58 STATISTICAL METHODS. 



Of the two antagonistic peculiarities united in the cross, 

 that which becomes visible in the soma is called by Mendel the 

 dominating, that which lies latent is called the recessive char- 

 acter. What determines which character shall be dominating 

 is still unknown, and the determination of this point offers an 

 enticing field of inquiry. In some cases the dominating form 

 is the systematically higher, in others it is the older or ances- 

 tral form. 



The law of dichotomy may now be developed. When a 

 mongrel (monohybrid) fertilization takes place the zygote con- 

 tains both the dominant quality (abbreviated d) and the re- 

 cessive quality (r). In the early cleavages d and r are both 

 passed over into both the daughter-cells; but apparently, at 

 the time of segregation of the germ-cells, the somatic cells 

 are provided with d only, while the germ-cells retain both 

 qualities. In the ripening of these germ-cells, probably in 

 the' reduction division, d and r come to reside in distinct cells, 

 so that we have 



of the female cells 50%d + 50%r, and 

 of the male cells 50%d + 50%r. 



If now mongrels are crossed haphazard, a male d cell may 

 unite with either a female d cell or with a female r cell; like- 

 wise a male r cell may unite with a female d or a female r cell. 

 Consequently in the long run we shall have of all the zygotes 



25%d, d + 50%d, r + 25%r, r, 



or 50% of the zygotes hybrid and 50% of pure blood, and of 

 the latter half exclusively maternal and half paternal. But 

 since the soma developed from the hybrid germ-cell has the 

 dominant character, we shall have 



75% of the cases with the dominant character; 

 25% " " " " " recessive 



and this agrees with various empirical results, of which the 

 following from Correns is instructive. A cross was obtained 

 between a variety of pea with a green (</) germ and one having 

 a yellow (y) germ. Yellow is dominating. 



