CHAPTER 33 



Genetics 



The genetic principles basic to the simpler types o£ inheritance dis- 

 cussed in the previous chapter have been understood for half a century 

 or more. In the intervening years research in genetics has been pursued 

 enthusiastically and a great many complicating factors have been dis- 

 covered and analyzed. In each case it has been found that the distribu- 

 tion of traits among the successive generations is a reproducible 

 phenomenon and that it can be explained as some variation of Mendel- 

 ian genetics. 



The relationship between the genes discussed in the previous chapter 

 and their traits is simple and clear: each gene produces a single trait. 

 Genetic research with many different kinds of animals and plants has 

 revealed that the relationship between gene and trait may be quite com- 

 plex. Several pairs of genes may interact to affect the production of a 

 single trait; one pair of genes may inhibit or reverse the effect of an- 

 other pair; or a given gene may produce different effects when the en- 

 vironment is altered in some way. The genes are inherited as units, but 

 may interact with one another in some complex fashion to produce the 

 trait. The relation between gene and trait, the mode of action of the 

 gene in producing the recognizably altered characteristic, has fascinated 

 geneticists for many years. This general field, called physiological 

 genetics or biochemical genetics, is being investigated very actively at 

 present. 



280. The Interactions of Genes 



Two or more independent pairs of genes may interact in any one 

 of several ways as they affect the phenotypic expression of a given trait. 

 The total number of genes which must be present and interact prop- 

 erly for the normal development of a given trait is quite large; several 

 dozen different genes affect the coat color of mammals such as rats, 

 rabbits or guinea pigs and nearly 100 different genes affect the size, 

 shape and color of the eves of the fruit fly. 



Complementary Genes. Two independent pairs of genes may be 

 interrelated in such a way that neither dominant can produce its effect 

 unless the other is present too. The presence of at least one dominant 

 gene from each pair produces one character; the alternate condition 

 results from the absence of either dominant or of both dominants. 



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