MENDELISM Such variations as those which we have described as discontin uous, which arise fully formed by mutation, seem always to be heritable, and to be handed on from parents to offspring in ac cordance to definite laws, those of Mendelian or alternate inheri tance. (See HEREDITY.) These laws were first formulated by Gregor Mendel, abbot of Brunn, in 1865, as the result of experi ments in the hybridization of garden plants, but they remained unappreciated until 1900.
The typical Mendelian experiment involves the selection of two individuals which differ by a single definite character and are members of lines which breed true for this character. These individuals are then crossed and the character of all the resulting first hybrid generation (f I ) examined. It will in all cases be found that the whole f i generation is uniform with respect to the character under consideration, and that in nearly all cases it resem bles one or other of the parents and does not exhibit a blend of their peculiarities. The character which appears in the first hybrid generation is said to be dominant, that which is hidden recessive. The whole of the individuals of the f z generation are allowed to mate at random, and the resulting second hybrid generation f 2 examined. It is found that the recessive character has reappeared in them, and that the individuals displaying it form one quarter of the whole. Thus the factor, whatever it be, which, when present in the fertilized egg, causes the appearance of the recessive character, must have been present in the individuals of the f i gen eration, uncontaminated by the simultaneous presence of that which causes the production of the dominant character. The com plete fusion of the reproductive cells which occurs at fertilization, and the nature of the cell divisions which form part of the develop ment of the animal, make it clear that all ordinary cells through out the body of the hybrid will contain the factor for both dom inant and recessive characters.
The nature of the phenomenon of segregation, the reappearance of both parental types in the second hybrid generation, can be in vestigated by breeding each member of that generation with its like. It is then found that recessive individuals bred together never produce dominants, rendering it clear that they do not contain the factor for the production of that character. The individuals which possess the dominant character are of two kinds ; one-third of them, when bred with their like, produce only dominants, the other two-thirds produce dominants and recessives in a three to one proportion.
This occurrence receives a complete explanation if, in the process of formation of the gametes, each receives either the factor for the production of the dominant or that for the recessive char acter, but not both, that is, that the gametes are "pure" with respect to these characters.
The observed ratio of dominant to recessive individuals in the f 2 second hybrid generation can only occur if the original hybrids produce equal numbers of the two kinds of gametes, and the mating of these gametes is entirely haphazard.
It follows, therefore, that the individuals which are produced in the experiment are of two kinds, those which produce only one sort of gamete, either dominant producing or recessive, which are hence called homozygous, and those which produce equal numbers of the two types of gametes, the heterozygous form. Furthermore, the morphology of the individuals presenting the dominant char acter will not enable us to distinguish the homozygous from the heterozygous animals.
It is thus essential to distinguish between the phenotype, the appearance of an animal, and the genotype, its constitution, as determined by that of the gametes from whose fusion it arose. The simplicity of this explanation of the Mendelian ratio soon led zoologists to look for structures of the germ cells which would provide a suitable mechanism.