whose parents differ in one respect is called a monohybrid; a dihybrid is one whose parents differ in two respects; and so on. Dihybrids and polyhybrids do not tend to split up into the two parent forms, as will be seen later.
Heretofore, plant breeders have been producing hybrids, and then by selecting to type each year from the progeny, trying to fix new types. Let us see what light Mendel's law throws on this practice. In the illustration given above, if the breeder had selected the type B of the progeny of the hybrid, he would have had a fixed type at once. Had he selected for type S, he would have had a mixture of the pure type S with the mixed type Sb. (Professor Bateson proposes the useful terms homozygote for pure types like S, and heterozygote for mixed types like Sb.) Next year the homozygotes S, would reproduce their kind only, while the heterozygotes would produce the three types B, Sb and S in the proportion 1:2:1. The second generation would therefore consist of S, 62.5 per cent.; Sb, 25 per cent.; and B, 12.5 per cent. This method of selection would never result in a fixed type unless the breeder should accidentally choose seed of the type S only. It has already been shown that the fixed type S could have been separated out at once by saving the seed of each selected plant separately, and observing which reproduced true to type. Nature fixes the type whether the breeder selects or not; heretofore, the breeder secured his fixed type by chance selection. With the knowledge of Mendel's law, he now selects his fixed type in a methodical manner, in the third generation.
Dihybrids are much more interesting, since they present the more usual case with which the breeder has to deal. With them, fixed types unlike either parent may be secured in the third generation. It frequently occurs that a breeder finds two characters in different varieties that he wishes to combine in a single variety. This is easily done when the characters obey Mendel's law. To illustrate this case I shall use characters which are of no particular importance, but for which I happen to possess experimental data. The principles are exactly the same for any characters that obey Mendel's law. Suppose we have a variety of wheat that has velvet chaff and another that has smooth heads (is not bearded) and that we wish to combine these two characters in a single variety. It is assumed that neither of the varieties has these two characters already; hence we have to deal with two pairs of opposite characters, namely, beards—no beards, and velvet—glabrous. We may, for brevity's sake, represent these characters by their initial letters, using small letters in cases where they are latent. In my work with wheats, beards have always been recessive, as stated above, and velvet chaff has always been dominant