76 HEREDITY [CH. 



colours, if the constitution of the parents with respect 

 to the colour-factors carried by them is known. Some 

 of these cases have been exceedingly difficult to elu- 

 cidate because it is often impossible by inspection to 

 determine the constitution of a given individual. This 

 must be tested by suitable matings with individuals 

 of colour lower in the series, and it is then found that 

 the results observed agree closely with expectation. 



A more surprising instance of 'reversion on 

 crossing* was discovered by Bateson in sweet-peas. 

 He found that within the white variety known as 

 ' Emily Henderson ' two distinct types exist, indis- 

 tinguishable in appearance, which when crossed 

 together give a purple closely resembling the wild 

 sweet-pea of Southern Europe. The purple rever- 

 sionary form in the first cross, (FJ, self-fertilised, 

 gives in the next generation, (F^), 9 coloured to 7 

 whites. The explanation is that some^plants of the 

 white form lack one colour factor (called by Bateson 

 '(7'); others lack the complementary factor ' R, 9 

 which if present with (?, would produce red pigment. 

 Since colour can only appear when both C and R are 

 present, each parental form is white, but when crossed 

 together C and R are combined in one plant and 

 coloured flowers result. The allelomorphic pairs are 

 C and its absence (c), and R and its absence (r) ; the 

 purple heterozygote is thus CcRr, and produces four 

 kinds of gametes OR, Cr, cR, cr. These mating at 



