Section 4 — Gene Action 



in the synthesis of hexoses and pentoses from 

 smaller compounds. 



4.23. Discrimination among Copying, Plasmid, and 

 Episome Models for Conversion-type Inheri- 

 tance in Maize. E. H. Coe, Jr. (Columbia, 

 U.S.A.). 



Specific models for conversion-type inheritance 

 or paramutation can be based on gene-extrinsic 

 or gene-intrinsic mechanisms. The basic obser- 

 vations in regular conversion-type systems (fail- 

 ure of normal segregation of parental phenotypes 

 in certain hybrids contrasted with normal se- 

 gregation in others) can be conceived to depend 

 upon (1) an independent but gene-maintained 

 material, (2) a gene-associated but transferrable 

 material, or (3) a gene-intrinsic feature that may 

 be forcibly copied. These conceptions are res- 

 pectively parallel to (1) the properties of plasmids 

 in many organisms, (2) the properties of episomes 

 in bacteria, and (3) special crossing-over models. 

 Criteria by which these systems might be distin- 

 guished in eukaryotes include time of conversion, 

 concurrence of chromosomal deletion with loss 

 of the conversion effect, infectivity, synchrony in 

 replication of the chromosome and the unortho- 

 dox system, association of the system with 

 particular alleles, and the mechanics of origina- 

 tion of the phenomenon. Because the regular 

 conversion-type event at the B locus in maize is 

 (a) completed late in the life cycle, (b) eliminated 

 by X-irradiation concurrently with linked 

 chromosomal markers, (c) apparently not in- 

 fective, (d) replicated synchronously with the 

 chromosome, (e) unrelated to allele function, and 

 (f) originating from a gene-independent system, 

 a dual model (a gene-associated but transfer- 

 rable material) is invoked. Specific predictions of 

 the dual model are being tested. A full report is 

 in preparation. 



4.24. Control of Activity at a Mutable Locus in Maize. 



Peter A. Peterson (Ames, U.S.A.). 



Control systems as formulated from a study 

 of mutable loci in maize are characterized by the 

 following features; the definable elements that 

 comprise the system vary in time and rate of 

 action upon a locus, undergo transposition to 

 various sites in the genome, and affect the qual- 

 itative expression of a locus. 



The mutable locus described here (ai colorless, 

 to A\ color on chromosome 3) consists of two 

 elements — / (Inhibitor) that controls the action 



of the gene in question when intimately associated 

 with it and En (Enhancer) that controls the action 

 of / by removing or inactivating /. The expression 

 of the / pattern is dependent on the presence of 

 En, however En cannot be recognized unless 

 there is an / controlled locus. Each of these 

 elements is identified by a pattern of mutability 

 as determined by the time (manifest in size of 

 mutant area) and frequency (manifest in number 

 of mutant areas) of mutation events. 



Recent experiments show that these elements 

 can also control the qualitative expression of the 

 locus. This is manifest in the wide assortment of 

 gene expression derived from certain of the 

 mutable alleles that have reduced levels of gene 

 action. The phenotypes in these cases range from 

 very light pales to dark pales that are heritable 

 and remain stable at each level in future gene- 

 rations. Experiments also indicate that additional 

 differences among the elements may be expressed 

 in their differential rate of change to other pattern 

 types or to colorless forms. 



These results show that a particular gene locus- 

 can be affected both quantitatively and qualitas- 

 tively by the controlling elements in the genome. 



4.25. Transposition of Mutability among Components 

 of a Compound Allele of the A 1 Locus in Maize* 



M. G. Neuffer (Columbia, U.S.A.). 



An unstable condition has arisen at the site 

 of the (3 component of the compound A b allele 

 (a (3) on chromosome 3 in maize. The mutability 

 is expressed as the loss of (3 function which per- 

 mits expression of the a phenotype (dilute seed, 

 red brown plant), since (3 is dominant to a in 

 these characteristics, and as frequent recovery 

 of (3 action to give sectors of the (3 phenotype 

 (full colored seed, purple plant). The system is 

 apparently autonomous as all known elements 

 are located at the A\ locus. Localization the effect 

 at (3 was confirmed by separation of the unstable 

 (3 ((3 m ) from x by crossing over in a heterozygote 

 of a (3 m with a homologous segment carrying 

 only a null component of the A\ locus. The (3 m 

 crossover has a recessive phenotype (colorless 

 seed, brown plant) with frequent sectors of |3 

 phenotype. 



Included among several variations from the 

 original case are those in which (1) the muta- 

 bility has left (3 and appeared at a to produce a m (3 

 and (2) the mutability remains at (3 but also has 

 appeared at a to produce a m |3 m . Kernels ex- 

 pressing the latter are phenotypically colorless 

 with dots of both dilute (a) and of full color ((3). 

 On these seeds it can be observed that changes 



44 



