14 THE THEORY OF THE GENE 



cells. This was revealed, as described above, by crossing 

 the hybrid (Fj) male to a female pure for the four reces- 

 sive genes. All of her mature eggs contain one set of four 

 recessive genes. Any egg fertilized by a sperm with one 

 set of the dominant wild type genes should give a wild 

 type fly. Any egg fertilized by a sperm with the four re- 

 cessive genes (which are the same as those in the female 

 here used) should give a black, purple, vestigial, speck 

 fly. These are two kinds of individuals obtained. 



Crossing-Over. 



The members of a linked group may not always be com- 

 pletely linked as in the case just given. In fact, in the F^ 

 female from the same cross, some of the recessive charac- 

 ters of one series may be interchanged for wild type 

 characters from the other series, but even then, since they 

 remain united more often than they interchange, they are 

 still said to be linked together. This interchange is called 

 crossing-over, which means that, between two corre- 

 sponding linked series, there may take place an orderly 

 interchange involving great numbers of genes. Since an 

 understanding of this process is essential to what fol- 

 lows, a few examples of crossing-over may be given. 



When a male Drosophila with the two recessive mutant 

 characters, yellow wings and white eyes, is mated to a 

 female with the wild type characters, gray wings and red 

 eyes, the daughters and sons have gray wings and red 

 eyes (Fig. 11 ). If one of the daughters is mated to a male 

 with the two recessive characters, yellow wings and white 

 eyes, there are four kinds of offspring. Two kinds are 

 like the grandparents, that is, they have yellow wings and 

 white eyes, or gray wings and red eyes. Together they 

 constitute 99 per cent of the offspring. The characters 

 that went in together have come out together in a much 

 higher percentage than expected from Mendel's second 



