MENDEL’S LAWS OF HEREDITY 391 
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 F; 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, 
1 dwarf wrinkled; which is a 9:3:3:1 ratio. This is the dihybrid 
ratio, the explanation of which may be indicated in Fig. 69. 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. 69 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. 69. The result is four phenotypes, as 
follows: Nos. 1, 2, 3, 4, 5, 7, 9, 10, 13 are tall smooth individuals; 
Nos. 11, 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 split 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, green individuals; and it is obvious that the 
