Hybridization. 1865.
4. Bateson, W. Mendel's Principles of Heredity. Cambridge: Cambridge University Press, 1909.