November 10, 1911] 



SCIENCE 



637 



fieulty at all when the chiaama type of cross 

 union between the homologous chromosomes is 

 grasped; the diiEculty arising rather from my 

 attempt to express in a sentence or two the 

 essence of the mechanism described by Jans- 

 sens. I said that the well-known twisting of 

 the chromosomes giving a spiral line of separa- 

 tion was followed by a splitting in a single 

 place. Emerson properly objects that if this 

 were strictly carried out some of the genes, 

 those at the nodal points, might be divided 

 quantitatively. In reality according to Jans- 

 sens the chromosomes break at the nodal 

 point and unite so that the two resulting 

 chromosomes consist of pieces (two or more) 

 of each of the original members of the pair. 

 If, then, the genes do not themselves split 

 when the chiasma is formed there is no oppor- 

 tunity offered for a quantitative division. 

 This is the mechanism that Janssens describes 

 as I understand it. 



A second point raised by Emerson is like- 

 wise not a serious difficulty, although we need 

 further facts in different animals and plants 

 concerning the nature of the chiasma type 

 before we can speak positively about the mat- 

 ter. Emerson asks how if the mechanism 

 explains the facts of coupling in those cases 

 where some interchange must be admitted (in 

 a case like that of Drosophila, for example), 

 can we account for the purity of certain 

 races where certain characters remain coupled 

 and never interchange? My answer is, first, 

 that the hypothesis was offered primarily to 

 account for those cases where the coupling is 

 not absolute and crossing must be admitted; 

 and second, that whether interchange does or 

 does not occur will depend primarily on the 

 nature of the chiasma type; whether, for ex- 

 ample, crossing takes place at certain levels 

 (stations) more likely than at others or 

 whether it is entirely a chance crossing. 

 Until cytologists have settled this matter we 

 may leave the question open ; but the very lati- 

 tude that this mechanism offers seems to me 

 to fit the situation far better than one that 

 admits of no such freedom; for the facts 

 themselves are diverse. That complete coup- 

 ling of several characters may exist, such as 



yellow, black and chocolate in mice is clear; 

 and the result in such cases may be due to the 

 region of the chromosome (that contains the 

 factors for these colors) holding together as a 

 unit when the chiasma forms; while in other 

 cases the union between a similar series of fac- 

 tors may not be so close, so that crossing is 

 more likely to take place. 



As to " what has become of the ' individu- 

 ality ' of the chromosomes " if interchange be- 

 tween homologous pairs be admitted, is a mat- 

 ter of very small consequence; since Boveri, 

 who is the chief exponent of the h3T)othesis of 

 individuality, has long since admitted such an 

 interchange in his definition; and since the 

 facts of Mendelian inheritance call for such 

 an interchange, if the chromosomes be ad- 

 mitted as the most likely vehicles of hereditary 

 factors. All that my hypothesis pretends to 

 account for is that groups of factors that enter 

 together tend to remain together. The chi- 

 asma type appears to explain how such union 

 may remain; perhaps some other mechanism 

 may be found that will do as well. The im- 

 portant point is thai the coupling (associa- 

 tion) of sex-limited characters that I have 

 found in Drosophila shows that the factors 

 must he referred to the same chromosome, and 

 if so there seems to he no escape from the 

 conclusion that interchange as well as associa- 

 tion must he admitted on the chromosome 

 hypothesis. 



Emerson has himseK suggested a view to 

 explain the remarkable cases of coupling that 

 he has found in corn.' His hypothesis re- 

 quires that in those cases where no inter- 

 change takes place the coupled factors lie in 

 homologous chromosomes, while in those cases 

 where interchange takes place the same or 

 similar factors are contained in non-homol- 

 ogous chromosomes. This may seem probable 

 or improbable, as one prefers, but in the case 

 of Drosophila, where the factors in question 

 are sex-limited and coupled with the sex chro- 

 mosome, we see that his hypothesis can not 

 hold, and that the facts can be explained with- 

 out need of such an hypothesis. 



^Annual Eeport Nebraska Agricultural Experi- 

 ment Station, 1911. 



