RULES AND RATIOS OF INHERITANCE 207 



in inheritance by any character depends on what other charac- 

 ter or characters it is mated with. 



Many examples of these relations are found in the follow- 

 ing pages. 



In the early study of genetics it was commonly believed 

 that each separably heritable characteristic, such as red eye 

 or vermilion eye or rudimentary wings in the fruit-fly, de- 

 pends on one gene only, and follows in its descent the distri- 

 bution of that gene. Any characteristic that gave in heredity 

 the simple Mendelian ratio (3 dominants to i recessive in 

 F2), or the simple sex-linked ratio, was considered a "unit 

 character," and was said to show "unit character inherit- 

 ance," depending supposedly on but a single "factor" or 

 gene. (The word factor came into use before the nature 

 and location of the genes were known; it is still much used 

 as a synonym for gene.) This single factor was supposed 

 in some way to represent in the germinal material the char- 

 acter to which it gave rise. It was thus called "the factor" 

 or "the gene" for that character, a form of expression which 

 still persists, though its implications are erroneous. 



Later it was discovered that this conception is a mistake, 

 since every characteristic is produced by the interaction of 

 many genes, and can be altered by changing any one of 

 them. The reason why certain characteristics give in heredity 

 the simple Mendelian or sex-linked ratio is that the two 

 parents differ in but one pair of the many gene pairs that 

 influence the character. If the two parents differ in more 

 than one of the gene pairs that influence the character, more 

 complex ratios result. Furthermore, the type of inheritance 

 shown by a given character (whether sex-linked, autosomal, 

 or of the Y type) depends on the location of the gene pair or 

 pairs in which the two parents differ. These important re- 

 lations, with the general principles underlying them, are il- 

 lustrated in the following sections, which take up in order 

 the types of inheritance, and the ratios that result when 



