300 



ANIMAL BIOLOGY 



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mentation are present. Furthermore, the progeny (F 2 ) of mulattoes 

 show different degrees of color ranging from pure white to pure 

 black owing to the sixty-four possible recombinations of genes 



according to the trihybrid formula: 

 the more 'black' genes present, the 

 darker the pigmentation of the skin. 

 The infrequent appearance of pure 

 whites or blacks in the F2 generation 

 is because the chances are slight that, 

 through segregation and independent 

 assortment, all the separate genes for 

 black or white will be brought to- 

 gether in a single gamete and, fur- 

 ther, that such a gamete at fertiliza- 

 tion will meet another similarly en- 

 dowed. (Fig. 188.) 



From these few examples, selected 

 almost at random from the wealth 

 of data at hand, it is apparent that 

 the various types of inheritance can 

 be satisfactorily interpreted on the 

 fundamental principles of segregation 

 and independent assortment. It is 

 merely necessary to bear in mind 

 that it is the genes, which condition 

 the development of characters, and 

 not the characters themselves that 

 behave as units; for now we know 

 that most characters are determined 

 by multiple genes, and that even in 

 cases where one gene seems to pro- 

 duce a character, other modifying 

 genes, complementary genes, etc., 



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8 2 10 



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Fig. 188. — The distribution 

 of the sixty-four possible recom- 

 binations in the F 2 generation may affect its development, Genes 



when three pairs of similar genes, g^e units in inheritance but are not 



cumulative in action, produce a •. • j^, x ^i^^™^ + 



., £ . c units in development, 



character; e.g., the olispnng 01 r 



mulattoes. The figures indicate Within the past few years genet- 



the number of genes (e.g., for icists have been able by the multiple 



black) present in the Individ- modifying gene, and similar 



uals represented by the column fe 1 • t_ • ? 



above. (From Waiter.) concepts to bring the inheritance ol 



