Mendel was the first to fashion, by means of a controlled pollination technique and careful statistical analysis of his results, a clear, analytic picture of heredity. His account of the experiments and his conclusions, published in 1866 (tr. Experiments in Plant Hybridization, 1926), were ignored during his lifetime. Rediscovered by three separate investigators (Correns, de Vries, and Tschermak) in 1900, Mendel's conclusions have become the basic tenets of genetics and a notable influence in plant and animal breeding.
Mendelism is the system of heredity formulated from Mendel's conclusions. Briefly summarized, as we understand it today by means of the science of genetics, the Mendelian system states that an inherited characteristic is determined by the combination of a pair of hereditary units, or genes, one from each of the parental reproductive cells, or gametes. In the body cells each pair of genes determines a particular hereditary characteristic (e.g., in the pea plant, a pair determining tallness or dwarfness).Mendel's First Law
The law of segregation (Mendel's first law) states that in the process of the formation of the gametes (see meiosis) the pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. Mendel found that when a pure strain of peas bearing one form of a gene (that is, a strain in which both members of the gene pair being studied are the same), inbred for many generations, was crossed with a pure strain carrying an alternative form of the gene, one of these forms consistently prevailed over the other in determining the visible characteristics of the offspring; he therefore termed the two forms dominant and recessive, and called the phenomenon itself the law of dominance. Given A as the dominant factor and a as the recessive, the offspring of the purebred strains having genes of the form AA and aa are hybrids, individuals each being Aa. When the hybrids are crossed, the offspring exhibit the characteristic in question in a ratio of three dominant to one recessive; i.e., the four possible combinations of the genes in Aa and Aa are AA, aA, Aa, and aa. By the same rule, when a hybrid is crossed with a purebred recessive (Aa with aa) the ratio is one to one. Breeders often use these ratios to trace the hybrid or purebred nature of the parent stock.Mendel's Second Law
The law of independent assortment (Mendel's second law) states that characteristics are inherited independently of each other; e.g., the dominant trait of yellow seed color in pea plants can appear in combination with either the dominant trait of plant tallness or the recessive trait of dwarfness. This law has been modified by the discovery of linkage in genetics.
See biography of Mendel by V. Ore (1984); see also R. C. Olby, The Origins of Mendelism (2d ed. 1985).
Gregor Johann Mendel (July 20, 1822 – January 6, 1884) was a German speaking Austrian Augustinian priest and scientist, and is often called the father of genetics for his study of the inheritance of traits in pea plants. Mendel showed that the inheritance of traits follows particular laws, which were later named after him. The significance of Mendel's work was not recognized until the turn of the 20th century. Its rediscovery prompted the foundation of the discipline of genetics.
Gregor Mendel, who is known as the "father of modern genetics", was inspired by both his professors at university and his colleagues at the monastery to study variation in plants, and he conducted his study in the monastery's garden. Between 1856 and 1863 Mendel cultivated and tested some 29,000 pea plants (i.e. Pisum sativum). This study showed that one in four pea plants had purebred recessive alleles, two out of four were hybrid and one out of four were purebred dominant. His experiments brought forth two generalizations which later became known as Mendel's Laws of Inheritance.
Mendel read his paper, "Experiments on Plant Hybridization", at two meetings of the Natural History Society of Brünn in Moravia in 1865. When Mendel's paper was published in 1866 in Proceedings of the Natural History Society of Brünn, it had little impact and was cited about three times over the next thirty-five years. His paper was criticized at the time, but is now considered a seminal work.
After Mendel completed his work with peas, he turned to experimenting with honeybees, in order to extend his work to animals. He produced a hybrid strain (so vicious they were destroyed), but failed to generate a clear picture of their heredity because of the difficulties in controlling mating behaviours of queen bees. He also described novel plant species, and these are denoted with the botanical author abbreviation "Mendel".
Elevated as abbot in 1868, his scientific work largely ended as Mendel became consumed with his increased administrative responsibilities, especially a dispute with the civil government over their attempt to impose special taxes on religious institutions.
At first Mendel's work was rejected, and it was not widely accepted until after he died. The common belief at the time was that Darwin's theory of pangenes were responsible for inheritance. The modern synthesis uses Mendelian genetics.
Mendel died on January 6, 1884, at age 61, in Brünn, Austria-Hungary (now Brno, Czech Republic), from chronic nephritis. Czech composer Leoš Janáček played the organ at his funeral. After his death, the following abbot burned all papers in Mendel's collection.
It was not until the early 20th century that the importance of his ideas was realized. In 1900, his work was rediscovered by Hugo de Vries and Carl Correns. Though Erich von Tschermak was originally also credited with rediscovery, this is no longer accepted because he did not understand Mendel's laws. Mendel's results were quickly replicated, and genetic linkage quickly worked out. Biologists flocked to the theory, even though it was not yet applicable to many phenomena, it sought to give a genotypic understanding of heredity which they felt was lacking in previous studies of heredity which focused on phenotypic approaches. Most prominent of these latter approaches was the biometric school of Karl Pearson and W.F.R. Weldon, which was based heavily on statistical studies of phenotype variation. The strongest opposition to this school came from William Bateson, who perhaps did the most in the early days of publicising the benefits of Mendel's theory (the word "genetics", and much of the discipline's other terminology, originated with Bateson). This debate between the biometricians and the Mendelians was extremely vigorous in the first two decades of the twentieth century, with the biometricians claiming statistical and mathematical rigor, whereas the Mendelians claimed a better understanding of biology. In the end, the two approaches were combined as the modern synthesis of evolutionary biology, especially by work conducted by R. A. Fisher as early as 1918.
Mendel's experimental results have later been the object of considerable dispute. Fisher analyzed the results of the F2 (second filial) ratio and found them to be implausibly close to the exact ratio of 3 to 1. Only a few would accuse Mendel of scientific malpractice or call it a scientific fraud — reproduction of his experiments has demonstrated the validity of his hypothesis — however, the results have continued to be a mystery for many, though it is often cited as an example of confirmation bias. This might arise if he detected an approximate 3 to 1 ratio early in his experiments with a small sample size, and continued collecting more data until the results conformed more nearly to an exact ratio. It is sometimes suggested that he may have censored his results, and that his seven traits each occur on a separate chromosome pair, an extremely unlikely occurrence if they were chosen at random. In fact, the genes Mendel studied occurred in only four linkage groups, and only one gene pair (out of 21 possible) is close enough to show deviation from independent assortment; this is not a pair that Mendel studied.