6 Darbishire, Mendelian Principles of Heredity. 



that the same was true of the yellow. That is to say, the 

 germ-cells of green peas will always produce green peas, 

 and that those of yellow will continue to give yellow. 



But what happens when a green pea is crossed with a 

 yellow? that is, when a 'green' germ-cell meets a 

 ' yellow ' germ-cell ? We know that a yellow-seeded 

 hybrid is produced ; but we want to know the condition 

 of its germ-cells. Mendel's hypothesis was that it contained 

 50% 'yellow' germ-cells and 50% 'green' ; no 'greenish- 

 yellow,' and no ' yellowish-green ' germ-cells, but equal 

 numbers of green-producing and yellow-producing 

 gametes ; he maintained that no fusion of characters (like 

 a chemical combination) takes place ; but merely a 

 mingling (like a mechanical mixture). 



The proportions in which the Z^'s, DR^s, and Ks, occur 

 in the offspring of hybrids is certainly accounted for by 

 this theory. Let us (putting aside sex for brevity's sake) 

 imagine what happens when a hybrid is cross- or self- 

 fertilized. Imagine, first, a 'green' germ-cell: it is an 

 even chance that it unites with a ' green ' or ' yellow ' 



a. Gy.G. 



b. Gx Y. 



Imagine a ' yellow ' : it has an even chance of uniting with 

 a * green ' or ' yellow.' 



c. YxG. 



d. Fx Y. 



Now h and c are the same ; so that the proportions in 

 which pure ' greens', pure 'yellows' and hybrids would be 

 produced, as the result of the random unions of the germ 

 cells of two hybrids, would be 16^ : 2(?F (hybrid) : i F, in 

 every four; or, of course, 25%/^ : 50% DR : 25% i?. And 

 this is exactly what happens as the result of actual 

 breeding. 



