Laughlin: Structure of Human Germ-Plasm 
the chromosome are known, their sym- 
bols may be inscribed in their true 
linear relationship upon the same spool 
in the germ-plasm abacus. Their 
genetic behavior will then be auto- 
matically demonstrated in manipulating 
the machine in the usual manner. With 
this understanding the tentative list 
may well stand, but the fact that the 
given assortment is arbitrary does not 
destroy its teaching value in demonstrat- 
ing the proven structure of the germ- 
plasm, the mechanical principles and 
mathematical possibilities in the segre- 
gation and recombination of traits. 
The following paragraphs describe 
the features of this abacus and their 
analogies in the human germ-plasm: 
1. The curved surface of each spoo! 
is divided vertically by black marks 
into two equal areas, each of which 
represents one of the two possible 
chromosomes, either of which the par- 
ent manufacturing them may contribute 
to the F, zygote. One of these chro- 
mosomes the said parent in turn 
received from his or her father, and 
the other from his or her mother. 
2. The front face of each chain of 
spools, in any combination, represents 
the number and genic composition of 
the chromosomes characteristic of the 
gamete contributed to the F, zygote 
by the parent proliferating it. If de- 
sired one ‘‘chromosome-face” may be 
labeled @ to indicate its paternal 
origin and the other Q to show its 
maternal origin. 
3. The two sets of spools are in par- 
allel and homologous position, as prob- 
ably is the case with chromosomes just 
before the formation of the equatorial 
plate when the gametes fuse to make 
the fertilized egg or zygote. But still 
more definitely this parallel and 
homologous position is exactly analo- 
gous to the chromosomal situation dur- 
ing para-synapsis, preceding the first 
maturation division. 
4. All of the potentialities of the two 
parents in reference to their chromo- 
somal combinations (barring crossing- 
over and other special phenomena) are 
here shown mechanically and may be 
187 
mathematically demonstrated by turn- 
ing the spools into their several pos- 
sible combinations. 
5. With this apparatus one may give 
a mechanical demonstration of the 
three normal genetic types of parents, 
and the consequent six types of Men- 
delian matings in reference to a single 
trait. The student examining the 
abacus here shown soon finds that, so 
far as breeding potentialities are con- 
cerned, the F, male parent (the father) 
is positively homozygous, that is, du- 
plex in reference to traits C, H, T, and 
R; the mother in reference to traits 
X and H. The father is heterozygous, 
that is, simplex in reference to traits X, 
P, S, I, K, D; the mother to traits P, 
O, I, K, R. The father is negatively 
homozygous, or nulliplex, in reference 
to traits g and o; the mother to c, g, s, 
t, and d. 
6. Let us next consider the possible 
types of matings. The gene H for 
the trait here shown in chromosome 
g presents an example in Mendelian 
Case’ 1 (DD* x DD = 100% DD:*) 
Genes X and R here listed in chromo- 
somes x and j, Case 2 (DD X Dr = 
50% DD and 50% Dr.) Genes C and 
T in chromosomes a and i, Case 3 (DD 
rr — 100% Dr.) “Genes PR. 1e and 
K in chromosomes b, f and h, Case 
AT Dr >< Dr 25% DD S07 Dr 
and 25% rr.) Genes S, O and D in 
chromosomes d. e and k. Case 5 (Dr 
x rr = 50% Dr and 50% rr.) Gene 
g in chromosome c, Case 6 (rr X& rr 
= 100% rr.) 
7. In most bi-sexual species, the 
cause of sex, so far as it has been 
traced in the reverse order of onto- 
geny, is found to lie in a chromosomal 
difference in the zygote and gamete. 
Sex-difference for the most part is the 
principal somatic difference within a 
species. It is therefore logical to ex- 
pect a greater difference between the 
chromosomes of a male-producing and 
a female-producing zygote than will be 
found to accompany any other con- 
trasted traits within the same species. 
In man the male is heterozygous, the 
female positively homozygous. In the 
