242 ESSENTIALS OF ZOOLOGY 



underlying principle of heredity that it can be said to be practically 

 universal in its application, it is often modified and complicated in 

 various ways. 



Multiple Allelomorphs. — Instead of the allelomorph consisting of 

 the usual two factors, a larger number of alternatives sometimes ap- 

 pear. Dominance in these cases occurs in a graded series, each mem- 

 ber, between the extremes, being dominant to the lower members and 

 recessive to the higher members of the series. A simple case of mul- 

 tiple allelomorphs is found in the inheritance of color in rats where 

 there are three factors ; namely, ordinary pigmentation, ruby-eyed 

 dilution, and albinism. In a cross betw^een heterozygous parents any 

 two of the three factors may be brought together in the offspring. If 

 ordinary pigmentation is present with either of the other two, it will 

 determine the color, since it is first in the series of dominance. If the 

 other two are the ones present, the color will be ruby-eyed dilution, 

 the second member of the series. 



A large series is presented by eye color in the common fruit fly, 

 BrosopMla inelanog aster, in which case there are eleven members 

 which are, in the order of their dominance : red, apricot, coral, ivory, 

 ecru, buff, tinged, blood, cherry, eosin, and white. Each parent may 

 have any two of these in its germ plasm and transmit either of them 

 to the offspring. 



Plural Genes.* — Several cases formerly interpreted to be simple 

 blending inheritance not conforming to Mendel's law have been ex- 

 plained by the discovery that there can be more than one allelo- 

 morphic pair concerned with the inheritance of the trait. Thus, in- 

 stead of there being one gene located at some definite place on a 

 chromosome, there are two or more genes variously located. Cases of 

 plural genes fall into two categories. In the one, each gene inherited 

 as a dominant, produces part of the result, and the effects are cumula- 

 tive. In the other type of case, the inheritance of one dominant gene 

 produces the entire effect. 



A common example of the cumulative type is the inheritance of skin 

 color in man. It has long been known that a cross between a white 

 man and a negro produces offspring of a medium shade called mulatto, 

 and that a cross between two mulattoes will produce offspring with a 

 range of color varying from intense black to a shade that may allow 



♦Other names used for this are multiple factors and duplicate grenes. I avoid the 

 first term because it is lil^ely to be confused by the beginning student with the term 

 multiple allelomorph and I object to the second because of the connotation of only 

 one repetition. 



