GAMETIC COUPLING IN YELLOW RATS. 179 



one sex. I may say, therefore, that the classification of two animals as 

 double recessives made in our prehminary paper was based on tests 

 which had produced 14 and 9 yellow young respectively. The only 

 possible alternative classification would have involved an expected 1 : 1 

 ratio of black-eyed to red-eyed young. The chances are overwhelm- 

 ingly great against the observed results being departures due to ran- 

 dom sampling from this expectation. The additional case of a double 

 recessive reported in this paper is so classified on tests which, to the 

 present time, have produced all together 28 yellow young. The num- 

 ber of young produced in each of the other tests is indicated below. 

 Tests taken to indicate that the parent was of the formula rrPP 

 produced only dark-eyed young (gray or black-coated), as follows: 



No. of young. ... 4 5 6 7 8 9 10 11 12 13 16 17 19 

 Cases 244522 13 123 1 2 = 32 



Tests showing the parents to be of the formula rrPp gave the follow- 

 ing numbers in 13 tests: 



Dark-eyed : Pink-eyed. . .2: 6 4:2 5:4 1:5 3:3 5:3 4:6 1:5 5:5 3:4 3:2 1:5 6:3 



Pink-eyed animals were classified as of formula RRpp on the basis 

 of the following tests, which yielded only dark-eyed young: 



No. of young.... 4 6 7 8 9 10 11 12 13 14 15 16 18 

 Cases 213222161311 2 = 27 



Pink-eyed animals were shown to be of formula Rrpp by the follow- 

 ing tests: 



Dark-eyed: Red-eyed... 10: 7 8:4 1:4 7:8 6:7 5:6 3:1 4:6 4:2 8:2 



Both red-eyed and pink-eyed yellow rats, when crossed with albinos, 

 produce an F : generation consisting exclusively of black-eyed (black 

 or gray) young. F 2 from the red-eyed cross consisted of black-eyed, 

 red-eyed, and albino young, and F 2 from the pink-eyed cross consisted 

 of black-eyed, pink-eyed, and albino young. If no linkage occurs the 

 expectation in each case is 9:3:4, and we at first supposed that this 

 was the ratio approximated. But a summary of all litters thus far 

 obtained indicates a probable linkage between albinism and the two 

 yellow variations. 



Thus, red-eyed non-agouti yellows mated with albinos from our plus- 

 selected hooded race produced 17 black F! young. These have given 

 us 58 F 2 young, of which 30 are black-eyed, 18 red-eyed, and 10 albinos. 

 A 9 : 3 : 4 ratio would call for 32.5 : 11 : 14.5. It is evident, therefore, 

 that we have too many red-eyed young and too few black-eyed and 

 albinos. Linkage (in this case, repulsion) between red-eye and albin- 

 ism would tend to increase the number of red-eyed and to decrease 

 the number of black-eyed without changing materially the expecta- 

 tion for albinos; hence, linkage seems probable. Linkage involving 

 1 cross-over to 3 non-cross-over gametes, or 25 per cent cross-over 



