Gregor Mendel - SpeedyLook encyclopedia

Johann Gregor Mendel , Monk and botanist Autrichien (July 22nd 1822 - January 6th 1884) is commonly recognized like the founding father of the Génétique. It is in the beginning of what is today called the Lois of Mendel, which define the way in which the Gène S are transmitted generations in generations.

Biography

Johann Gregor Mendel is born the July 22nd 1822 in Heinzendorf (Hynčice, district of Nový Jičín), small village of Moravie, in a family of peasants. Endowed for the studies, the young boy is very quickly noticed by the priest of the village which decides to send it to continue its studies far from at his place. In 1840, it joined the Institute of philosophy of Olomouc in order to follow two years preparatory there to the entry to the University. In September 1843, Mendel is received with the noviciate of the monastery of Brno; it will be ordered priest in 1848. As of its arrival with the monastery, Mendel feels all that a cultural medium particularly stimulant can bring to its aspirations. It devotes all its spare time to the study of the natural science. In parallel, it ensures of the scientific lesson in the colleges and colleges of the surroundings.

Mendel leaves in 1851 to follow the courses of the Institute of physics of Christian Doppler; it studies there, in addition to the obligatory matters, the Botanique, vegetable physiology, the Entomologie, the Paléontologie. During two years, it acquires all the methodological bases which will enable him to carry out its experiments later. During its stay with Vienna, Mendel is brought to be interested in the theories of Franz Unger, professor of vegetable physiology. This one recommends the experimental study to include/understand the appearance of the new characters at the plants during successive generations. He hopes thus to solve the problem raised by the Hybridation at the plants.

Of return to the monastery, Mendel installs an experimental garden in the court and sets up an experimental design aiming at explaining the laws of the origin and of the formation of the Hybride S. After ten years of meticulous work, Mendel thus posed the theoretical bases of the genetics and modern heredity. In 1868, Mendel is elected higher of its convent. Obliged to devote much of its time to the duties of its load, it gives up its research very thorough on the hybridization of the plants. It is invested then in other fields more compatible with its obligations, in particular the Horticulture and the Apiculture. It also impassions for the Météorologie which will be the field that it will have longest studied, of 1856 until its death in 1884.

Laws of Mendel

First law: Uniformity of the hybrid in first generation
Second law: Segregation of the characters in second generation
Third law: Disjunction independent of characters in F2

Former knowledge

Work, before Mendel, to try to include/understand the mechanisms of heredity was a failure. The reason is that the hybrideurs worked as they had always worked i.e. by tests and errors. They crossed individuals presenting of the different characters and chose in the descent those which corresponded best to the desideratas. However these procedures, very effective in addition in selection since the prehistoric origin of the breeding and agriculture, did not allow a predictivity of the results and thus the statement of laws.

The whole of the scientific community of the time supported the model of the Hérédité by mixture where the characters had by an individual were intermediate between those of these two parents (the crossing of a white relative and a black relative giving for example a gray or white and black individual).

Methods

Mendel will choose the parents in a different way. First of all it adopts like experimental model the garden peas ( Pisum sativum ), plants with flowers whose natural reproduction is done by Autofécondation, making it possible to control the Hybridation and to quickly produce a great number of descendants.

It chooses to study the edible pea heredity presenting seven characters of which each one can be found in two alternative forms, easily identifiable:

Form and color of seed, color of the envelope, forms and color of the pod, position of the flowers and length of the stem.

the first experiment which it will describe in its article consists in only studying the results of hybridization obtained for one of the pairs of characters. For example, the “shape of the pea” (phenotypical character governs by only one Gène) which exists according to two alternatives: smooth seed or wrinkled seed (phenotypical expression of each of the 2 Allèle S of gene that Mendel names factor).

the peas reproducing naturally by autofecondation, it thus manages to select pure Lignées of which all the individuals always have the same alternative form, that is to say a parental line with smooth seeds (which one will call P1, for the continuation of the reasoning) and the other with wrinkled seeds (that one will call P2). They are thus individuals Homozygote S for gene considered, they have one type of allele.
the crossing is done by depositing Pollen of a flower of the P1 line on the Pistil of a flower of the P2 line (to which it had removed the cheesecloth S to avoid all risks of autofecondation). It takes the care to carry out reciprocal fecundations (pollen of P2 on pistil of P1) to see whether the results are identical.
the individuals obtained by crossing of P1 and P2 are thus hybrids (which one notes usually F1). A second generation called F2 is produced by natural reproduction (autofecondation) of F1.
It studies successively pure lines differing by only one character (monohybridism) then two (dihybridism) and finally three (trihybridism).

Results

For the totality of the studied characters, 100% of the hybrids obtained are identical. For example, the crossing of a pea with smooth seeds (P1) and of a pea with wrinkled seeds (P2) always gives a F1 generation where all the individuals are peas with smooth seeds. The factor “seeds wrinkled” is thus recessive compared to the factor “seeds smooth” (which is described as dominating). It is the first law of Mendel known as of uniformity of the hybrids of first generation.
In F2 (generation obtained by autofecondation of F1), one can show by experiments of Crossing-test, the existence of three different genotypes:

50% of hétérozygotes (an allele dominating associated with a recessive allele) identical to the parents (F1 = hybrid),
25% of homozygotes dominant, phenotype identical to that of F1,
25% of homozygotes recessive of phenotype different from that of F1.

It is the second law of Mendel or law of disjunction of the alleles which is the result of the Méiose.

In dihybridism, the composite distribution of the 2 characters (four phenotypes) is the combination of two independent monohybridic distributions 3/4 “ and 1/4 ”

that is to say 9/16 “ 3/16 ” 3/16 “ 1/16 ” .

It is the third law of Mendel known as of independence of the characters which is not applicable to dependant genes.

The results of trihybridisms (8 phenotypes) are predicted easily: 27 “ 9 ” 9 “ 3 ” 9 “ 3 ” 3 “ 1 ” .

In conclusion, Mendel proposes that the hereditary characteristics of alive are controlled each one by dual control (a pair of alleles) and that only one on two is transmitted to the descendant by each relative. It is the base of the Génétique which will start at the beginning of the 20th century. At the same time, with the first steps of a quantitative biology will develop the Statistique S. It publishes its work in 1865 in “Experiments in Hybridization Seedling”.

Redécouverte of the laws of Mendel

At the beginning of the 20th century, the Netherlander Hugo de Vries, the German Carl Erich Correns and the Austrian Erich von Tschermak redécouvrent the laws of heredity independently, and recognize in Mendel their discoverer. Specialized in research on heredity, Gregor Mendel (1822-1884) had stated, after a series of experiments on the hybridization of pea, a certain number of laws on the transmission of the distinctive characters. Its results quickly are retorted and validated.

However one period of scientific Controversy initiated mainly by William Bateson and Karl Pearson followed in connection with the importance of the theory mendélienne.

In 1918, Ronald Fisher uses the genetics mendélienne to establish the theoretical base of the modern synthesis of evolutionary, but critical biology nevertheless the methods: particularly the results of F2 (second generation) which cannot be exactly 3 per 1. He shows Mendel to have enjolivé his results (by not knowing the importance of the test as a blind man) but this dissension on the methods cannot deny the importance of the phenomenon highlighted by Mendel: the disjunction of the alleles at the time of the meiosis and their recombinations at the time of fecundation.

Sources

http://www.infoscience.fr/histoire/portrait/mendel.html
http://www.fundp.ac.be/bioscope/1886_mendel/mendel.html

The Université Mendel of Brno is re-elected in 1994 in its honor.

Simple: Gregor Mendel

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