Nos. 618-619] SHORTER ARTICLES AND DISCUSSION 365 



zygotes fail to develop normally after implantation in the 

 uterus," they suggest "that, in mice there may be a 'lethal fac- 

 tor,' similar to those so well known in Drosophila, which is so 

 closely linked to the factor for yellow that they are practically 

 at the same locus and there is consequently no crossing-over." 



If the postulated factors are indeed so closely linked that 

 crossing-over never occurs, would it not be simpler and even more 

 logical to assume that but a single allelomorphic pair of factors 

 is involved? Such an assumption necessitates a second, namely, 

 that one of the factors of this pair is dominant in one of its ex- 

 pressions (yellow) and recessive in a second (lethal). 



Morgan- has recently made a similar assumption for the mu- 

 tant sex-linked factor in Drosophila which causes "notch" in 

 the wings. He states that it is "dominant in regard to the wing 

 character but recessive in its lethal effect. A female with notch 

 wings carries the gene in one of her X-chromosomes and the nor- 

 mal allelomorph in the other X-chromosome. Half of her sons 

 get the former and die, the other half get the latter X-chromo- 

 some and live. As there are no lethal bearing males, the females 

 must in every generation be bred to normal males." 



If Morgan is right in his assumption, there seems to be no rea- 

 son, a priori, why factors having both dominant and recessive 

 expressions should be limited to sex-chromosomes. 



Stating that crossing-over has not been observed in a certain 

 number of generations, even though the number of both individ- 

 uals and generations is large, is, however, a very different matter 

 from proving that crossing-over never occurs. Just where the 

 evidence ceases to be negative and becomes positive, perhaps de- 

 pends upon the ratio between the space occupied by a factor 

 and the total length of the chromosome bearing it, in its relation 

 to the law of probability, and to possible factors restricting 

 crossing-over. 



The data necessary for such a calculation does not appear to 

 be available for either mice or Drosophila. Which interpretation 

 is assumed must therefore for the present be a matter of per- 

 sonal preference. The continued failure to find crossing-over 

 will in each case tend to make the one involving a single allelo- 

 morphic pair of factors more probable. 



William A. Lippincott 

 Kansas State Agricultural College, 

 January, 1918 



2 Amek. Nat., Vol. LI, No. 609, September, 1917, pp. 513-544. 



