140 Applied Biophysics 



pair of homologous chromosomes has the genes ABcdeFgH 

 and in the other the genes a b C d E f g h, may produce gametes 

 which possess for example A B c d E f g h or a b C d E F g H as 

 well as chromosomes like one or other parental homologue. 

 This orderly rearrangement comes about by crossing over during 

 meiosis, the relative frequency of rearrangement occurring 

 between two particular genes being a measure, technically known 

 as the linkage value, of their distance apart on the chromosome. 

 All the genes or loci present in one chromosome together con- 

 stitute one linkage grou]), the number of possible groups in an 

 organism being equal to the haploid number of chromosomes. 

 For a further account of genetics particularly in relation to man 

 the reader is referred to Ford.^ 



Stability of Chromosomes and Genes 



Apart from the process of crossing over, whereby the chromo- 

 somes may recombine their differences, the chromosomes are 

 highly stable structures. However, clianges do occur very rarely, 

 resulting in alterations in the linear order of the genes within 

 one chromosome or linkage group, or exchange of blocks of genes 

 between two non-homologous chromosomes or linkage groups. 

 The frequency of these structural changes, spontaneously very 

 rare, is greatly increased by various radiations. Similarly the 

 genes themselves also possess a high degree of stability. They 

 have a capacity of self-reproduction which is one of the most 

 important characteristics of living matter. All the evidence 

 indicates that they reproduce exactly, and that, if any change 

 occurs within one of them, the gene reproduces in its changed 

 form. 



Changes in genes do occur spontaneously, but usually the 

 frequency of such mutations is very small. The normal frequency 

 is of the order of one change per million genes per nuclear 

 division cycle, and may be smaller even than this for a great 

 many genes. A few genes are highly mutable, with a rate of 

 about one per thousand or ten thousand genes per nuclear cycle. ^ 

 There is, however, no indication that they are fundamentally 



