ILLUSTRATING THE STRUCTURE AND 
MATHEMATICS OF THE HUMAN GERM-PLASM 
Harry H. LAuGHiin, Sc. D. 
Eugenics Record Office, Cold 
N ABACUS is a very simple 
piece of apparatus, while the 
germ-plasm is very complex. 
Still it is possible by such an 
apparatus to illustrate the basic geog- 
raphy of the chromosome, and also to 
demonstrate the elementary mathemati- 
cal principles involved in the segrega- 
tion and recombination of genes. 
The germ-plasm abacus consists es- 
sentially in two rows of spools so 
arranged that each spool has a definite 
and homologous companion. There 
should be as many spools in each string 
as there are chromosomes in the 
gametes of the particular species which 
the apparatus demonstrates. Until 
more is known in the case of man con- 
cerning the shapes and relative lengths 
of the individual chromosomes, it will 
not serve any useful purpose to differ- 
entiate spool-shapes in this simple 
apparatus. Later it may be possible 
to make each spool a model in shape 
and relative size of the particular 
chromosome which it represents, so 
that in such case we would have a 
much more perfect mechanical model 
of the human germ-plasm. 
The machine here pictured is built ot 
one-half inch strips of wood; the sur- 
face of the frame, without the handle, 
measures 51% by 18 inches. The spools, 
or chromosome-models, are made of 
l-inch cylinders 1 inch long and 
mounted on iron rods, so that they may 
be turned or spun with ease, but each 
has a catch which meshes into a notch 
in the rod so that the spool is much 
more apt to stop at an exact half-turn. 
This notch-and-catch contrivance is 
Spring Harbor, New York 
convenient but is not essential, for 
there are only twenty-four spools to be 
spun, and after brushing or raking them 
for the purpose of securing random or 
chance assortment, the few small ad- 
justments necessary to secure exact 
alignment require only a few moments. 
If there are twelve’ chromosomes in 
each human gamete, then there are 
twelve linkage-groups of traits in man. 
The breaking-up of these groups would 
be proportional to the rate of crossing- 
over in gametogenesis. Whether the 
genes for the particular twelve traits 
indicated on this particular abacus lie 
in separate chromosomes we do not yet 
know. In future years the instructor 
in genetics, in manufacturing a germ- 
plasm abacus, will keep pace with the 
erowth of knowledge of the linear 
geography of the human gene, just as 
he will of the shape and relative size 
of the several chromosomes. But it is 
useful and perfectly proper to present 
this tentative arrangement, because the 
traits indicated have been studied to a 
considerable extent, and, so far as 
known, none has been shown to be cor- 
related with another here listed. But 
by the laws of chance future knowl- 
edge will prove several cases in which 
more than one of this random list lie 
in the same chromosome, and conse- 
quently other chromosomes will be 
blank so far as the present list is con- 
cerned, but by linkage and crossing- 
over studies the blanks will be readily 
filléd by other genes properly located. 
When two or more human genes are 
demonstrated to lie in the same chro- 
mosome, and their relative positions in 
*Cytologists lave not yet agreed on the number of chromosomes in man. According to 
von Winiwarter (1912) the diploid number is 47 in the male and 48 in the female. 
Accord- 
ing to Wieman (1917) the diploid number is 24 in both sexes, but one pair of which are the 
idiochromosomes -ry. 
(See also Guyer, Montgomery, Jordan, Stevens and Evans.) 
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