240 
GENETICS: CLAUSEN AND GOODSPEED 
HEREDITARY REACTION -SYSTEM RELATIONS— AN 
EXTENSION OF MENDELIAN CONCEPTS 
By R. E. Clausen and T. H. Goodspeed 
DIVISION OF GENETICS, COLLEGE OF AGRICULTURE AND DEPARTMENT OF 
BOTANY. U MI VERSIFY OF CALIFORNIA 
Received by the Academy. March 11.1916 
The most important as well as the most consistent and intelligible 
series of Mendelian conceptions are those which Morgan and his asso- 
ciates have formulated on the basis of their extensive studies of heredity 
in the common fruit fly, Drosophila ampelophila. During the progress 
of their investigations they have observed the origin of over a hundred 
factor-mutations in this species, and they have determined the hereditary 
interrelations of a large number of them. They have established, for 
the fruit fly, the validity of the fundamental conception of Mendelism 
that the imits contributed by two parents separate in the germ cells of 
the offspring without having had any effect on one another, that long 
and intimate association in the same chromosomal mechanism does not 
modify the fundamental constitution or relations in the hereditary 
mechanism of the units of which it is made up. They have also demon- 
strated that the known behavior of the chromosomes furnishes a most 
satisfactory basis for an explanation of the distribution of hereditary 
units to the germ cells. Furthermore, from the linkage-relations dis- 
played among the factors, Morgan has succeeded not only in demon- 
strating that the number of groups of factors corresponds to the number 
of pairs of chromosomes, but he has also succeeded in preparing a map 
of the relative Knear positions of the factors within the chromosomes. 
It, therefore, follows that, so far as heredity is concerned, the chromo- 
somes are made up of a linear series of loci which bear at least some 
specific relation to one another as is indicated by this aggregation into 
chromosomes. Hereditary modifications of characters in the individual 
depend upon changes in the loci, a particular type of change in some 
particular locus corresponding to each different character-modification. 
Now, since a changed locus maintains the same formal relations with 
the other loci in the system as it does in its normal unchanged con- 
dition, it is clear that the chromosome conception of heredity furnishes 
a consistent explanation of the fundamental nature of allelomorphism 
and of the mechanistic basis of Mendelian segregation. Further the 
evidence of somatogenesis seems to indicate that the hereditary units 
form a physico-chemical reaction-system of which the elements, the loci 
of the hereditary system, bear more or less specific relations to one 
