i68 BREEDING 



written down each. time. The result of any given 

 pair of throws may, of course, be one of three things : 

 head-head, head-tail, and tail-tail. And the total 

 result of a thousand throws will not be very far 

 from 250 head-heads, 500 head-tails, and 250 tail- 

 tails. 



In this illustration the matter of sex has been 

 left out, so that the equal frequency of the four types 

 of unions of germ cells seen in the first column of 

 the Table on p. 167 has not been illustrated. This 

 can easily be done. One of the persons who tosses the 

 coins may be designated the " male," and the other 

 the " female." If the sex of the tosser be taken into 

 account and recorded, it will be found that the four 

 following events will occur equally frequently. Male 

 will be denoted by the symbol (J, and female by 

 the symbol ?: — 



Head tossed by (J and Head tossed by $ 

 Head tossed by (^ and Tail tossed by 5 

 Tail tossed by (J and Head tossed by 5 

 Tail tossed by ^ and Tail tossed by 5 



Before we leave this illustration of the manner 

 in which the Mendelian ratio 1 : 2 : 1 is brought about, 

 a matter of great theoretical and practical importance 

 must be dealt with. 



This ratio 1 : 2 : 1 in the second hybrid generation, 

 which is the essential and diagnostic feature of 

 Mendelian inheritance, is brought about by the 

 random union of male germ cells (50 per cent, of 

 which contain the factor for the dominant, and 50 



