Mendel's Law 37 



immediate progeny would show the dominant character. 

 The real situation would be revealed, however, when this 

 progeny was inbred, for one-half would be homozygous 

 (pure breeders) and the other half would be heterozygous 

 (hybrid breeders). 



Thus far we have considered only what is called the 

 monohybrid ratio, that is, the ratio obtained from one 

 pair of contrasting characters, such as tallness and 

 dwarfness. The next step is to consider the dihybrid 

 ratio. Mendel also used contrasting seed characters, 

 finding, for example, that smoothness in seeds is domi- 

 nant to a wrinkled condition. Introducing this pair of 

 contrasting characters into the situation we have been 

 considering, the dihybrid ratio will be the result. Cross- 

 ing 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 F2 generation, however, the following ratio will 

 appear: 9 tall smooth, 3 dwarf smooth, 3 tall wrinkled, 

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

 dihybrid ratio, the explanation of which may be indi- 

 cated in fig. 5. The question may be raised why the 

 characters for tallness and smoothness are not repre- 

 sented on the same chromosome. If they were, the 

 result would be a simple monohybrid ratio, except that 

 the tall individuals would always be smooth-seeded as 

 well, and dwarfs would be always wrinkled-seeded. 

 The possibihty of one chromosome carrying two differ- 

 ent determiners will be considered later, but at present 

 we shall assume that these' determiners are on different 

 chromosomes. 



