166 HEREDITY AND ENVIRONMENT 



found evidence that the hereditary value of 

 different chromosomes is different. (6) Mc- 

 Clung, Stevens and Wilson have discovered 

 that the determination of sex is associated 

 with the presence or absence of a particular 

 chromosome, the X chromosome, in the sper- 

 matozoon which fertilizes the egg. If an 

 Qg'g is fertilized by a sperm which lacks the 

 X chromosome a male is produced, if fertilized 

 bj"^ the other type a female results. (7) Fin- 

 ally Morgan has found that there is a linkage 

 of certain somatic characters with sex in the 

 fruit fly, Drosophila, which can be readily ex- 

 plained by assuming that the determiners for 

 these characters are in some way associated 

 with the sex chromosome. 



We have in these facts a remarkable corre- 

 lation between the distribution of the chromo- 

 somes and the occurrence of certain characters 

 of the adult animal. The association of ma- 

 ternal and paternal chromosomes in fertiliza- 

 tion and their segregation in the maturation of 

 the germ cells is parallel to the association of 

 Mendelian characters in the zygote and their 

 segregation in the gametes; if the distribution 



