Crossing-over took place between white and yellow only once in a hundred times. Other characters show different values, but the same value under the same conditions is obtained from the same pair of characters.
Fig. 67. Map of four chromosomes of D. ampelophila locating those factors in each group that have been most fully studied.
If we assume that the nearer together the factors lie in the chromosome the less likely is a twist to occur between them, and conversely the farther apart they lie the more likely is a twist to occur between them, we can understand how the linkage is different for different pairs of factors.
On this basis we have made out chromosomal maps for each chromosome (fig. 67). The diagram indicates those loci that have been most accurately placed.
The Evidence from Interference
There is a considerable body of information that we have obtained that corroborates the location of the factors in the chromosome. This evidence is too technical to take up in any detail, but there is one result that is so important that I must attempt to explain it. If, as I assume, crossing over is brought about by twisting of the chromosomes, and if owing to the material of the chromosomes there is a most frequent distance of internode, then, when crossing over between nodes takes place at same level at a-b in figure 68, the region on
each side of that point, a to A and b to B, should be protected, so to speak, from further crossing over. This in fact we have found to be the case. No other explanation so far proposed will account for this extraordinary relation.