240 



PRINCIPLED OF ANIMAL BIOLOGY 



the purpose of this chapter, however, the order of discussion may be 

 reversed. The results of breeding experiments will be more readily 

 understood if the mechanism of heredity, so far as it is now known, is 

 described first. 



The Genes or Factors.-(^Each inherited character of an animal or 

 plant may be regarded as being produced by a definite something in the 



YelloN Bodij 

 Nhife £ye 



I Grai^ Body 

 Red Eye 



NormalNing 



\NOPmalk/ing 



Vermilion 

 E-ye 



Miniature 

 i^ino 



Redtye 

 Miniature 



Rudimen- 

 tary t^ina 



Forked 

 Brist/es 



CompJeteEye. 



Rudimen- 

 tary lA/in<j 



Forked 

 Bristles 



Bar £ye 



Fig. 189. — Diagrams of two homologous chromosomes of the fruitfly Drosophila. 

 Some of the genes are represented, and are in their proper order through the length of the 

 chromosome. Corresponding genes are located at the same level of the two chromosomes. 



undivided germ. This "something" has been given various names, 

 of which gene and factor are the most commonly used. A fertilized egg 

 contains two genes for each inherited character of the adult that will 

 develop from it; that is, two for color of hair, two for length of stem, two 

 for shape of comb, and so on. These statements would need some quali- 

 fication in an extended study of heredity, but for the present purpose 



