194 READINGS IN BIOLOGICAL SCIENCE 



of eye color in men and concluded that, while two dark-eyed parents may 

 be hybrids in regard to eye color and thus may have children both with 

 dark and blue eyes, the character blue-eyed, being recessive, is always pure. 

 Hence two blue-eved parents will have only blue-eyed children. A few 

 months later I learned that a divorce had taken place in that small town. I 

 was surprised and resolved to be very careful even with scientifically 

 proved statements in the future. 



Even more important is the Mendelian analysis of hereditary diseases. 

 If we learn that the predisposition to a certain disease is inherited through a 

 dominant gene, as diabetes, for instance, then we know that all persons 

 carrying the gene will be sick. In this case all carriers can be easily recog- 

 nized. In the case of recessive diseases, feeblemindedness,* for instance, we 

 know that the recessive gene may be covered by the dominant gene for 

 health and that the person, seemingly healthy, may carry the disease and 

 transmit it to his children. 



With every year the influence of Mendel's modest work became more 

 widespread. The theoretical explanation given by Mendel was based upon 

 the hypothesis of a mechanism for the distribution and combination of the 

 genes. To-day we know that exactly such a mechanism, as was seen by the 

 prophetic eye of Mendel, exists in the chromosome apparatus of the nucleus 

 of the cells. The development of research on chromosomes, from the obser- 

 vations of the chromosomes and their distribution by mitosis to the dis- 

 covery of the reduction of the number of chromosomes in building the sex 

 cells and finally to the audacious attempt to locate the single genes within 

 the chromosomes, is all a story, exciting as a novel and at the same time one 

 of the most grandiose chapters in the history of science. A tiny animal, the 

 fruit fly, Drosophila, was found to be the best object for genetical research. 

 The parallelism between the behavior of the chromosomes and the mecha- 

 nism of Mendelian inheritance was studied by hundreds of scientists, who 

 were trying to determine even the location of the different genes within 

 the different chromosomes and who started to devise so-called chromosome 

 maps. 



Correns, Baur and Goldschmidt in Germany; Bateson and his school in 

 England; Devries in Holland; Nilsson-Ehle in Sweden, are the outstanding 

 geneticists of the first decade after 1900. But soon the picture changed. The 

 Carnegie Institution for Genetic Research in Long Island, under the leader- 

 ship of Davenport and later under Blakeslee, became one of the world's 

 centers of genetic research. In 19 10, T. H. Morgan, then at Columbia Uni- 

 versity, later at the California Institute of Technology, started his investi- 

 gations with the fruit fly, Drosophila, and founded the largest and most 

 active school of geneticists. The U.S. Department of Agriculture with its 

 network of experimental stations connected with more than a hundred agri- 



• Not all feeblemindedness is inherited. Some cases are due to accidents or falls, 

 some to disease. — Edt 



