THE MECHANISM OF HEREDITY 55 



an L sperm is % x % = 1/4, and the chance of an L egg 

 meeting an 8 sperm is y 2 x % = %. Similarly, the chance 

 of an S egg meeting an L sperm is 1/4 and of an L egg 

 meeting an S sperm 14. The sum of the possibilities 

 is then 



L -\-L = 1/4 



S + L = 1/2 

 S + S =1/4 



One important thing to be remembered, however, is 

 that the law of chance expresses the probability when in- 

 finitely large numbers are concerned. When small num- 

 bers are dealt with, departures from the expected ratios 

 are obtained. It is a matter of common sense that al- 

 though the chances of throwing heads with a penny are %> 

 yet in a small number of throws exactly % of them may 

 not be heads. A regular ratio of these departures is to be 

 expected which accords with the law of error. 



Typically, experiments such as this are the basis for 

 Mendel's Laws of Inheritance, the first of which may be 

 stated as follows : Unit factors contributed by two parents 

 having definite roles in tlie development of characters, 

 separate in the germ cells of the offspring without having 

 influenced each other. Although Mendel himself knew of 

 no mechanism by which such a process could take place, 

 although his theory was evolved wholly as a fitting inter- 

 pretation of the facts obtained by breeding, what can be 

 more reasonable than to suppose that the germ cell factors 

 reside in the chromosomes and that the separation of the 

 chromosomes at the reduction division in gametogenesis 

 furnishes the segregation of factors required? 



The interaction of a pair of homologous factors in a 

 hybrid allelomorphs they are called does not always 



