LINKAGE

Non-crossovers                                          Crossovers

White round                 Red bar             White bar                  Red round

28 per cent.          28 per cent.     22 per cent.         22 per cent.

56 per cent.                                 44 per cent.

Here a large amount of crossing over appears, about
44 per cent. In fact, so freely do the factors inter-
change that without sufficiently large and accurate
numbers the linkage might entirely escape detection.
 

The Mechanism of Crossing Over

If it be admitted that the Mendelian factors are
carried by chromosomes it can not be denied that
interchange between homologous chromosomes must
occur, for sex linked factors cross over from each
other, and yet are known to be in the same pair of
chromosomes, since they all follow the X chromo-
some in its distribution. The evidence allows for no
other interpretation. But why should crossing over
take place so rarely between certain factors and so
often between others? We can make use here of
certain information in regard to the chromosomes
that gives a very simple answer to the question. In
the early germ cells, before the maturation period
begins, the chromosomes appear to be scattered in
the nuclei, and the homologous chromosomes in
many cases show no tendency to lie together, although
in some animals, e.g. in many flies, the members of a
pair are often found side by side. In this early period
the germ cells divide as do other cells and thereby
increase in numbers.    But at the termination of this