SEGREGATION. 



9 



races ? What does this hybrid transmit to its offspring ? Let us con- 

 sider the case of the hybrid primrose having purple flowers. The 

 facts are, as found by experiment, that this purple hybrid produces 

 three kinds of progeny. About one-fourth of the seed produced by 

 this hybrid produces plants having red flowers like those of the red- 

 flowered parent of the hybrid. Another fourth have white flowers, 

 while the remaining half have purple flowers. Furthermore, the 

 red and the white flowered plants of this second generation will repro- 

 duce only red or white progeny, as the case may be; that is, they 

 behave exactly like pure red or pure white races. On the other 

 hand, every one of the purple-flowered plants will produce in the 

 next generation three kinds of progeny as before. One-fourth of 

 the progeny of these purple-flowered plants will have red flowers, 

 one-fourth of them white flowers, and half of them purple flowers. 

 This experiment was continued by an English florist for fifteen 

 years, always with the same result. The purple always split up 

 into one-fourth red, one-fourth white, and one-half purple, while 

 the reds and the whites thus produced always behaved like pure 

 races of red or white. From these facts we infer that in self -fertilized 

 species an individual which is hybrid with reference to a particular 

 pair of characters tends to produce progeny one-fourth of which 

 is of pure race like one of the parents of the hybrid, another fourth of 

 pure race like the other parent, while the remaining half is hybrid 

 like the original hybrid itself. 



Mendel suggested that the cause of these peculiar phenomena is 

 that the hybrid produces two kinds of ovules and two kinds of pollen, 

 the one kind of ovule and one kind of pollen being exactly like those 

 of one of the parents of the hybrid so far as the one character under 

 consideration is concerned, the other kind being like those of the 

 other parent. Let us see how this hypothesis fits the facts. 



Suppose the hybrid does produce two kinds of ovules in equal 

 numbers, one of which carries the potentiality of the red flower 

 color, the other that of the white, and two kinds of pollen differing 

 in a similar manner. Let us designate the ovules and pollen car- 

 rying red by the letter R, and those carrying white by W. Let us 

 first consider what happens to the ovules of type R. These ovules 

 are offered both R and TF pollen in equal quantities. The chances 

 are, therefore, that half the R ovifles wiU be fertilized by R pollen 

 and the other half by TF pollen. In the first of these cases, we have 

 R ovules fertilized by R pollen, Avhich would, of course, give pure 

 red individuals. For convenience, we may designate these individ- 

 uals resulting from the fertilization of R ovules by R pollen as RR 

 individuals. Since half of the ovules produced by the hybrid are 

 supposed to be of type R and since half of these are fertilized by R 



165 



