MENDELISM 433 



easy to illustrate subsequent results with the counters. The 

 result of mating the hybrids in F2 can be illustrated in exactly 

 the same way as that of mating the Fi hybrids. And it is 

 readily seen that it is no more possible to get anything but 

 Dwarfs from the Dwarfs in F2 than it is possible to get any- 

 thing but pairs of white counters by drawing from two hats 

 each containing nothing but white ones, and that it is no more 

 possible to get anything but Tails from those Tails, i.e. the 

 pure ones, which resulted from the union of like gametes than 

 to get anything but pairs of reds from drawings from two hats 

 containing only reds. 



The animal or plant resulting from the union of two germ- 

 cells or gametes — and all animals and plants which reproduce 

 sexually result from such a union — is called a zygote ; that 

 resulting ;from the union of like gametes or germ-cells is 

 called a homozygote ; that from the union of unlike gametes 

 a heterozygote. 



The heterozygote in the case which we have been con- 

 sidering as an example was only distinguishable from the 

 dominant homozygote by the fact that when allowed to self- 

 fertilise it produced Dwarfs as well as Tails. But sometimes 

 the result of crossing two pure strains is different from either 

 of them — that is to say, the heterozygote has a character 

 peculiar to itself. This makes it possible to distinguish between 

 the heterozygotes and the dominant homozygotes in F2 other- 

 wise than by breeding from them. 



The phenomenon may be illustrated by the famous case 

 of the Andalusian fowl. If you buy a pair of these birds from 

 the very best source you will find that they will not breed 

 true, but that they will produce besides the Andalusians 

 blacks and splashed whites, and that on the average these 

 three forms will be produced in the proportion 1 black, 2 Anda- 

 lusians, 1 splashed white. If you were not acquainted with 

 Mendelian phenomena, but were guided by the principles of 

 heredity which were universally held in pre-Mendelian days 

 (and are still by those who are not Mendelians), you would 

 sell the blacks and whites for eating and breed from the 

 Andalusians, in the belief that, as they at least, to your own 

 personal knowledge, had an Andalusian father and mother, 

 they would be more likely to breed true than a pair of Anda- 

 lusians of unknown and, as you would expect from the result 



