2 72 POPULAR SCIENCE MONTHLY. 



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 pro- 

 poses the useful terms homozygote for pure types like 8, and hetero- 

 zygote for mixed types like Sb.) Next year the homozygotes 8 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 gen- 

 eration 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 



