i84 BREEDING 



less and less as the size of a family containing no 

 recessive increases. 



By far the most convenient way, as stated at 

 the beginning of the last paragraph, of testing the 

 zygotic constitution (that is, of determining whether 

 it is heterozygous or homozygous) of an individual 

 bearing the dominant character is to mate it with 

 a recessive. There are two advantages of this over 

 any other way ; the first and most practically impor- 

 tant is that an individual bearing the recessive 

 character can only be one thing, ER, in contra- 

 distinction to one bearing the dominant character, 

 which can, of course, be DD or DR ; so that the 

 recessive can be used at once, without any pre- 

 liminary time-wasting testing which is necessary to 

 determine the constitution of an individual bear- 

 ing the dominant character. The recessive wears 

 its gametic constitution on its sleeve, and no further 

 credentials are required. The second advantage of 

 testing by mating with a recessive is that the result 

 of mating a hybrid with a recessive is the production 

 of twice as many recessives as are produced by the 

 union of two hybrids, 50 per cent, as against 25 per 

 cent. ; and consequently twice as great a likelihood 

 of the form tested throwing a recessive, if it is a 

 hybrid, as there would be if it were mated with a 

 hybrid. 



My hybrid mice of the first hybrid generation 

 were mated with albinos, not to test their hybridity, 

 for that was obviously not in question, but to test 

 the truth of the statement that hybrids mated with 



