PRINCIPLES OF GENETICS 17 



female with gray wings and white eyes the daughters 

 have gray wings and red eyes (Fig. 12). If one of the 

 daughters is mated to a male with the two recessive 

 mutant characters, yellow wings and white eyes, there 

 are four kinds of flies produced. Two of these are like the 

 two grandparents, and constitute 99 per cent of the out- 

 put. Two are new combinations, or crossovers, one with 

 yellow wings and white eyes and the other with gray 

 wings and red eyes. Together they make up 1 per cent of 

 the second generation. 



These results show that the same amount of crossing- 

 over takes place irrespective of the way in which the 

 combinations of the same characters enter the cross. If 

 the two recessives enter together, they tend to hold to- 

 gether. This relation was called coupling by Bateson and 

 Punnett. If one of the recessives enters from one parent 

 and the other recessive from the other parent, they tend 

 to come out separately (each in combination with the 

 dominant that went in with it). This relation was called 

 repulsion. It is clear, however, from the two crosses that 

 have just been given, that these relations are not two 

 phenomena, but expressions of the same one, namely, that 

 the two linked characters that enter a cross, quite irre- 

 spective of their dominance or recessiveness, tend to hold 

 together. 



Other characters give different percentages of cross- 

 ing-over. For example, when a male Drosophila with the 

 two mutant characters, white eyes and miniature wings 

 (Fig. 13), is mated to a wild type fly with red eyes and 

 long wings the offspring have long wings and red eyes. 

 If one of the daughters is mated to a male with white 

 eyes and miniature wings the offspring are of four kinds. 

 The two grandparental types constitute 67 per cent and 

 the two cross-over types 33 per cent of this generation. 



A still higher percentage of crossing-over is given in 



