264 



PHYSICAL BASIS OF HEREDITY 



appearance of even ordinary mutants, unless they be 

 dominant, must come two or more generations after the 

 mutation has taken place ; for, the evidence indicates that 

 mutation appears in only one chromosome at a time.^ 

 In the case of sex-linked genes, however, any mutation 

 that takes place in one of the X-ohromosomes of the mother 

 is revealed if the egg containing it gives rise to a son, 

 because he has but one X-chromosome and that comes 

 from his mother. 



The delayed occurrence then of mutants in balanced 

 stocks is not different from the delay in other stocks — 



9 



V- ■ H 



Hf t" Bf -j-s Twin Hybrids. 



Fig. 116. — Diagram illustrating balanced lethals and twin hybrids. 



only when the recombinations occur in balanced lethal 

 stocks they must have been preceded by crossing over, 

 which diminishes the number of mutants that appears. 

 The number of mutants that appears is determined by 

 the distance of the genes for the character from the 

 nearest lethal gene. 



One of the most interesting features of Lamarck's 

 primrose arises when it is bred to certain other species 

 or varieties. It gives rise to two kinds of offspring 

 called Twin Hybrids, to which De Vries gives the names 

 Iceta and velutina (Fig. 115). Now it is a feature 



■If in Bclf-fertilizing forms a mutation takes place so early in the 

 germ-plasm that it gets into both eggs and sperm the new character may 

 appear at once (see ante). 



