I04 



Outline of Genetics 



using any such formula can usually be avoided through 

 the following expedient. Instead of inbreeding the Fi, 

 it can be back crossed with the double recessive parent 

 (dwarf white). This parent race is perfectly homozy- 

 gous, so that it produces only the one type of gamete 

 (in spite of crossing over, which must be taking place 

 here also). The results of this back cross are represented 

 in fig. 20. It is obvious that the phenotypic ratio 

 obtained (9 tall red:i tall white: i dwarf red: 9 dwarf 



T 

 R 



R 



Fig. 20 — Population resulting from mating of gametes of Fi (shown 

 above) with gametes of recessive parent (only one type of gamete, shown 

 at left) in a case of linkage with lo per cent crossing over. 



white) corresponds exactly to the ratio among the types 

 of gametes produced by the Fj. In this way the cross- 

 over value is quite apparent, and no computation neces- 

 sary. Back crosses wdth the recessive parent will always 

 provide results which are easier to interpret than are the 

 F2 ratios. 



As investigations were made of additional pairs of 

 linked genes, it was discovered that each such pair had 

 a characteristic and rather constant cross-over value. 

 For example, assuming that a third gene A is located on 

 the same chromosome w^ith T and Ry by means indicated 



