CHAPTER XVIII 



MENDEL'S LAWS OF INHERITANCE 



Thus far we have taken for granted that the new individuals of each 

 generation will be like the parents that produced them and that the 

 peculiar characters by which we distinguish one race from another will 

 be perpetuated. Generally speaking, this is true, and the realization of its 

 truth has given rise to such proverbs as "like begets like" and such 

 metaphors as "a chip off the old block" and such folklore stories as that 

 of the ugly duckling. The biologist has also recognized the general truth 

 of "like begets like" in defining a kind or species of organism as "a group 

 of like individuals that naturally perpetuate themselves by reproduction." 

 Nevertheless, it is easily seen that offspring are not exact duplicates of 

 their parents. Only very rarely is it difficult to see well-marked differences 

 between full brothers or full sisters; some children appear to "take after 

 their father"; others "take after their mother"; some are intermediate 

 or are not very like either parent. Yet we do not hesitate to say that, on 

 the whole, like does beget like. 



We are here encountering two apparently contradictory phenomena 

 that have been termed heredity and variation. By heredity is meant the 

 passing of like qualities from one generation to the next. Many of these 

 qualities are common to all the members of the race ; others, not common 

 to all the race, are likely to be common to or very frequent in a given 

 parent-offspring sequence within that race. By variation, on the other 

 hand, we mean all departures from a complete identity of qualities — the 

 differences that permit us to distinguish between two individuals of the 

 same race or of the same parent-offspring sequence. 



The practical breeder, convinced of the general truth that like begets 

 like, has long utilized these phenomena of heredity and variation. Select- 

 ing the most desirable variants from a litter, a herd, or a crop, he has 

 utilized them for the parents of the next generation, eliminating less 

 desirable individuals from the reproductive sequence. This process, 

 repeated generation after generation, has led to the development of many 

 distinct breeds of domesticated animals and plants, each characterized 

 by the accumulation into a common inheritance of a desirable combina- 

 tion of formerly more variable qualities, and the elimination of other 



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