372 PRINCIPLES OF EMBRYOLOGY 



certain chromosomes in Oenothera) in which no crossing-over occurs. 

 Further, the defmition involves us in proving a negative. Shall we be 

 satisfied if no crossing-over occurs in one thousand individuals, in ten 

 thousand, one million, or how many (cf pseudo-alleles, p. 375) ? 



Another criterion could be based on the breakages induced in chromo- 

 somes by x-rays or mutagenic chemicals. This would suppose that breaks 

 must occur between genes and not through them and that the breaks 

 would therefore suffice to separate the individual genes from one another. 

 Such a criterion is, however, not easy to use in practice. Thirdly, we might 

 try to base a defmition of the gene on its physiological action, a gene 

 being the smallest element which behaves as a unit in the developmental 

 activity of the chromosomes. We shall see, however, that the evidence 

 shows that this defmition conflicts with the others, since parts of the 

 chromosome which behave as separate units from the point of view of 

 crossing-over may nevertheless influence one another's developmental 

 activity. It may be possible to interpret these influences as secondary 

 interactions of neighbouring genes, but stiU the situation makes it difficult 

 to provide a perfectly simple and clear-cut defmition. 



If, however, one looks more closely at the material structure of the 

 chromosome which one is talking about, the reasons for this difficulty 

 become easier to understand. The chromosome consists largely of protein 

 which, throughout most of the life-cycle of the cell, is combined with a 

 greater or lesser amount of nucleic acid. Now the basic structure of both 

 protein and nucleic acid consists of a hnear arrangement of small units. 

 More is known about the proteins. The basic element here is a polypep- 

 tide link, the sequence in which is 



O 

 C 



\ /\ / 

 CH NH 



R 



These are arranged in series which may contain many hundred individual 

 links. Within such series there is a hierarchy of periodicities of different 

 scales. The amino-acids (R) attached to the polypeptide chain may, for 

 instance, be arranged in a repeating pattern, the repeat unit covering a 

 fairly small number of individual links. Then there may be rather larger 

 units corresponding to the unit cells of protein crystals. The protein 

 molecules, as they may exist in solution, are a larger unit again. Virus 



