No. 646] 



LINKAGE IX I'EEOMYSCUS 



415 



of F2 individuals derived from simple Fj x Fj matings 

 is too small to give a representative diliybrid ratio. The 

 really important tests have been made witli 'Vxtrnctod" 

 albinos and pallids of the generation. 



Matings have been made (1) between " cxi incifil " al- 

 binos and "pure" pallids {i.e., those known to be free 

 from the factor for albinism), (2) between extracted 

 pallids and pure albinos, and (3) between extracted pal- 

 lids and extracted albinos. There were likew^ise a num- 

 ber of matings in which the pedigrees were less simple.^ 

 On the assumption of a w^holly independent segregation 

 of these factors, our Fg pallids (of simple pedigree) 

 should have a 2/3 chance of being heterozygous for al- 

 binism, while our albinos should have a 3/4 chance of 

 being either homozygous or heterozygous for pallid.^ ° 



Eighteen F2 mice were involved in these tests. The 

 total number of offspring derived from these was 135, 

 the number per parent ranging from 3 to 26. By no 

 means all of these parents, taken singly, have thus far 

 given birth to a sufficient number of young to prove their 

 genetic composition with any certainty. But the cumula- 

 tive testimony of all of these matings is overwhelming. 

 Not a single pallid mouse and only tw^o albinos have ap- 

 peared among the 135 young which have thus far been 

 born. Had there been a normal proportion of ' ' carriers ' ' 

 among the parents, these matings should have yielded 37 

 albinos and 18 pallids, as the most probable "expected" 

 numbers. That all of the offspring with two exceptions 

 (these being sibs) were of the wild type is evidence of a 

 high degree of linkage (in this case "repulsion") be- 

 tween the albino and the pallid factors," 



» Back-crosses and heterozygous albinos figured in some of these pedi- 

 grees. In these cases the odds are different from those which hold for indi- 

 viduals derived from the simpler types of mating. They have, however, 

 been computed for every animal used. In about half of the "extracted" 

 albinos, for example, there was only a 5/8 chance that the individual 

 carried the pallid factor. 



10 It is a safe assumption that the double recessive form would be albino. 



11 It might be supposed that the testimony of 18 parent mice, even if all 

 of these were shown conclusively to be lacking in "cross-over" gametes, 



