The appearance of the different groups can now be further explained in terms of the Mendelian theory. According to the conception of the individual formulated by Mendel, each cell of the body contains an ingredient derived from each of the sexual cells or gametes which united at the moment of fertilization of the ovum by the spermatozoon to form the individual. But when the adult in his or her turn forms sexual cells or gametes, these ingredients separate again, half the gametes containing one of the pair of factors, half containing the other. This process certainly takes place during the rearrangement of the nuclear substance or chromosomes at the cell divisions which result in the formation of the ripe sexual cells. It is called the “segregation of the gametes.” In the present case the unit character producing Group II will be first considered. As already explained, the factors concerned may be called A and a, and the individual of Group II may be constituted by AA or Aa, and the gametes, therefore, may contain either A or a, but not both. The individuals resulting from the union of the gametes derived from Aa adults may then be constituted in three ways--AA, Aa, or aa. Similarly for the unit character producing Group III, the factors concerned may be called B and b, and the individual of this group may contain BB or Bb. The gametes then contain either B or b, and the individual resulting from their union may again be constituted in three ways--BB, Bb, or bb. In computing the results, however, it must be remembered that most, or perhaps all, people are hybrids, so that both unit characters are present simultaneously, and all the factors must be taken into account. It is easily seen that the gametes derived from a hybrid individual must contain one of the following combinations: AB, Ab, aB, or ab, and consequently the individuals formed from them must have one of the following constitutions: AB--Ab, Ab--aB, aB--ab, ab--ab, AB--AB, AB--aB, Ab--ab, aB--aB, AB--ab, Ab--Ab. This includes all the possible combinations that can result from the chance union of the gametes, and it is now clear which blood groups result from which combinations, if it be remembered that A is dominant to a, B ” ” ” b, and that Group I results from the _presence_ of both A and B. ” II ” ” ” ” ” A only. ” III ” ” ” ” ” B ” ” IV ” ” ” _absence_ ” both A and B. Thus Group I may be constituted by AB--AB. AB--aB. AB--Ab. AB--ab. Ab--aB. Group II may be constituted by Ab--Ab. Ab--ab. ” III ” ” ” ” ab--aB. aB--aB. ” IV ” ” ” ” ab--ab.