THE NATURE OF THE GENE 397 



inert regions differ from the euchromatic regions only in some general 

 condition which overlies the same basic diiferentiation into band and 

 interband. 



The centromeres are probably quite differently constituted from the 

 rest of the chromosome. They seem to be unable to transmit torsional 

 stresses, since the directions of coiling at metaphase are apparently 

 independent of one another in the two arms of a chrom.osome with a 

 central centromere; and similarly interference in crossing over does 

 not extend across a centromere. It has also been shown that v/hen 

 centromeres divide at metaphase they do not always split along a 

 plane parallel to the length of the chromosome, but may occasionally 

 be divided transversely or at any angle. ^ All these facts tend to suggest 

 th.at the centromeres, unlike the rest of the chromosome, are not 

 fibrous structures. 



2. The Nature of the Gene 



Before we can discuss the chemical nature of the gene, we must 

 reconsider the definition of the word.^ The Mendelian factor was 

 originally defined simply as an entity which obeyed Mendel's laws and 

 had an action in determining the characters of the adult organism. This 

 definition could apply to whole chromosomes, or large sections of 

 chromosomes. With the discovery of linkage and crossing-over, the 

 definition of the factor, or, as it was now called, the gene, became 

 narrowed down by adding the property that genes act as units in 

 crossing-over, which occurs between them but not through them. At 

 the present time, this definition has become unsatisfactory because 

 there are cases to which it is inappHcable. We know that genes may be 

 guarded from crossing-over, for instance in the parts of the Y chromo- 

 some which have no homologue or in the complexes in a ring-forming 

 heterozygote such as Oenothera. Moreover, we know of inert sections 

 of the chromosomes, which appear to consist of particles much like 

 those in the other regions of the chromosome, sharing with them the 

 properties of multipHcation and attraction, but lacking any effect on 

 development; we may wish to stretch our definition to cover inert 

 genes. Finally, the position effect shows that genes which may cross 

 over independently of one another may not be independent in their 

 developmental actions ; in this case the two parts of the definition do 

 not tally. 



It is clear that the old picture of the chromosome, as a Unear array 

 ^ Upcott 1938. - Cf. Darlington 1938. 



