INDEPENDENT MEN DELI AN INHERITANCE 77 



egg and sperm cells in F 2 and these give red-eyed females half of which 

 are homozygous (WX}(WX) and half heterozygous (WX)(wX) and 

 equal numbers of red-eyed and white-eyed males (WX)Y and (wX}Y 

 respectively. 



In the reciprocal cross, Fig. 36, the white-eyed female contains two 

 X-chromosomes each bearing a factor for white eyes. Her genetic 

 constitution, therefore, is (wX}(wX). All the eggs from such a female 

 will be of the genetic constitution (wX) they contain an X-chromosome 

 bearing a white-eye factor. When such eggs are fertilized by an X- 

 bearing sperm cell from the male, the female produced will be of the 

 genetic constitution (WX)(wX). It will be red-eyed because of the red- 

 eyed factor carried by the X-chromosome of the sperm. On the other 

 hand, when such an egg is fertilized by a F-bearing sperm cell, the male 

 thus produced will be of the genetic constitution (wX)Y. It will be 

 white-eyed, because of the white-eye factor in the .^-chromosome of the 

 egg-cell. Breeding two such F\ individuals together will result in the 

 F 2 distribution shown in the diagram. Females will be produced half 

 of which are of the genetic constitution (WX)(wX) and half (wX)(wX), 

 hence red-eyed and white-eyed respectively; and males half of the genetic 

 constitution (WX)Y and half (wX)Y, hence red-eyed and white-eyed 

 respectively. The peculiar relations exhibited in the inheritance of 

 white-eye color in Drosophila, therefore, admit of a logical chromosome 

 interpretation, if we assume that the factors involved are borne by the 

 X-chromosomes. The type of inheritance which is apparently dependent 

 on factors borne in the sex-chromosomes is called sex-linked inheritance. 

 It will be treated more fully in Chapter XI. 



Mathematical Adequacy of Mendelism. Mendelian principles do 

 not apply to isolated phenomena of inheritance alone, but they are of 

 general significance. It is consequently of interest to know how well 

 experimental results agree with theoretical expectations when Mendel- 

 ian analyses are rigidly applied. Particularly is this true of the mathe- 

 matical relations involved, which have often been used to confute the 

 arguments of Mendelian interpretations. We shall accordingly con- 

 sider the results of Mendel's original investigation from this standpoint, 

 and a few other cases which have been investigated in particularly large 

 progenies and under circumstances which practically eliminate personal 

 bias. 



Mendel's investigations with peas included a consideration of seven 

 pairs of contrasted characters as follows: 



1. The Difference in Form of Ripe Seeds. These, are either round 

 or roundish, the depressions, if any, occur on the surface, and are at most 

 only shallow as in the indent type; or they are irregularly angular and 

 deeply wrinkled. 



