450 



THE MAINTENANCE OF SPECIES 



In actual practice, if a combination of three or more characters 

 is desired, one character at a time in either pure dominant or recessive 

 form is obtained. By this method, since the expectation of either a 

 pure dominant or a pure recessive in a monohybrid is one out of four, 

 early reahzation of the desired combination is likely. 



Unit Characters and Factors 



A great deal has been learned about heredity through the experi- 

 mental breeding of plants and animals since Mendel's laws became 

 available. Many of the facts gained, however, are at first sight in 

 apparent contradiction to these laws, but the value of the fundamental 

 concepts of dominance, independent assortment, and segregation in the 



^ ,. interpretation of inherit- 



nSreQIiary OOmailU wcice remains unques- 



Determiners Characters tioned. Any adequate 



\_) .«...___^ consideration of the ap- 



parent departures from 

 the clear-cut conclusions 

 of Mendelism would re- 

 quire many more pages 

 than are available in this 

 book. 



For one thing, Men- 

 del's experiments led him 

 to the idea of Unit 

 Characters, each spon- 

 sored by a single germinal 

 determiner. There is 

 now abundant evidence 

 that whatever it is in the 

 germplasm that, under suitable environmental conditions, becomes 

 eventually expressed as a single character, it is often made up of more 

 than one unit. This discovery has led to the development of the factor 

 hypothesis, which implies that there is usually, if not always, an in- 

 terplay between different hereditary factors in determining the con- 

 tribution which inheritance furnishes to the formation of a character 

 in an individual. Moreover, a constellation of interacting hereditary 

 factors may be responsible, in certain instances, for the expression of 

 more than one visible character. 



Modified Ratios. The existence of factors, or fractional rather 



B 



Diagram of the relation between hereditary 

 determiners and resulting somatic characters. 

 A, three or more determiners may combine to 

 produce a single visible character, or B, a single 

 hereditary determiner may find expression in a 

 number of difTerent somatic characters. 



