Heterozygous Populations in Plants 141 



pair of genes, when crossed, should produce two types of off- 

 spring phenotypically. When the organism is a dihybrid, four 

 kinds of offspring should result. For a polyhybrid heterozygous 

 for n pairs of genes, the number of kinds of offspring should be 

 2^^. If two people heterozygous for the same pair of genes in 

 each of man's 24 chromosome pairs should mate, they could 

 produce 2-^ or 16,777,216 different kinds of offspring. There is 

 no wonder, therefore, that children do not look exactly like one 

 parent. The possibilities for diversity among offspring are great 

 in a population as heterozygous as that of human beings. 



Heterozygous Populations in Plants 



Many plants and animals also are highly heterozygous and 

 produce a wealth of different types of offspring. For that 

 reason, plant breeders often have to resort to vegetative propa- 

 gation in order to maintain a certain type. Let us suppose that 

 a hybridizer crosses two different varieties of apples and obtains 

 among the offspring one plant that is a superior type of apple in 

 every respect. He naturally wishes to propagate this new va- 

 riety and to sell it commercially. Because the parents differed 

 by many genes, however, this new variety would be highly 

 heterozygous. Therefore, the offspring which it would produce 

 from seed would show great variation; probably only a very 

 few out of a very large number of plants would be sufficiently 

 like the heterozygous parent to be worth raising. These plants, 

 too, would undoubtedly fail to breed true for they would also 

 be heterozygous for a number of genes. After many generations, 

 the hybridizer, if he were still living, would perhaps succeed in 

 establishing a true-breeding strain of this excellent variety, 

 but at a tremendous cost in labor and acreage that could be 

 devoted to other purposes. 



Rather than try to establish such a true-breeding strain, he 

 would reproduce this desirable apple vegetatively, by taking 

 buds and grafting them on to stocks of inferior types. Since 

 the branches which developed from the bud would be like the 

 tree from which the buds were taken, they would produce the 

 same kind of apples that were found on this superior plant. 

 Those branches would be highly heterozygous and would not 

 breed true any more than the original plant. They would pro- 

 duce superior fruit, however, and that is all that would be re- 



