70 THE CONTROL OF LIFE 



the second filial generation ? If we suppose, for sim- 

 plicity's sake, that there are 16 of them, 12 of these must 

 be Tall and 4 dwarf. But of the 12 Tails, 9 must have 

 Yellow seeds and 3 green seeds. And of the dwarfs, 

 3 must have Yellow seeds and 1 green seeds. So the 

 formula will run :— 9TY -f- 3Tg + 3dY.+ Idg. 



One of the diagrammatic examples of a unit-character 

 in Man is night-blindness. It is a peculiarity of the 

 eye, apparently associated with a lack of visual purple 

 in the retina. Those affected by the peculiarity find 

 it difficult to see in dim light. There are records of 

 an occurrence of night-blindness in one Jean Nougaret 

 in 1647, and of its regular recurrence in a certain pro- 

 portion of his descendants for more than three centuries. 

 No normal member of the lineage who married a normal 

 type had any night-blindness among his descendants. 

 The character is either there or not there. 



The inheritance of eye-colour is on Mendelian lines, 

 and the mode is also illustrated in the case of brachy- 

 dactylism, where all the fingers are like thumbs in having 

 two joints instead of three. What is known as presenile 

 cataract seems to be a Mendelian character, and so are 

 certain kinds of feeble-mindedness and epilepsy. A 

 reference to eye-colour may be useful. 



When there is abundant pigment in the iris, the eyes 

 are brown ; when the factor or determiner for brown is 

 not part of the inheritance the eyes are blue, i.e. with 

 little pigment in the iris. The children of two parents 

 with brown-eyed ancestry will have a double dose or 

 duflex inheritance of brown. The children of blue-eyed 



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