The Law of Segregation 241 



flies that appeared in Morgan's cultures was one very dark 

 fly which is called ebony, and a dark gray one called sooty. 

 The hybrids were intermediate in pigmentation. When 

 these were bred together there were segregated not merely 

 the sooty and ebony types but also several grades of gray 

 lying between. Nevertheless, it was possible to sort these 

 intermediates in a way that showed a reappearance of an- 

 cestral traits in the Mendelian 3 : i ratio. 



Extracted Pure Lines . 



Further experiments with the third and later genera- 

 tions, carried on by Mendel, and in more recent times by 

 many investigators in all parts of the world, reveal a certain 

 uniformity in the appearance of the alternative traits in 

 successive generations. This uniformity can be stated in a 

 simple mathematical formula, pointing to a uniform opera- 

 tion of some mechanism in the reproductive process. It is 

 found, for example, in the case of the peas, that the recessive 

 type (green seed) which reappears in the second generation 

 is just as " pure " as the grandparent from which the type 

 is derived. The offspring of such an individual will never 

 include yellows unless at some point in the line there is again 

 a crossing with the dominant type. That is to say, whereas 

 a pure type can give rise only to its own kind of individuals 

 hybrids can give rise to both kinds. The practical difficulty 

 has always come from the fact that, because of dominance, 

 it has been impossible to distinguish by appearance the pure 

 dominant from the hybrid dominant (see Fig. 86) . 



The " phenotype " proclaims the presence of one of a 

 pair of ancestral traits, but not the presence of the other. 

 Breeding exposes the fact that the hybrid individual has both 

 hereditary capacities and that it is genotypically different 

 from the dominant parental form which it resembles. A 

 pure dominant or a pure recessive derived after segregation 

 from hybrid parents is called an " extracted " individual. 

 Every recessive individual derived from hybrid parentage 



