SOME REMARKS ABOUT THE DISTANCE BETWEEN THE GENES 2.i3 



casi'. If now in these cases the contact of the chroniosonies lakes 

 phice at corresijoiulin},' points, e. j,'. Ijetween AH and Ab, hetwecn 

 BC and he and bctwci'ii (.1) and cl) tlie assuni|)li()n must l)e made 

 llial the ninnher oi crossing oNcr at the three |)oints will lake |)hi((' in 

 the rekdion 1 : 2 : 3. Instead ol assuniinj^ the j)ercentaj,'e of crossini^ 

 o\er between contiguous genes to ])e proportional U) the distance» 

 Ix'tween them the presumption may be made that the distance between 

 the different genes is the same in all the cases, and that the percentage 

 of crossing over is proportional to the difference of affinity between 

 the genes. 



The hypothesis developed above bears resemblance in some 

 points to the view expressed by Goldschmidt as to the »distance» 

 between the genes. Goldschmidt (1917, pag. 83) writes as follows: 

 »Er (Morgan) sieht in den un])ekannten, quantitativ bestimmten Kräften 

 des Austauschs eine entsprechende Entfernung der Faktoren im 

 Chromosom. Es ist aber doch klar, dass man jede Proportion geo- 

 metrisch als Entfernungen auf einer Geraden darstellen kann. Wenn 

 diese Darstellung also in gegebenen Fall stets mit Tatsachen über- 

 einstimmt, so beweist das nicht etwa, dass nun wirklich Entfernungen 

 auf einer Geraden hinter der Erscheinung als Ursache stehen, sondern 

 es beweist nur, dass irgend welche Kräfte im Spiel sind, deren rela- 

 tiven Effekt als Entfernungen auf einer (ieraden dargestellt werden 

 können». 



The percentage of crossing over between A and C {B being pre- 

 sent) equals the sum of the percentage of crossing over between 

 A and B and C. The percentage value may, however, be less than 

 this sum if B is absent, as the degree of affinity between A and C may 

 either equal the sum of the affinity degrees between A and B and B and 

 (I, or it may be less than this sum. Corresponding phenomena are not 

 unknown in Drosophihi mehnwgdster. Whether or not these ])heno- 

 inena may become better explained on the basis of the above developed 

 hypothesis cannot yet, however, be ascertained. 



SUMMARY. 



It has been shown by Morgan and co-workers that the genes in 

 Drosophihi nwlanogaster are situated in a row behind one another in 

 a definite order. It is believed that the distance between contiguous 

 genes is different, and that the different number crossing over obtained 



