BASES OF CHANGE 



between the units to rearrange, differences such as we actually see 

 bet\vecn the parts of the cliromosomes. These differences between 

 units cannot, therefore, have arisen by the changes of arrangement 

 themselves. 



Now crossing-over is a change in arrangement between genes. 

 Changes within genes should, therefore, have no direct effects on their 

 own crossing-over. Many genes, in fact, have an indirect or physio- 

 logical effect on the process of chromosome pairing and on the 

 observed recombination. For example, a so-called asynaptic gene 

 often (in maize, pea, or Drosophila) produces a general reduction in 

 the crossing-over of all the chromosome pairs when homozygous 

 by causing incomplete pairing with results that we shall see later. 

 But of direct effects produced in the heterozygote these residual 

 intragenic mutations have none. They are apparently changes in 

 indivisible points or loci on the linkage map, and are accordingly 

 known as point mutations. 



Somatic Mutation 



Instances are known, in Antirrhinum and elsewhere, of two 

 allelomorphic genes or two homologous chromosomes mutating in 

 the cell simultaneously to the same new allelomorphs. Such events 

 suggest a chemical determinacy in the mutation even if they arise 

 in two steps. These instances are, however, very rare and it is a 

 remarkable fact, or even principle, that allelomorphs are (with 

 these few exceptions) independent in their mutations. The original 

 form of any change or mutation in the body o{ a zygote must 

 therefore be a heterozygote. And the original form of any hetero- 

 zygote must be a mutation. Only later can the heterozygote 

 appear as we characteristically fmd it, that is as a cross between 

 two homozygotes, as a hybrid in the restricted pre-mendelian 

 sense. 



It is also worth noting that most mutations occur somatically, 

 that is in the nuclei of a growing body. Hence the new type of cell 

 is not merely heterozygous. It is also incomplete so far as the body 

 is concerned : it is a patch. In plants where the germinal tissue is not 

 separated early in development a change may or may not affect the 



no 



