35° EVOLUTION, GENETICS, AND EUGENICS 



Thus far we have considered only what is called the monohybrid 

 ratio, that is, the ratio obtained from one pair of contrasting charac- 

 ters, such as tallness and dwarfness. The next step is to consider the 

 dihybrid ratio. Mendel also used contrasting seed characters, find- 

 ing, for example, that smoothness in seeds is dominant to a wrinkled 

 condition. Introducing this pair of contrasting characters into the 

 situation we have been considering, the dihybrid ratio will be the 

 result. Crossing a tall, smooth-seeded individual with a dwarf 

 wrinkled-seeded individual it is evident that all of the Fi or first hybrid 

 generation will be tall, smooth-seeded individuals, since both of these 

 characters are dominant. In the F, generation, however, the follow- 

 ing ratio will appear: 9 tall smooth, 3 dwarf smooth, 3 tall wrinkled, 

 I dwarf wrinkled; which is a 9:3:3:1 ratio. This is the dihybrid 

 ratio, the explanation of which may be indicated in Fig. , The 

 question may be raised why the characters for tallness and smoothness 

 are not represented on the same chromosome. If they were, the 

 result would be a simple monohybrid ratio, except that the tall indi- 

 viduals would always be smooth-seeded as well, and dwarfs would be 

 always wrinkled-seeded. The possibility of one chromosome carrying 

 two different determiners will be considered later, but at present we 

 shall assume that these determiners are on different chromosomes. 



Fig. 59 shows that we are dealing with two homozygotes, each pro- 

 ducing only one kind of gamete, so that all the hybrid progeny will 

 be similar, both genotypically and phenotypically, that is, with the 

 same germinal constitution and the same appearance. By inbreeding 

 these F, individuals, it will be seen that four kinds of gametes are 

 involved. Crossing these four kinds of gametes the resulting com- 

 binations are indicated in Fig. 59. The result is four phenotypes, as 

 follows: Nos. I, 2, 3, 4, 5, 7, 9, 10, 13 are tall smooth individuals; 

 Nos. II, 12, 15 are dwarf smooth; Nos. 6, 8, 14 are tall wrinkled; 

 No. 16 is dwarf wrinkled. This is the 9:3:3:1 ratio. 



It will be noticed that Nos. 1, 6, 11, 16 are homozygotes and there- 

 fore will breed true; but the rest are heterozygotes, either for one pair 

 of characters or for both, and these would spUt into various types upon 

 further breeding. 



The next step is the trihybrid ratio. Mendel found yellow seeds 

 dominant over green seeds, and if this pair of characters is included 

 with those used above the trihybrid result can be observed. The 

 experiment would consist in crossing tall, smooth, yellow individuals 

 with dwarf, wrinkled, srreen individuals; and it is obvious that the 



