MENDELISM is the scientific theory relating to the distribu tive mechanism of organic inheritance, promulgated in 1866 by the Abbot G. J. Mendel, to interpret certain phenomena revealed by the experimental breeding of plants. Born in a peasant family in 1822, Johann Gregor Mendel became a monk and eventually abbot in the Augustinian monastery at Briinn, in Moravia. He carried on his experiments in the garden of the monastery, where he died in 1884. Mendel took for his problem the question as to the manner in which true-breeding varieties within a species are related. He concentrated his attention upon the mode of inherit ance of sharply contrasted pairs of characters. His experimental material was the culinary pea and the pairs of characters with which he dealt are shown below. His method was that of hy bridization, keeping accurate pedigree records, and counting the number of individuals in each generation and the numbers of dissimilar kinds. In all cases the result was the same ; the first cross (the first filial generation or F,) exhibited only one of the two alternative characters which had distinguished its parents. The character that prevailed Mendel called the dominant mem ber of the pair, that which was suppressed, the recessive. The F, plants were allowed to become self-fertilized and the seeds were harvested separately and sown separately, producing the second filial generation, F,. This generation was mixed, consisting of indi viduals exhibiting the dominant character and others exhibiting the recessive, and in every four on the average there were three with the dominant to one with the recessive.
The F, individuals were allowed to become self-fertilized and it was found that, whilst every individual exhibiting the recessive character bred true, of those which had exhibited the dominant, in every three on the average there was one which bred true and two which yielded a ratio of 3 :1 as had the F, individuals.
Having thus collected by observation and controlled experimen tation a sufficient number of data to enable him to recognize the orderliness and regularity of his results, Mendel then constructed a working hypothesis referring to the phenomena of segregation and the independent assortment and recombination of factors to account for the observed facts. He postulated that corresponding to every inherited character there are determiners or factors (now often designated genes) in the gametes. Each gamete car
ries a factor for each and every heritable character that the future individual may exhibit and thus an individual arising from the union of two gametes has a double set of factors, each gamete a single set. Characters can be classified as alternative (allelomor phic), e.g., stem length can be either tall or dwarf, and allelo morphic characters are alternative characters of one and the same structure or function. A tall pea can have received the factor for this character by way of both male and female gametes, being duplex for this factor and homozygous for the character, or can have received this factor by way of one gamete and the alter native factor for dwarf stem by way of the other, being simplex for each of these factors and heterozygous for the character it exhibits, in this case tall because, for reasons as yet unknown, tall is dominant to dwarf. An individual exhibiting the recessive char acter of a pair must necessarily be nulliplex for the dominant factor, duplex for the recessive factor, and homozygous for the character it displays. The individual being constitutionally du plex, and the gamete simplex, it follows that into each gamete elaborated by an individual there must pass one or other of each pair of factors in the hereditary constitution. In respect of the factors for all characters for which the individual is homozygous, all gametes will be factorially similar, but in respect of those for characters for which the individual is heterozygous, there will be two sorts of gametes, one carrying the factor for one character of a pair, the other the factor for its alternative, and the two kinds will be elaborated in equal numbers. If, of the two characters, one is dominant, if at the time of fertilization there are equal numbers both of male and of female gametes carrying the dominant and recessive factors respectively (as will be the case for the gametes of the F,), and if fertilization is at random, then chance will yield on the average in every four, three individuals exhibiting the dom inant character of a pair and one exhibiting the recessive. Men del's law of segregation refers to this clean separation of factors during the formation of the gametes. It can be illustrated by the following scheme.