36 HEREDITY IN RABBITS, RATS, AND MICE. 



non-cross-overs. But in the production of the 45, only non-cross-over 

 gametes had functioned. The total gametes involved accordingly are 

 18 cross-overs and 18 4- (2 X 45) = 108 non-cross-over gametes; 

 total 126. As 18 is 14.28 per cent of 126, the indicated percentage of 

 cross-overs is 14.28 per cent. 



Among 75 F2 albinos which were tested, 20 produced pink-ej^ed 

 young (as well as dark-eyed ones), while the remaining 55 produced 

 only dark-eyed young. Reasoning as before, there were e\'idently 

 involved in this case 20 cross-over gametes and 20 -|- (2 X 55) = 130 

 non-cross-overs, total 150. But 20 is 13^ per cent of 150; hence the 

 indicated percentage of cross-overs is 13^. 



Combining the tests of F2 pink-eyed and of F2 albinos, we have in 

 tests involving 276 Fi gametes an indicated percentage of 13.76 

 cross-overs. 



The pink-eyed parents, which in the course of these tests had been 

 found to carry albinism, were now mated with each other, and the 

 albino young which they produced when so mated were used in 

 building up a race of double recessives, for all albinos so produced 

 must of necessity be homozygous for pink-eye. These double reces- 

 sives were next mated with Fi dark-eyed animals obtained by the 

 original cross of pink-eyed with albino, or with dark-eyed individuals 

 of similar genetic constitution which had resulted from the test 

 matings. The interpretation of the results obtained from these back- 

 cross matings is the same as that given by similar matings in the case 

 of rats. The Fi parent would form gametes of the four sorts CP, cP, 

 Cp, and cp, of which cP and Cp would represent the original com- 

 binations found in pure albinos and pure pink-eyed respectively, and 

 so would be non-cross-overs, but CP and cp would arise only by crossing- 

 over. Of the four types of gamete, CP alone would produce a dark- 

 eyed zygote, if mated with a double recessive, cp. But this is one of 

 the two cross-over types. Hence the number of dark-eyed young 

 produced in mating Fi animals with double recessives should indicate 

 half the total percentage of cross-overs. By matings of the sort just 

 described, 3,142 young have been produced, of which 222 were dark- 

 eyed. Doubling this number, we have 444 as the probable number of 

 cross-over gametes in 3,142 gametes produced by the Fi parents, an 

 indicated percentage of 14.13. This agrees very well indeed with the 

 13.76 per cent indicated by the test-matings of F2 pink-eyed and 

 albinos. It seems safe to assume, therefore, that the average cross- 

 over percentage is close to 14 per cent. For the corresponding char- 

 acters in rats, the indicated percentage of cross-overs is considerably 

 higher, ^^z, 21.1, but it should be borne in mind that the estimate is 

 based on a much smaller series of observations in the case of rats and 

 that further observations may alter it materially. 



