24 MENDELISM CHAP. 



dominant and recessive elements respectively. If now 

 the F! plant be crossed with the pure recessive, we are 

 bringing together a series of gametes consisting of equal 

 numbers of dominants and recessives with a series con- 

 sisting solely of recessives. We ought from such a cross 

 to obtain equal numbers of dominant and recessive in- 

 dividuals, and further, the dominants so produced ought 

 all to give both dominants and recessives in the ratio 3 : i 

 when they themselves are bred from. Both of these ex- 

 pectations were amply confirmed by experiment, and cross- 

 ing with the recessive is now a recognised way of testing 

 whether a plant or animal bearing a dominant character 

 is a pure dominant, or an impure dominant which is carry- 

 ing the recessive character. In the former case the off- 

 spring 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 ob- 

 tained when two individuals differing in a single pair of 

 characters are crossed together and with the interpreta- 

 tion 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 bear- 

 ing coloured flowers was crossed with a dwarf plant 



