292 GENETICS 



defectiveness. But since In the two stocks these have been 

 acquired independently, the recessive genes will in most cases 

 be present in different pairs in the two stocks. Thus the 

 genes of the two stocks show the condition indicated in the 

 diagram of figure 60; each has numerous recessive gene 

 pairs, so that each has a number of weaknesses and defects. 

 When such stocks are crossed, the genes from the different 



P 



M 



Figure 60. Diagram to illustrate how parents (P and M) showing 

 many recessive defective characteristics may produce offspring with 

 none. The spindle-shaped bodies represent genes; those represented in 

 white are recessive, producing defective characteristics. The offspring 

 (F) have a normal dominant gene in every pair, and are therefore 

 without any of the parental defects. 



sources supplement each other, each stock contributing a 

 vigorous dominant gene for each pair that is weak and re- 

 cessive in the other. In consequence the offspring (Fi) are 

 far superior to the parents. 



This great improvement in Fi when two different stocks 

 or varieties are crossed is common in domestic animals and 

 plants; it is commonly spoken of as hybrid vigor, or heter- 

 osis. Two varieties of maize when crossed commonly pro- 

 duce offspring that are stronger and more vigorous than the 

 parent varieties, and give a higher yield of grain. The two 

 original varieties may both be short, spindling, weak, with 

 almost no yield of grain ; their offspring are tall and vigorous 



