86 Monohybrids 



ing a plant and raising the offspring would accomplish the same 

 result. To be certain of including a sufficient number of reces- 

 sives to establish the nature of the unknown within the realm of 

 probability, however, the seedsman would have to grow a much 

 larger number of plants from a selfing than from a testcross. 

 The use of seed from a self-pollination would require more land 

 than seed from a testcross, and that land might be used more 

 profitably for another purpose; it also would require more labor 

 to pot and set out the additional plants required by this method. 

 Usually, therefore, the testcross method is more practical from 

 an economic viewpoint. However, the best method to use is also 

 determined in part by the nature of the plant under considera- 

 tion. Each flower of wheat produces one seed. This plant is 

 self-fertihzed with no difficulty, but the labor involved in making 

 over a hundred hand pollinations is a great item of expense. For 

 wheat, the expense of making the crosses might outweigh that of 

 the additional land and labor necessary for a test by self-fertiliza- 

 tion and might make testcrossing impractical. The situation 

 would be different for a plant like tobacco where one hand pol- 

 lination would produce several hundred seeds; for it the test- 

 cross method would be more desirable. 



Species of animals in which the individuals are of one sex only 

 are tested by the testcross method as it is obviously impossible 

 to self-fertilize them. It would be possible to test an unknown 

 dominant animal by crossing with a known heterozygote, but 

 such a method would be no simpler than to cross with a recessive 

 and would require a greater number of offspring so as to be sure 

 to include a reasonable number of recessives. 



Incomplete Dominance 



When a homozygous dominant is crossed with a homozygous 

 recessive, the Fi is phenotypically like the dominant parent and 

 the F2 splits into three dominants to one recessive if dominance 

 is complete. When dominance is incomplete, however, the Fi 

 does not resemble either parent and the phenotypic ratio in the 

 F2 is identical with the genotypic ratio. In the four-o'clock, a 

 red-flowered plant, WW, crossed with a white-flowered plant, 

 WW, would give a pink-flowered Fi, Ww; this, when selfed, would 

 show a segregation in the F2 into 1 red (WW) : 2 pink (Ww) : 1 

 white (ww). 



