204 GENETICS 



were pure yellow. In a Mendelian monohybrid cross, 

 as has been previously pointed out, the expectation is 

 that in the second generation one fourth of the offspring 

 will be recessives {DR X DR = DD -\-2DR-\- RR) , but 

 when yellow mice are bred together, the percentage of 

 recessives approximates one third instead of one fourth. 

 This apparent exception to the Mendelian ratio finds 

 an explanation, however, when it is assumed that selec- 

 tive fertilization takes place in such a cross, and thus, 

 since a D gamete never unites with another D gamete, 

 but always with its opposite, R, pure yellow mice are 

 unknown. 



This supposition is further supported by the fact 

 that the litters of young from yellow mice are, on an 

 average, only three fourths as large as normal litters 

 of mice, which is exactly what would be expected 

 if one fourth of the possible gametic combinations 

 {DD) fail to produce offspring. 



Castle's tentative explanation of the determina- 

 tion of sex at least breaks away from the old concep- 

 tion that the sperm-cell produces male offspring and 

 the egg-cell, females. It agrees, too, with Darwin's 

 idea that both sexes are present in each individual 

 with one sex latent. In certain parthenogenetic 

 rotifers, aphids and daphnids, both sexes are plainly 

 present in the female, since two kinds of easily dis- 

 tinguishable eggs are produced, one of which develops 

 into males and the other into females without fer- 

 tilization or any kind of a union with a sperm-cell. 



