Gene Action 



153 



Furthermore, equal increases in quantity of 

 different genes do not necessarily produce 

 proportional effects on the different reac- 

 tions which lead to gene-dependent prod- 

 ucts. 



The developmental effects of partial quan- 

 titative changes in genie content are usually 

 much more striking than those in poly- 

 ploidy. The most important case of this na- 

 ture is, of course, the quantitative change in 

 the number of X-chi'omosomes in bisexual 

 organisms. The change from one X-chromo- 

 some to two, with the autosomal genes kept 

 constant in diploid quantity, results in the 

 development of the other sex characterized 

 by striking differences in morphology and 

 physiology. Changes in the quantity of the 

 autosomes likewise upset the developmental 

 balance, resulting in intersexual differentia- 

 tion (Standfuss, '14; Bridges, '22; reviews in 

 Goldschmidt, '31, and Fankhauser, this vol- 

 ume), or in other morphological changes 

 (Sinnott, Houghtaling, and Blakeslee, '34; 

 Bridges, '22). 



Changes in the quantity of small chromo- 

 somal sections or of individual genes some- 

 times have no, or hardly observable, effects, 

 but at other times produce striking results. 

 In Drosophila deficiencies at many loci, in 

 heterozygous condition, lead to the appear- 

 ance of normal individuals. On the contrary, 

 a heterozygous deficiency at the facet locus 

 results in abnormal, notched wings, and 

 heterozygous deficiencies in various chromo- 

 some regions result in "Minute" slender 

 bristled flies with greatly prolonged develop- 

 ment (see Bridges and Brehme, '44; Brehme, 

 '41). In these latter cases a single quantity 

 of a normal allele is incapable of main- 

 taining the normal process of development 

 ("haplo-insufficiency") (Mohr, '32). Increase 

 beyond the diploid quantity of normal al- 

 leles, by use of duplications of chromosomal 

 sections, is usually of less effect than de- 

 crease. This may partially be due to the fact 

 that the relative change in developmental 

 balance is more severe when the quantity 

 of a gene is reduced from 2 to 1, than if it 

 is increased from 2 to 3. In part, the lack of 

 effect of increase in the quantity of a gene 

 from the diploid to the triploid seems to 

 depend on a situation in which the diploid 

 effect involves a threshold of activity which 

 cannot be surpassed. The cause of the exist- 

 ence of such thresholds must be searched for 

 in limiting factors imposed by other genie 

 or non-genic components of the cells or of 

 the developing systems. 



QUALITATIVE VARIABILITY OF GENIC 

 CONTENT 



Many genes are known to occur in differ- 

 ent states, their allelic forms. Any allele has 

 high stability, but rare mutations may trans- 

 form one allele into another. While in some 

 cases actual absences (deficiencies) of the 

 gene in the chromosome may simulate the 

 effect of an allele, the regular existence of 

 multiple alleles at most, if not all, loci sug- 

 gests that alleles are qualitatively different 

 representatives of the genie material at each 

 locus. 



The different alleles of a gene are recog- 

 nized as such by their different action. Most 

 studies of genie action in development con- 

 sist in the description of the effects of sub- 

 stitution of one allele for another. If, for 

 example, the effect of the creeper allele in 

 the fowl is being investigated, the direct ob- 

 ject of inquiry is the difference between the 

 development of an egg that contains this al- 

 lele and another that contains, instead, the 

 "normal" non-creeper allele. 



Gene action in development may be stud- 

 ied from different aspects. The geneticist 

 may desire to trace the developmental chain 

 of events back to the primary agents, the 

 genes, in the hope of learning about their 

 immediate action and their elementary con- 

 stitution. The student of development may 

 use the different alleles as differentiating 

 tools by means of which he can introduce 

 internal variables into the course of develop- 

 ment in order to gain insight into its proc- 

 esses. The two modes of inquiry are, of 

 course, closely related and have been jointly 

 subsumed under such designations as physio- 

 logical and developmental genetics (Gold- 

 schmidt, '20, '27, '38; Stern, '40; Wright, '16, 

 '34, '41, '45a, b), or in regard to more de- 

 limited fields, biochemical genetics (Beadle, 

 '45) and immunogenetics (Irwin, '47). 



The chain of events which leads from 

 genes to phenotypic properties must vary 

 in length from case to case. By and large, 

 intracelkdar biochemical properties will be 

 connected with the genes by fewer links than 

 morphological properties of organs or organ 

 systems. In no case do we yet know with 

 certainty that only a single step intervenes 

 between a gene and a recognizable gene- 

 dependent product. It has, however, been 

 suggested with plausibility that some cellular 

 antigens owe their specificity to direct genie 

 action (Haldane, '37). There exists a one- 

 to-one correspondence between the presence 



