350 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



to a consideration which is more directly significant for the physiology 

 of development. 



In the earlier days of genetics it was customary to speak of unit 

 characters in heredity, because certain contrasted characters, rather 

 clearly defined, furnished the data for the Mendelian ratios. Cer- 

 tain students of genetics inferred that the Mendelian units responsible 

 for the selected character were genes producing only a single effect. 

 This was careless logic. It took a good deal of hammering to get 

 rid of this erroneous idea. As facts accumulated it became evident 

 that each gene produces not a single effect, but in some cases a 

 multitude of effects on the characters of the individual. It is true 

 that in most genetic work only one of these character effects is 

 selected for study — the one that is most sharply defined and sep- 

 arable from its contrasted character — but in most cases minor dif- 



FiGCRB 4. — Diagram of Oenothera chromosomes Illustrating the configuration of chromo- 

 somes (c and (1) when there has been an exchange between different chromosomes as 

 indicated in the figure by the black and white. In a and a' an exchange between 

 chromosomes I and II is shown. In b the chromosome group is drawn, in which there 

 are two interchanged chromosomes and two whole chromosomes. In c the coming to- 

 gether of these four chromosomes is shown, and in d the results of the opening out of 

 this cross into a twisted ring. Chromosome pairs, that came in together, pass to 

 opposite poles. 



ferences also are recognizable that are just as much the product of 

 the same gene as is the major effect. In fact, the major difference 

 selected for classification of the contrasted character-pairs may be 

 of small importance for the welfare of the individual, while some 

 of the concomitant effects may be of vital importance for the 

 economy of the individual, affecting its vitality, its length of life, 

 or its fertility. I need not dwell at length on these relations be- 

 cause they are recognized today by all geneticists. It is important, 

 nevertheless, to take cognizance of them, because the whole problem 

 of the physiology of development is involved. 



The coming together of the chromosomes at the maturation 

 division, and their subsequent movement apart to opposite poles of 

 the meiotic figure, insures the regular distribution of one set of 

 chromosomes to each daughter-cell and the fulfilment of Mendel's 

 second law. These movements have the appearance of physical 

 events. Cytologists speak of these two phenomena as attraction and 



