234 EVOLUTION, GENETICS, AND EUGENICS 



the total, or almost total, exclusion of the other. No intermediate 

 forms appeared. 



Mendel called the character that prevailed dominant, and the 

 character that was suppressed, or apparently suppressed, recessive. 

 And the first big result was that crosses between a plant with the 

 dominant character and a plant with the recessive character yielded 

 offspring all resembling the dominant parent as regards the character 

 in question. Let us for shortness call the parents D and R, and the 

 first result may be expressed thus: DXR = D. 



It must be carefully noted that the complete dominance which 

 Mendel observed has been shown in other cases to be the exception 

 rather than the rule. Thus a cross between a " Chinese " primula with 

 wavy crenated petals and a "star" primula with fiat simply notched 

 petals is intermediate between the two parents; and yet, as the next 

 generation shows, the case is one of Mendelian inheritance. 



In many cases the hybrid, while on the whole dominant, may show 

 some influence of the recessive character but not nearly enough to 

 warrant us in speaking of a blend. Thus, when white (dominant) 

 Leghorn poultry are crossed with brown (recessive) Leghorn, most of 

 the offspring have some "ticks" of colour. When these are inbred 

 they produce a quarter brown (extracted recessives) and three- 

 quarters pure white or white with a few ticks. The dominance is not 

 quite perfect. 



The Law of Splitting or Segregation. — In the next generation the 

 cross-bred plants (products of D and R, or R and D, but all apparently 

 like D) were allowed to fertilise themselves, with the result that their 

 offspring exhibited the two original forms, on the average three domi- 

 nants to one recessive. Out of 1,064 plants, 787 were tall, 277 were 

 dwarfs. 



When these recessive dwarfs were allowed to fertilise themselves 

 they gave rise to recessives only, for any number of generations. The 

 recessive character bred true. 



When the dominants, on the other hand, were allowed to fertilise 

 themselves, one-third of them produced " pure " dominants, which in 

 subsequent generations gave rise to dominants only; and two-thirds 

 of them produced once again the characteristic mixture of dominants 

 and recessives in the proportion of 3 : 1. 



The general results may be expressed in the scheme. The 

 result of the hybridisation is a generation (F,) like the dominant 

 parent. They may be represented by the symbol D(R), for they 



