20s 



THE AMERICAN NATURALIST 



[Vol. LIV 



Thus in a cross of two yellows, one of them probably hetero- 

 zygous for both black and brown and the other for brown only, 

 the offspring totalled fourteen yellow and four brown, the ex- 

 pectation being twelve yellow, three black and three brown. 

 The yellow descendents of this mating when bred to browns are 

 expected to give two yellows (one heterozygous for both black 

 and brown and one heterozygous for brown only) to one black 

 to one brown. The actual numbers obtained were eighteen 

 yellow, one black, and ten brown, and the expected numbers 

 fourteen yellow, seven black and seven brown. Dunn has sum- 

 marized these data according to the percentage of young of 

 each sort produced, as follows: 



Per cent, expected . 



In the total number of young observed, the chances are equal 

 that the 16.6 per cent, expected black young might go as high as 

 30.8 per cent, or as low as 2.4 per cent. It is therefore apparent 

 that neither the 3.4 per cent, black nor the 34.5 per cent, brown 

 are significantly outside the limit of probable variation due to 

 chance. 



Although not including chance fluctuation as one of the three 

 theories capable of explaining his observed facts, Dunn evi- 

 dently feels the need for larger numbers of young before con- 

 sidering random sampling eliminated. 



It is interesting however to see just what evidence there is in 

 Dunn's data that black and yellow are linked. Apparently the 

 only facts in support of this hypothesis is the deficiency of 

 blacks and the slight excess of browns referred to. Significant 

 evidence for the expected excess of yellows carrying both black 

 and brown as compared with those carrying brown only is not 

 obtained. Of seventeen such yellows tested, ten carried both 

 black and brown and seven brown only — exact equality or 8.5 of 

 each is the Mendelian expectancy. The excess of yellows carry- 

 ing black and brown is 8.8 per cent, as against non-yellow browns 

 of 17.9 per cent. The deficiency of yellow carrying brown only 

 is 8.8 per cent, as against a deficiency of blacks of 13.3 per cent. 

 The sum of the departures from the expected equality in yellows 

 is 17.6 per cent., while in non-yellows it is 31.2 per cent., or 

 almost twice as much. The discrepancies in the yellow indi- 



