130 THE THEORY OF THE GENE 



Tetraploidy as a Means of Increasing the Number 



of Genes in a Species. 

 One of the most interesting considerations connected 

 with tetraploids from an evolutionary standpoint is the 

 opportunity they may seem to furnish for increasing the 

 number of new genes. If new and stable types arise 

 throuo-h doublino: the number of the chromosomes, and 

 if, after doubling, the four like chromosomes should be- 

 come different in the course of time, so that two become 

 more like each other, and the other two also become more 

 like each other, the tetraploid would then resemble geneti- 

 cally a diploid, except in so far as many of the genes 

 remained unchanged. Many like genes would then be 

 present in four chromosomes of each set, and the expec- 

 tation for the F2, when an individual is heterozygous for 

 only one pair of genes, would be a Mendelian ratio of 15 

 to 1 instead of 3 to 1. Such ratios have in fact been found 

 (wheat, shepherd's purse) but whether tetraploidy ac- 

 counts for the result or whether doubling has occurred in 

 some other w^ay remains to be determined. 



On the whole, it seems that until we know something 

 more as to the way in which new genes arise — if they do 

 now arise — it is rather hazardous to take advantage of 

 tetraploidy as a general explanation to account for a 

 change in number of the genes. It is true that in monoe- 

 cious plants new types may arise in this way, yet it is 

 improbable that, in animals with separate sexes, tetra- 

 ploidy could become established (except in parthenoge- 

 netic species), because, as has been pointed out above, the 

 tetraploid is lost by crossing to an ordinary or diploid 

 individual and not easily recovered afterwards. 



