t 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.

[Illustration: FIG. 68. Scheme to indicate that when the members of a pair
of chromosomes cross (at a-b) the region on each side is protected
inversely to the distance from a-b.]

What advantage, may be asked, is there in obtaining numerical data of this
kind? It is this:--whenever a new character appears we need only determine
in which of the four groups it lies and its distance from two members
within that group. With this information we can predict with a high degree
of probability what results it will give with any other member of any
group. Thus we can do on paper what would require many months of labor by
making the actual experiment. In a word we can predict what will happen in
a situation where prediction is impossible without this numerical
information.

_The Evidence from Non-Disjunction_

In the course of the work on Drosophila exceptions appeared in one strain
where certain individuals did not conform to the scheme of sex linked
inheritance. For a moment the hypothesis seemed to fail, but a careful
examination led to the suspicion that in this strain something had happened
to the sex chromosomes. It was seen that if in some way the X chromosomes
failed to disjoin in certain eggs, the exceptions could be explained. The
analysis led to the suggestion that if the Y chromosome had got into the
female line the results would be accounted for, since its presenc