Chapter 20 

 GENE ACTION 



It was pointed out in earlier chapters that inherited characters 

 are the result of the interaction of certain alleles with all the 

 other genes of the organism and with the environment. Thus we 

 say that one plant has white flowers because it has genes for 

 white flowers and another has red flowers because it has the 

 allele for white flowers. These two alleles, superimposed on the 

 same genetic background and in plants which develop under 

 the same environmental conditions, will produce two strikingly 

 different results. Why? Unfortunately, this question is not 

 easily answered on the basis of our present information. We 

 have a good understanding of the manner in which genes are 

 transmitted from one generation to another and we know the 

 phenotypic and genotypic ratios to be expected from certain 

 types of crosses, but the reasons why certain genes produce 

 certain phenotypes while their alleles produce others are still 

 much of a puzzle. For a number of genes we have some evi- 

 dence that may indicate a clue to this problem, but there is 

 much still to be learned of the action of genes in general. This 

 phase of genetics is closely allied to both embryology and bio- 

 chemistry. 



In considering gene action, we must realize that the indi- 

 vidual is a unit and not merely an aggregation of a large num- 

 ber of smaller units. The individual usually starts out as a fer- 

 tilized egg. This zygote divides and the two cells which result 

 from that division divide. Many cell divisions follow until the 

 mature plant or animal is produced. As the cells divide, dif- 

 ferentiation takes place, for some cells assume different shapes 

 and sizes. This differentiation in form is accompanied by a 

 differentiation in function. Thus the individual becomes gradu- 

 ally differentiated until the final, mature form is attained. Some 

 of the steps in this differentiating process occur early and affect 



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