CROSSING OVER 



89 



Fia. 36.— Back-cross of Fi female (out of black vestigial by wild) to black vestigial male. 



in the same chromosome, or whether they enter the cross 

 in opposite chromosomes— their likelihood of interchange 

 is exactly the same. If the F^ male had been back-crossed 

 (Fig. 34) only two kinds of offspring would have been 



