268 HEREDITY AND ENVIRONMENT 



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. 



Seoo 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 the last lecture, 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 

 of the spermatozoa receive such a determiner, 

 the other half of them being without it. If 

 then, an egg is fertilized by a sperm without 

 one of these determiners, a male results; but 

 if an egg is fertilized by a sperm with one of 

 these determiners, a female is produced. This 



