[202] The reader will find a critical discussion of the presence and absence theory on page 220 of Morgan, Sturtevant, Muller, and Bridges, _The Mechanism of Mendelian Heredity_. New York, 1915.
When two pure breeds of parents differ in one character, _e. g._, two varieties of beans, one with a violet the other with a white flower, the cross between the two species (the F{ 2} generation) has pale violet flowers, approximately intermediate between the two parents. If these hybrids are bred among themselves the offspring is called the F{ 2} generation. According to Mendel's law the hybrids of the first F{ 1} generation all have two kinds of eggs in equal numbers, one kind representing the pure breed of the parents with violet, the other of the pure breed with white flowers. The same is true for the pollen cells. Hence the following possible combinations must appear in the offspring when the pale violet hybrids are inbred:
violet white .... eggs |\ /| | X | |/ \| violet white .... pollen
The four possible combinations are: (1) violet--violet; (2) violet--white; (3) violet--white; (4) white--white. The first will result in pure violet flowers, the fourth in pure white, and the second and third in pale violet flowers. Since all four combinations will appear in equal numbers when the number of crossings is sufficiently large the numerical result will be:
Fifty per cent. of the F{ 2} generation will be pale violet, 25 per cent. violet, and 25 per cent. white. The violets and whites each will breed true when bred among themselves since they are pure, and produce only one type of eggs and pollen. The pale violets are hybrids and will again produce the two types of eggs and pollen, that is, if bred among themselves will again give violets, pale violets, and whites in the ratio 1:2:1. This the experiment confirms.
As has been stated, it not infrequently happens that all the hybrids of the first generation are alike. In such cases the one character is "recessive," _i. e._, overshadowed or covered by the other the "dominant" character, which alone appears in the hybrids. Thus when Mendel crossed peas having round seeds with peas having angular seeds all the hybrids had round seeds. The round form is dominant, the angular recessive, _i. e._, all the hybrids have round seeds. When these hybrids were bred among themselves the next generation produced round and angular seeds in the ratio of 3:1 (5474 round to 1850 angular). The explanation is as follows. Let R denote round, A angular character; the pure breeds of parents have the gametic constitution RR and AA respectively. When crossed, all the offsprings have the constitution RA and since A is recessive this hybrid generation resembles the pure RR parents. The F{ 1} generation produces two kinds of eggs R and A and two kinds of pollen R and A in equal numbers, and these if inbred give the following four combinations in equal numbers: