'Heredity 



THE DISTINCTIVE individuality of each 

 living thing depends not only upon parental 

 heritage but also upon environmental ex- 

 perience. However, the potency of the envi- 

 ronment in shaping the form and function 

 of each individual will be disregarded for 

 the present, and all attention will be focused 

 upon the factors of heredity. 



IDENTIFYING THE MACHINERY OF 

 HEREDITARY TRANSMISSION 



Heredity depends upon the protoplasmic 

 continuity between parent and offspring. But 

 all that a parent generally contributes to the 

 offspring is a small sample of its own proto- 

 plasm, usually in the form of a single gamete 

 cell. 



Excluding asexual modes of reproduction, 

 which will be considered separately, only the 

 gametes establish continuity between parents 

 and offspring, and consequently the mecha- 

 nism of hereditary transmission must operate 

 across this exceedingly slender protoplasmic 

 bridge. 



The processes of sexual reproduction indi- 

 cate very clearly that the hereditary potency 

 of the cytoplasm is not nearly as great as that 



of the nucleus. In a vast majority of organ- 

 isms, the male gamete contains virtually no 

 cytoplasm as compared with the large amount 

 present in the egg — and yet the male and 

 female gametes contribute equally to the 

 hereditary qualities of the offspring. This 

 indicates that the machinery of inheritance 

 lies mainly in the nuclei — or more particu- 

 larly in the chromosomes — of the gamete 

 cells. The chromosomes, in fact, are the only 

 entities that are always passed on in equal 

 quantities from parents to offspring in sexual 

 organisms generally. 



Modern genetics began to develop very 

 rapidly in 1910 under the leadership of 

 Thomas Hunt Morgan, and since that time 

 geneticists in all parts of the world have 

 cooperated in establishing the chromosome 

 theory of heredity. In fact there now exists 

 a vast amount of experimental evidence 

 that proves conclusively that (1) chromo- 

 somes are the essential agencies in the trans- 

 mission of hereditary traits; (2) each chro- 

 mosome is made up, essentially, of a linear 

 series of definitely localized units, called 

 genes; (3) each gene is a decisive factor in 

 determining one or more hereditary qualities 

 in every individual organism. 



475 



