INTRODUCTION 



oats, Ai'cna sativa, which bleach and die in strong sunlight, but, in 

 a weaker light, seem normal, i.e. have a normal phenotype. 



The phenotype is the product of reaction of the genotype and 

 the environment. Either can change it, and it is not to be traced to 

 either separately. Thus, too, the study of differences in both heredity 

 and environment, by permitting us to separate their effects, must 

 be the foundation of our knowledge of genetics. 



The differences between individuals are not the only ones with 

 which heredity is concerned. All organisms undergo development 

 which consists of growth together with the origin of differences 

 between the parts, differentiation as we call it. Heredity depends 

 on a repetition of this development. Similar genotypes give or deter- 

 mine similar sequences of development. Heredity at once determines 

 that individuals are alike and that their parts are unlike. This is the 

 paradox that baffled our forebears. We can now resolve it by 

 examining the materials and processes concerned. 



When we cut the tail off a worm or the top off a dandelion, what 

 is left grows again to replace the lost part: it regenerates. In spite 

 of their differentiation, the parts of the animal or plant have some- 

 thing inborn in them which is still the same. Similarly, fragments, 

 cuttings and grafts, of particular animals and plants, Hydra, the 

 pondweed Elodea, or a variety of apple or pineapple, can be propa- 

 gated throughout the world with great and predictable uniformity. 

 These vegetative individuals, or clones, demonstrate heredity in its 

 simplest form. They have, as a rule, and so far as we can make out, 

 the same genotype. And since the whole of each clone is derived from 

 a fragment of one individual, we see that the differentiated parts of 

 that individual must have had the same genotype. 



With sexual reproduction the matter is different. Here a new 

 individual arises from the fusion of two germ cells, usually from the 

 fertihzation of an egg by a sperm. No two individuals formed in this 

 way are absolutely alike. In fact, if we fmd a pair of twin children 

 (or eight armadillos) who are alike, we assume that they have been 

 derived from single fertihzed eggs by mere fragmentation and 

 growth. And we call them identical twins (or octuplets). 



What is it that remains constant in vegetative individuals, yet is 

 liable to change in sexual reproduction? To know this, we must 

 look at the cells of which plant and animal bodies are composed. 



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