66 



PHYSICAL BASIS OF HEEEDITY 



of 16 individuals is expected. Whenever possible, there- 

 fore, the back-cross experiment is preferable to the 

 inbred F^ cross. 



In animals and in plants with separate sexes it has 

 been found that both F^^ males and F^ females give when 

 back-crossed, identically the same results, showing that 

 free assortment takes place in both sexes. 



Fig. 30. — Miniature wing, a; and dumpy, b; and minature dumpy, c. 



There is a corollary to the cross involving two pairs 

 of factors that is interesting, because it gives an explana- 

 tion of the phenomena of atavism. The wild vinegar fly, 

 Drosophila melanog aster (Fig. 4), has long wings. It 

 gave rise, through mutation, to a race with miniature 

 {mm) wings (Fig. 30, a), and also to another race with 

 short wings (Fig. 30, h) called *^ dumpy'' {dd). If a 

 female miniature (mmDD) is crossed to a dumpy male 

 (MMdd), all the offspring (MmDd) have long wings like 



