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 Mendelian 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 chromosomes 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.[1]
- ↑ 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.
