318 



EVOLUTION AND GENETICS 



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grains only one of the sixty-four can be of the depth of color of 

 the original dominant parent. The construction of a Punnett 



square for this case will indicate that 

 there are only six degrees of red pres- 

 ent, in addition to the one white 

 individual, and that most of the sixty- 

 four chance combinations fall within 

 the three intermediate groups of the 

 seven (Fig. 183). 



Ear length in rabbits was studied 

 by Castle with similar results. The 

 cross between lop-eared and short- 

 eared parents gives an Fi generation 

 with intermediate ears, while the F2 

 generation produced no individuals 

 showing the ear length of either of 

 the grandparents. It has been pointed 

 out by various writers that if four 

 factors are involved in the determina- 

 tion of car length, only one individual 

 out of two hundred and fifty-six in the 

 F2 generation can be expected to re- 

 semble either grandparent, while with 

 six factors such resemblance can occur 

 only once in 4096 times! As Castle 

 concludes: "it would be remarkable if 

 under such conditions the extreme size 

 were ever recovered from an ordinary 

 cross." 



Inheritance of this type has been 

 called blending inheritance, but it is 

 to be distinguished carefully from the 

 type of blending referred to in Chap- 

 ter XV. The pink four-o' clocks cited 

 as an example in that case are due to 

 the fact that neither the red nor the 

 white color factor dominates the other 

 in heterozygous individuals, and the 

 result is a complete expression of the Mendelian 1:2: 1 monohy- 

 brid ratio. When cumulative factors are involved, however, the 



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G R 4 3 2 1 

 Fig. 183.— The distribution of 

 the .sixty-four possihihtie.s in 

 the F2 generation when three 

 similar determiners act to 

 produce a given character. 

 The numbers indicate the 

 number of determiners pre- 

 sent in the individuals repre- 

 sented by the column above. 

 (From Walter.) 



