Review: Mendelism Up to Date 21 
by the public, for a glance over the 
great body of semi-popular writings on 
genetics, particularly as concerned with 
man, will show that the primitive and 
charmingly simple idea of. ‘‘one char- 
acter, one determiner”’ still prevails.? 
In addition to this fundamental re- 
vision of the hypothesis by which the 
observed facts of Mendelian heredity 
are explained, we have some extensions 
that are of great importance. The first 
to be considered here is connected with 
the now widely-accepted hypothesis that 
the chromosomes furnish the basis for 
Mendelian heredity. 
With the adoption of this hypothesis, 
it was obvious from the beginning, as 
the authors point out, “that there was 
one essential requirement of the chromo- 
some view [of heredity], namely, that 
all the factors carried by the same 
chromosome should tend to remain to- 
gether. Therefore, since the number of 
heritable characters may be large in 
comparison with the number of pairs 
of chromosomes, we should expect ac- 
tually to find not only the independent 
behavior of pairs, but also cases in which 
characters are linked together in groups 
in their inheritance. Even in species 
where a limited number of Mendelian 
units are known, we should still expect 
to find some of them in groups.”’ 
LINKAGE OF FACTORS 
Bateson and Punnett, in 1906, made 
the discovery which according to Mor- 
gan meets this need; the phenomenon 
is now called linkage. Its importance 
in the theory can be understood from 
the fact that in the fruit-fly (Droso- 
phila), the principal material for work 
at Columbia University, more than a 
hundred separate factors have been 
studied in heredity, and they have 
been found to be linked together in 
only four systems (corresponding to the 
four chromosomes of the fly), one of 
which contains, so far as is now known, 
but two factors. 
After it was found that factors were 
linked together the question naturally 
arose, Do they always remain linked, 
or do they sometimes break away and 
form new combinations? Experimental 
breeding has shown that the latter is 
the case. Another item is therefore 
added to the outline of Mendelism: 
crossing over, in which a character leaves 
the group to which it is normally linked, 
and unites with the alternative group. 
This behavior can be plausibly explained 
through the chromosome mechanism; 
for at one period in their history the 
chromosomes unite and then split. It 
is evident that a crossing over of various 
A B C D 
CROSSING OVER 
Diagram representing the way factors may 
cross from one chromosome to another. 
At the level where the black and white 
rod cross at (a) they fuse and unite 
separating as shown in (d). The details 
of the crossover are shown in (b) and (c). 
From Morgan et al. (Fig. 5.) 
factors might be quite possible at this 
time. 
Starting with this fact of crossing over 
the authors reached another conception, 
which is one of the newest developments 
in heredity and which seems to give us 
such a precise knowledge of this phase 
of inheritance that many workers con- 
template it aghast, holding their breaths 
and hardly daring to believe that we 
can have traveled so far in the explor- 
ation of a territory that was not long 
ago thought pathless. This conception 
which has been lately introduced by 
Morgan and his associates is that of 
the linear arrangement of factors. 
2 It is worth noting that a large part of the attacks on Mendelism are based on this older inter- 
pretation of it. When critics prove to their own satisfaction, in many pages of evidence, that the 
idea of ‘‘one character, one determiner,’’ which they consider to be Mendelian, is perfectly mon- 
strous, the advanced Mendelist merely smiles and agrees with them. The factorial hypothesis 
is untouched by most of the shots at ‘‘ Mendelism”’ in general. 
