Genetics for medical students / by E. B. Ford.
- Ford, E. B. (Edmund Briscoe), 1901-1988.
- Date:
- [1961]
Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)
Credit: Genetics for medical students / by E. B. Ford. Source: Wellcome Collection.
97/216 page 89
![THE ACTION OF THE GENES 89 is sufficient to raise enzyme production to the saturation level even in single dose. Rare genes will always be judged by their reaction as heterozygotes, and those will be favoured which even in that phase do not lower enzyme production below the norma] saturation level, so as to evoke an imper¬ fect development of the characters concerned. That is to say, Haldane envisages dominance arising through a change in the gene itself, through selection of one rather than of another member of a multiple allelomorph series. It may, however, be questioned how far multiple allelo¬ morphs are available for this purpose at each locus. Further¬ more, the level of the threshold value up to which an enzyme acts will be determined by the general constitution of the organism and so, in part, by other genes; thus it will be sus¬ ceptible of modification by selection operating on the gene- complex in the manner proposed by Fisher. It should also be noticed that experimental proof of Fisher's method of dominance modification has now been obtained in a number of instances. It has indeed been possible under experimental conditions to select the heretozygous expression of the characters produced by a given gene until they have become nearly dominant in one line and nearly recessive in the other. In these circumstances, it has been established, by crossing with the normal form, that the gene itself has remained unaltered (Fisher, 1935, 1938; Ford, 19406). It has been stressed that the genes of an organism act together to form a gene-complex adjusted by selection to give a favourable result. The chances therefore are exceed¬ ingly remote that purely random changes in the genes, such as arise by mutation, shall so fit in with the balanced system already in existence as to promote harmonious working. The probability is indeed almost overwhelming that the effects of mutations will be disadvantageous, a circumstance to which attention has already been directed. The considera¬ tion that mutation is a recurrent phenomenon also contri¬ butes to this result. Were organisms living in a constant environment it would seem almost incredible that mutations having advantageous effects should ever now arise. All that](https://iiif.wellcomecollection.org/image/b18028639_0098.JP2/full/800%2C/0/default.jpg)
