246 Experimental Transformation of Species 



I. Each pair of contrasting characters in a plant is pro- 

 duced by a pair of ahernative determiners, and each unit 

 character is independent of other characters. For example, 



seed color is inher- 

 ited independently of 

 height of the plant, 

 shape of the seed, and 

 so on. 



2. Of each pair of 

 contrasting characters, 

 one is dominant and the 

 other recessive. He 

 supposed that the pres- 

 ence of the determiner 

 for dominant inhibits 

 the development of the 

 recessive. 



3. Each germ cell 

 contains only one or 

 the other determiner 

 for a given pair of 

 contrasting characters. 

 By the " purity of 

 the gametes " Mendel 

 meant that any given 

 tg^ or sperm could de- 

 termine the develop- 

 ment of either the 

 dominant or the reces- 

 sive, although the hy- 

 brid parent body bear- 

 ing the germ cells had 

 the capacity to produce 

 two kinds of " pure " 

 germs. 



Of these three principles the second and third have been 

 confirmed by later researches. While the modern student 



Fig. 63. The Principle of Independ- 

 ent Assortment 



Pigmentation in guinea pigs is dominant over 

 albinism; short hair is dominant over long hair; 

 and rough coat is dominant over smooth coat. 

 When two " pure " individuals like those shown 

 are mated, the offspring will all be like the lower 

 animal, but with a rough coat. On mating the 

 hybrids together in suflScient numbers, the segre- 

 gation will result in producing every possible com- 

 bination of these three sets of characters: dark- 

 short-rough; dark-short-smooth; dark-long-rough; 

 dark-long-smooth; white-short-rough; white-short- 

 smooth; white-long-rough; white-long-smooth — 

 eight in all (2^). The proportions will be such 

 that for each pair of contrasted characters there 

 will be three dominants to one recessive. From 

 photographs by Professor W. E. Castle; in Gruen- 

 berg, Elementary Biology, published by Ginn & 

 Company. 



