46 Outline of Genetics 



the recessive homozygote will produce only recessive 

 homozygotes. 



It is interesting to consider what would happen if a 

 heterozygote were crossed with a homozygous recessive. 

 It should be obvious that one half of the progeny would 

 be pure recessives, while the other half would be hetero- 

 zygotes, that is, there would be a i : i ratio. 



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 dwarf- 

 ness. The next step is to consider the dihyhrid ratio. 

 Mendel also used contrasting flower colors, finding, for 

 example, that red flower is dominant to white flower. 

 Introducing this pair of contrasting characters into the 

 situation we have been considering, the dihybrid ratio 

 will be the result. Crossing a tall, rei-floweredjndivid- 

 ual with a dwarf, white-flowered individual, it is evident 

 that all the Fi or first hybrid generation will be tall, red- 

 flowered individuals, since both of these characters are 

 dominant. In the F2 generation, however, the following 

 ratio will appear: 9 tall, red; 3 tall, white; 3 dwarf, red; 

 and I dwarf, white. This 9:3:3:1 is the dihybrid ratio, 

 the explanation of which is indicated in fig. 5. The 

 question may be raised why the characters for tallness 

 and redness are not represented on the same chromo- 

 some. If they were, the result would be a simple mono- 

 hybrid ratio, except that the tall individuals would 

 always be red flowered, and the dwarfs would always be 

 white flowered. The possibility of one chromosome 

 carrying two different determiners will be considered 

 later, but at present we shall assume that these deter- 

 miners are on different chromosomes. 



