274 HEREDITY AND ENVIRONMENT 



In a few instances a character may be domi- 

 nant at one time and recessive at another. 

 Thus Davenport found that an extra toe in 

 fowls is dominant under certain circumstances 

 and recessive under others. Tennent found 

 that characters which are usually dominant in 

 hybrid echinoderms may be made recessive if 

 the chemical or physical nature of the sea water 

 is changed. Such cases seem to show that 

 dominance may depend sometimes upon en- 

 vironmental conditions, sometimes upon a par- 

 ticular combination of hereditary units. 



Sex and Sex-Limited Inheritance 



Sex and sex-limited inheritance may be con- 

 sidered here since they involve questions of 

 dominance, certain characters remaining un- 

 developed in one sex which are fully developed 

 in the other. There is good evidence, as was 

 shown in Chapter II, that sex is a Mendel- 

 ian character, in which the female has a double 

 dose of the determiner for sex, whereas the 

 male has only a single dose. Consequently in 

 the formation of the gametes every egg re- 

 ceives one sex-determiner, while only one-half 



