MIRACLES OF SCIENCE 



the short ancestor, and two that are mixed domi- 

 nants, like their parents. 



What holds for tallness and shortness holds also 

 for each opposing pair of unit characters. Yellow 

 seeds, for example, are dominant to green seeds; 

 all hybrids of the first generation show yellow seeds; 

 but of their offspring, one in four will show the re- 

 cessive trait of greenness of seed, and will breed true 

 to that trait generation after generation, quite as if 

 there had been no strain of yellow seeds in its an- 

 cestry. 



Add that the different pairs of unit characters hold 

 their respective qualities of dominance or of reces- 

 siveness regardless of association with other charac- 

 ters, and we see that the manifestations of Mendelian 

 heredity may be exceedingly diversified, yet that a 

 clear understanding of "Mendel's Law" as just out- 

 lined may make their interpretation clear where 

 without this knowledge for a guide they might seem 

 exceedingly mystifying. Professor R. G. Punnett, 

 of Cambridge University, gives a simple but effective 

 illustration of the way in which a knowledge of Men- 

 del's law might aid a practical breeder. He supposes 

 the case of a gardener who has two varieties of plant 

 each possessing a desirable character and who wishes 

 to combine these characters in a third form: 



"He may, for example, possess tall green-seeded 

 and dwarf yellow-seeded peas, and may wish to raise 

 a strain of green dwarfs. He makes his cross and 

 nothing but tall yellows result. At first sight he 

 would appear to be further than ever from his end, 

 for the hybrids differ more from the plant at which 



194 



