io THE POPULAR SCIENCE MONTHLY 



place them in the third chromosome without any pretensions as to 

 which of the pairs of chromosomes are numbered II. and III. 



The arrangement of these characters in groups is based on a general 

 fact in regard to their behavior in heredity, viz., A member of any 

 group shows linkage with all other members of that group, but shows 

 independent assortment with any member of any other group. In 

 Drosophila ampelophila there are five pairs of chromosomes. According 

 to Stevens, the sex chromosomes are attached to one of the other pairs. 

 Three of the five pairs are occupied according to our view by the three 

 groups of linked factors that we have studied. There are as yet two 

 more pairs of chromosomes than there are groups of linked factors. 



On the chromosome hypothesis we can readily see that if the factors 

 that stand for characters lie in the chromosomes, those that lie in the 

 different chromosomes should give independent assortment, and the 

 ratios obtained in breeding experiments should be the expected Men- 

 delian ratios. On the other hand, it may appear that the factors 

 that lie in the same chromosomes should always march together through 

 successive generations. If this were true the linked characters would 

 be absolutely linked to each other. Experience shows, however, that 

 the linked characters are not absolutely linked, but that to a greater or 

 a less degree, according to the factors involved, interchanges must in 

 some way take place. Here fortunately there is a cytological relation 

 that may be utilized to explain how interchanges between like chromo- 

 somes may take place. 



It has been observed when the homologous pairs of chromosomes 

 unite before maturation of the egg and sperm that they twist around 

 each other. In consequence, parts of each chromosome may come to lie 

 on one side of the twist and other parts on the other side. If at times 

 the chromosomes break at the crossing point, and each then unites with 

 that part of the other chromosome that lies on the same side, the new 

 chromosomes that emerge later from the pair will be made up of parts 

 of each chromosome to the extent to which breaking has taken place 

 at some of the crossed levels. 5 



s The twisting of the chromosomes has been described by a number of 

 writers. Janssens has observed that at the time when the pairs are about to 

 separate, cross-bridges between the pairs (more strictly between the halves of 

 the pairs) can be seen. Whether these cross-bridges are the result of the kind of 

 crossing referred to in the text can not be discussed here. Janssens points out 

 that the mechanism of interchange between homologous chromosomes, by means 

 of the cross-bridges furnishes an interesting explanation of those cases where 

 the number of distinct allelomorphic pairs of characters is greater than the pairs 

 of distinct chromosomes. The evidence seems to me to indicate furthermore 

 that independent assortment occurs when factors lie in different chromosomes, 

 while the interchange between homologous chromosomes accounts only for the 

 relatively small proportion of crossing-over. Only when the factors lie very far 

 apart is there a numerical approach to the independent assortment of factors 

 lying in different chromosomes. 



