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INTRODUCTION 



THE AIM AND SCOPE OF GENETICS 



Genotype mid Eiiviroiiiiiciit Nucleus and Cytoplasm 

 Individuals, Cells and Populations 



When we sow an acorn we expect to raise an oak; when we 



breed dogs we expect to get puppies; and even when we culture a 



typhoid bacillus we expect to produce more bacteria of a kind that 



will cause typhoid. In a word, we expect that like will beget like. But 



the resemblance we look for is not absolute or unconditional. One 



oak is not exactly like another ; nor is one dog like another ; nor even, 



invariably, one typhoid bacillus. The causes of these unlikenesses 



are sometimes obviously external. Lack of iron, or lack of light, or 



:oo much light, may make the green leaves of a plant turn yellow. 



Disease, starvation, or training may alter the shape, habits, or abilities 



:>{ an animal. The causes of other unlikenesses, as of the likenesses, 



ire internal, although less obviously so. These inborn causes have 



o be discovered. By comparison and experiment they have to be 



eparated from the external causes. They have to be defmed as 



naterials or processes whose behaviour and effects we can predict 



nd control. This is the aim and scope of genetics. 



Our first task is thus to separate external from internal causes. 



.eaves may be yellow on account of the conditions under which the 



Jant is grown, that is on account of the environment. But we also 



now plants that have yellow leaves under any conditions. These 



lUSt be yeUow on account of their internal or inborn properties. 



V^hat makes these inborn properties? Popularly we say heredity. 



echnically, we must be stricter and say genotype (the type which 



reates). The genotype, according to Johannsen, is what gives the 



bserved development, the appearance, of a plant or animal in a 



articular environment. This appearance we call the phenotype (the 



'pe which is seen). Differences between phenotypes (such as our 



sllow and green leaved plants) may be due to differences between 



motypes, to differences between environments, or even to dif- 



rences in both acting together. For example, there are strains of 



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