BIOLOGY IN HUMAN AFFAIRS 



chanical problem of heredity has been solved as definitely 

 as the mechanical problem of molecular structure. Let us 

 recapitulate. Every organism, whether man or beast, 

 whether flowering plant or fern, starts life as a combination 

 of more or less independently inherited genes. The genes 

 are the units of heredity, the physical connection between 

 one generation and its successor, and are the basis of all 

 developmental changes. They are self-perpetuating bodies 

 which grow and divide throughout long periods of racial 

 history without exhibiting discernible variation in the 

 functions they perform; yet in rare cases any given gene 

 may take on a new constitution and thus, if a functional 

 germ cell results, provide new raw material for the sieve of 

 natural selection. The gene pattern of every individual is 

 definite and distinctive for that individual, and is ordinarily 

 made up of two complete sets of genes, one contributed by 

 the egg and one by the sperm. Each mature egg or sperm 

 possesses one complete set of genes, chosen from these two 

 sets by means of a regularly constituted method of gene 

 distribution. Normally, the essential features of this 

 distribution are as follows: Any homologous pair of chro- 

 mosomes may exchange one or more genes, provided the 

 exchange is equable and is carried out in obedience to 

 regulations. The entire packet of genes of each respective 

 pair of chromosomes is then passed to the two daughter 

 cells which become the mature reproductive cells — through 

 a second equatorial division having no known genetic 

 significance — by the free assortment of one member of each 

 pair. 



The emphasis which has been placed upon gene patterns 

 should not be construed as meaning that no other factors 

 have determinative value in the final result that is called 



H. Morgan (Princeton University Press, Princeton, 1916), or "The Genetical 

 Theory of Natural Selection," by R. A. Fisher (The Clarendon Press, Oxford, 

 1930). 



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