CHAPTER XXIII 



GENETIC CHANGES AND THE CHROMOSOMES 



In one way our views concerning heredity have been consid- 

 erably simplified by the discovery that blending inheritance 

 may be included in the category of Mendelian inheritance. 

 One mechanism will now suffice for all kinds of inlicri- 

 tance, this mechanism being found in the chromosomes. 

 In them, we may reasonably suppose, is found the material 

 basis of every inherited character. When the inheritance 

 is of the simplest kind, involving presence or absence of 

 color or some similar character, we assume that a genetic 

 change has occurred in a single, definite locus in a particular 

 chromosome, and that this single change is responsible for 

 the observed inherited variation. Other characters depend 

 on two or more genes, which may lie at different loci in the 

 same chromosome, or even in different chromosomes. Thus 

 the gray coat of a rabbit is an inherited character which de- 

 pends on at least five different genes, each of which appar- 

 ently lies in a different chromosome. These are (1) a color 

 factor, (2) a black factor, (3) an extension factor, (4) an 

 agouti factor and (5) an intensity factor. Each of these 

 factors or genes behaves as an independent unit in trans- 

 mission. We know of their existence only because each of 

 them has been observed to occur in two or more alternative 

 forms. For a gene which remains unchanged remains un- 

 known. We do not know how many undiscovered genes are 

 concerned in producing the gray coat of a rabbit, nor in what 

 linkage-systems (chromosomes) they he. These few have 

 revealed themselves by their striking variations. By various 

 combinations of the different forms of these five genes, we 

 get all the known color varieties of gray, black, yellow, and 

 white rabbits. When it comes to the inheritance of size 

 differences among rabbits, we suppose that genes affecting 



205 



