132 REVERSION AND ALLIED PHENOMENA 



5. " Skipping a Generation " 



It is often remarked in human inheritance that a child re-exhibits 

 the peculiarity of a grandfather or grandmother, which the parents 

 did not show. A Mendelian interpretation of this is in some cases 

 possible. " If the two grandfathers have blue eyes and both grand- 

 mothers brown eyes, then the parents may both have simplex brown 

 eyes * ; they will both form germ-cells of which 50 per cent, have 

 and 50 per cent, lack the determiner to form brown iris pigment. 

 From such brown-eyed parents one child in four will have blue 

 eyes like the grandfathers. This is atavism. Cases of atavism 

 can, in general, be explained on the same ground as atavism to 

 blue-eyed grandparents" (Davenport, 1910, p. 292). 



In case of sex-limited characters, such as bleeding or haemophilia, 

 the phenomenon of "skipping a generation" may be illustrated. 

 For the haemophilia is usually transmitted through unaffected 

 daughters to grandsons. This may be comparable to other cases 

 of sex-limited inheritance, e.g. in certain strains of sheep where 

 the horns are confined to the males. 



It is likely that skipping a generation is less frequent than it is 

 supposed to be ; thus features which the parent thinks he never 

 had may have been plain enough when he was of the same age as 

 his son now is. Moreover, in the case of characters that blend it is 

 an obvious possibility that a grandson should sometimes show his 

 grandfather's pattern. Finally, some cases of the disappearance of 

 exceptional ability and the return to mediocrity come within the 

 rubric of " filial regression." 



But our present point is that there seems little utility in calling 

 " skipping a generation " a " reversion," or even an atavism. 



A drone-bee arises from an unfertilised egg; it has a mother 

 and two grandparents, but no father. But it seems rather absurd 

 to call its resemblance to its grandfather either atavistic or rever- 

 sionary. This is a reductio ad absurdum, for the drone-bee would 

 resemble its father if it had one 1 



* " Ordinarily when parents are similar, each unit character of the 

 offspring develops from two similar determiners one paternal and one 

 maternal. Thus in its origin any unit character is duplex. When, 

 however, the determiner is found in only one of the parents the character 

 is simplex." This will be clearer after the chapter on Mendelism has been 

 read. 



