264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



Mendel noticed another fact. The round-seeded parent had yellow 

 seed color, while the wrinkled parent plant had green seeds. Among 

 the grandchildren four types appeared, with seeds round yellow and 

 round green, wrinkled yellow and wrinkled green. Some of you will 

 remember their proportions : 9:3:3:1. But that is a minor matter. 

 The lever for further insight is the childishly simple fact that the 

 parental traits round and yellow which came from one parent, and 

 wrinkled and green which came from the other, had not always re- 

 appeared together in the combination in which they had been intro- 

 duced into the cross, but had also appeared in the new combinations 

 round green and wrinkled yellow. This fact reveals that each parent 

 does not transmit a unified lump of hereditary matter, one whose 

 joint consequences are in one case roundness and yellowness and in 

 the other wo-inkledness and greenness. Rather it shows that the hered- 

 itary matter of an individual is broken up not only into the two 

 contributions of his parents, but that each contribution itself consists 

 of separate and separable units. Thus the concept of the hereditary 

 makeup as an assembly of many independent units was born. Just 

 as the atomic theory had reduced the ephemeral multitude of chemical 

 phenomena to the eternal properties of a limited number of atoms, so 

 the theory of hereditary elements had reduced the multitude of specific 

 individual appearances and disappearances of traits to the existence 

 of a limited number of combining and disjoining elementary biological 

 constituents. 



This knowledge, not appreciated during Mendel's life, became our 

 property at the beginning of the present century. What grew out of 

 it are adventures of our time. Mendel knew that the separable 

 hereditary contributions were contained in the plant's egg and 

 pollen, or in the animal's egg and sperm, but he did not speculate 

 about their nature or their specific location. A few further facts than 

 those he knew have permitted us to place the hereditary elements into 

 the small but microscopically clearly visible bodies of the cell's nu- 

 cleus, the chromosomes. Still more facts, demonstrated first in sweet- 

 peas and fruitflies, have been the clues to the hereditary architecture 

 of the chromosomes. While the chromosomes are contained in the 

 nucleus in a haphazard fashion, like string beans in a bag, each 

 chromosome itself is a well-arranged construction in which specific 

 hereditary units occupy specific places. Thus we have accurate maps 

 of the chromosomes of some organisms in which many of their 

 thousands of different hereditary elements are assigned their loca- 

 tions : showing which one of the various chromosomes is the carrier 

 of each specific element ; picturing the linear order in which the ele- 

 ments are arranged within each chromosome ; and giving the seriation 

 of the elements and their relative distances within the chromosomal 

 lineup. 



