64 PHYSICAL BASIS OF HEREDITY 



Fj fly has the formula vVeE. Independent assortment 

 of the two pairs of factors 



v_ E_ 



T T 



gives four kinds of germ-cells both in males and fe- 

 males, thus : 



vE Ve VE ve 



Any one of the four kinds of egg may be fertilized by any 

 one of the same four kinds of sperm giving the same result 

 as in the case of the peas, viz., four kinds of F2 individuals 

 in the ratios of 9 : 3 : 3 : 1. In practical tests the occur- 

 rence of or the possession of a race with both recessives 

 in it is highly desirable for use in making a back-cross to 

 Pj (instead of inbreeding F^'s), because the numerical 

 results obtained by back-crossing furnish, for a smaller 

 number of individuals, more significant data. For exam- 

 ple, if a tall pea with colored flowers is crossed to a 

 short pea with white flowers, and the F^ individuals 

 (SsWiv) are back-crossed to short white peas (sw), the 

 expected ratio will be 1:1:1:1, because the four kinds 

 of gametes in Fi {8W, Sw, sW, sw) will then reveal 

 themselves in the offspring, since the double recessive 

 individual (siv) used for back-crossing (having only 

 recessive gametes) will not ''cover up'^ any of the factors 

 coming from the F^ hybrid. For instance, as shown 

 in our type example, the F^ gametes are SW, Sw, 

 sW, siv. The only kind of gamete produced by the 

 double recessive, short white, is sw. When this meets 

 each of the above gametes only four kinds of combina- 

 tions are possible, viz., SWsw, Swsw, sWsiv, swsw; and 

 these zygotes, containing only the same dominants as the 

 F^ gametes, will reveal what the kinds of gametes were. 

 In practice an approximation to a 1 : 1 : 1 : 1 ratio is much 

 more likely to be evident than an approximation to a 

 9 : 3 : 3 : 1 in which only one double recessive individual out 



