22 MENDELISM 



CHAP. 



recognised way of testing whether a plant or animal 

 bearing a dominant character is a pure dominant or 

 an impure dominant which is carrying the recessive 

 character. In the former case the offspring will be 

 all of the dominant form, while in the latter they 

 will consist on the average of equal numbers of 

 dominants and recessives. 



So far we have been concerned with the results 

 obtained when two individuals differing in a single 

 pair of characters are crossed together and with 

 the interpretation of those results. But Mendel also 

 used plants which differed in more than a single pair 

 of differentiating characters. In such cases he found 

 that each pair of characters followed the same definite 

 rule, but that the inheritance of each pair was 

 absolutely independent of the other. Thus, for 

 example, when a tall plant bearing coloured flowers 

 was crossed with a dwarf plant bearing white flowers 

 the resulting hybrid was a tall plant with coloured 

 flowers. For coloured flowers are dominant to white, 

 and tallness is dominant to dwarfness. In the 

 succeeding generation there are plants with coloured 

 flowers and plants with white flowers in the pro- 

 portion of 3: I, and at the same time tall plants 

 and dwarf plants in the same proportion. Hence the 

 chances that a tall plant vyiH have coloured flowers 

 are three times as great as its chance of having 

 white flowers. * And this is also true for the dwarf 

 plants. As the result of this cross, therefore, we 

 should expect an F^ generation consisting of four 

 classes, viz. coloured tails, white tails, coloured 

 dwarfs, and white dwarfs, and we should further 

 expect these four forms to appear in the ratio of 



