DEPARTMENT OF GENETICS.^ 



C. B. Davenport, Director. 

 GENERAL STATEMENT. 



In this Department the year ending August 31, 1923, has been productive 

 of a variety of interesting results. On the purely experimental side progress 

 has been made in the induction of mutation, in the further analysis of chro- 

 mosome variation in relation to somatic variation, in ascertaining the external 

 factors that modify the sex-ratio in Cladocera, and in unraveling the compli- 

 cated interrelation of the internal secretions that influence fecundity. 



In the field of human genetics progress has been made in analyzing the 

 hereditary factors that help determine body build and bone defects and in 

 determining relative values in the national population of the different racial 

 elements contributed to it by immigration. 



Of all our work, that which seems most fundamental is with the chromo- 

 somes. When in 1885 Weismann first used the term germ-plasm to signify 

 the totality of the chromosomal complex characteristic of an organic species, 

 the new term with its implications must have seemed, even to many biologists, 

 unwarranted and speculative. To-day it is perhaps the principal object of 

 investigation in genetical research and the chromosome has been raised from 

 an exceptionally esoteric and technical position to one of acknowledged 

 utmost importance to man and all of his interests. Gradually the concep- 

 tion is forcing its way that we are, under ordinary conditions of environment, 

 "what our chromosomes make us." 



This germ-plasm Weismann regarded as singularly protected from environ- 

 mental influences, and certainly recent experience has fortified this conclusion. 

 Nevertheless, change (evolution) does occur; the chromosomes do change. A 

 minute change in the gene constitution of a chromosome, and still more the 

 addition or subtraction of a single chromosome, will ordinarily be associated 

 with a uniform and definite change in the soma. That change is, obviously, 

 in part due to peculiarities in the constitution of the organism itself, but it 

 is now quite certain that it is possible, by abnormal conditions of tempera- 

 ture, by the X-ray, probably also by alcohol, sera, centrifuging, and other 

 conditions, to modify not only the processes of cell division and chromo- 

 some separation, but also the composition of the individual chromosome. 

 This is, certainly, induced or directed evolution. 



While the immediate consequence of such induced mutations is not always 

 a new species, with its properties of multiple differentiating characters, 

 inter-sterility, and constancy of traits, there is one class of inducible mutation 

 that meets these conditions, as Dr. Blakeslee has pointed out. That is the 

 class of tetraploidy, in which the number of chromosomes is doubled. Follow- 

 ing out this suggestion. Dr. Belling has investigated a number of plant species 

 and finds that in one of them the chromosomes are, indeed, four of a kind, 

 leading to the inference of the origin of this species by tetraploidy — i. e., the 

 failure, somewhere in its history, of the divided chromosomes in some par- 

 ent cell to get into distinct daughter nuclei. In consequence of this failure 



1 Situated at Cold Spring Harbor, Long Island, New York. 



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