200 OUTLINES OF EVOLUTIONARY BIOLOGY 



in the parent organisms, and if we suppose that each one contains 

 only one of the two contrasted characters with which we are 

 experimenting, and if we further suppose that the two kinds are 

 produced in equal proportions and that they unite at random, 

 then, in accordance with the law of chance, we should expect 

 to get in the offspring the proportion of one with the one 

 character, two with both characters and one with the other 

 character, exactly as Mendel found in his experiments. 1 There 

 seems to be no other possible explanation of the Mendelian 

 phenomena. 



The conclusions to be drawn from these results are of 

 fundamental importance. In the first place, we learn that small 

 individual characters may be separately represented in the 

 germ cells and separately transmitted from parent to offspring. 

 This indicates that the entire organism may be built up of a 

 number of "unit characters," and if we can once establish 

 the general occurrence of such unit characters we shall have 

 taken a long stride towards the understanding of the laws of 

 heredity. 



In the second place, we may conclude from these experiments 

 that, as regards the unit characters with which we are dealing, 

 we have a complete segregation amongst the germ cells or 

 gametes, each of which carries only one of each pair of contrasted 

 characters (technically termed allelomorphs), and in this respect 

 it makes no difference whether the germ cell be male or female. 

 Thus we arrive at what is sometimes termed the doctrine of the 

 "purity of the gametes" purity, that is to say, with regard to 

 each character of any such contrasted pair, and without reference 

 to the other innumerable characters which must be represented 

 in the germ cells in order that the whole complex structure of the 

 organism may be developed from them. 



We may now examine an example of a class of practical 

 problems which may be solved by the application of such simple 

 Mendelian principles. 



Fowl-fanciers are well acquainted with a particular breed of 

 fowl known as the blue Andalusian. It has long been known 

 that this kind of fowl cannot be made to breed true. Some of 



1 A little consideration will show that if the contrasted characters are repre- 

 sented in equal proportions of both male and female gametes, the facts that a 

 male must always unite with a female, and that the actual number of sperm cells 

 produced is usually very much larger than the number of ova, will not affect the 

 result. 



