14 MENDELISM 



hybrid, but that they are produced in equal numbers. Of 

 four ovules, two will be dominant and two recessive ; and 

 the same will hold true for the pollen grains. 



But how will equal numbers of dominant and recessive 

 germ-cells produce offspring in which the dominants exceed 

 the recessives by three to one ? For four individuals, we 

 shall require four pollen grains and four ovules. Let us 

 assume that a hybrid produces four ovules which are fertilized 

 by four of its own pollen grains. What is most likely to 

 happen, on the assumption that the dominant and recessive 

 germ-cells exist in equal numbers, is shown by this diagram — 

 Of the two dominant pollen grains-one will unite with a 

 dominant ovule (a) ; the other with a recessive ovule (6) ; 

 and of the two recessive pollen grains one will meet the 

 remaining dominant ovule (c), and the other the recessive 

 ovule (d). That is, all the possible combinations will occur. 

 Other combinations than the above four might occur, and 

 sometimes do so, — for instance, the pollen grain might 

 unite with the second dominant ovule ; but in the great 

 majority of cases the result is as above stated, and so we 

 get three individuals [a, 6, and c) produced by at least one 

 dominant germ-cell, and the result will be that they appear 

 like the dominant parent ; the fourth will be recessive. 



Ovules. Pollen grains. 



• < • • 



a • 



O 



o ° 



"2 o 



o <- 

 Germ-cells. Resulting individuals. 



The result, you observe, is the same whether it be the 

 pollen or the ovule that is dominant. It is, however, clear 

 from the diagram that the three dominant offspring, though 



