828 TEXTBOOK OF ZOOLOGY 



appears. Dominance in these cases occurs in a graded series, each 

 member, between the extremes, being dominant to the lower members 

 and recessive to the higher members of the series. A simple case of 

 multiple 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 between 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 larger series is presented by eye color in the common fruit fly, 

 Drosophila melanog 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. 



Multiple Genes. — Several cases formerly interpreted to be simple 

 blending inheritance not conforming to Mendel's law have been 

 explained 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 

 multiple 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 

 person and a pure-blooded negro produces offspring of a medium 

 shade called mulatto, and that a cross between two mulattoes will pro- 

 duce offspring with a range of color varying from intense black to 

 a shade that may allow the person to pass for white. This is ex- 

 plained by the fact that there are two allelomorphic pairs concerned 

 with the inheritance of color, the dominant of either of which will 

 produce a certain amount of pigment per square millimeter of 

 skin surface. The negro of pure skin inheritance is homozygous, 

 having the dominant genes of each pair. Using P and P' to repre- 

 sent the dominant factors of these two allelomorphs and p and p' the 



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