Opment of Tarsal Claws in Heterozygotes1

ONSIDERABLE attention has been drawn to the various cases of apparent C multiple alleles which upon critical examination have been found to exh bit a low order of recombination, as if they occupy spatially separable, adjacent gene loci. Such mutants, termed pseudoalleles, exhibit what have been considered to be the physiological criteria of allelism ; namely 1) they exhibit similar phenotypic effects, and 2) their heterozygotes exhibit dominance of one or the other member or intermediate phenotypes. In addition, position effects are noted in several cases, as follows: if one considers a pair of pseudoallelic loci, a+ and b+, and their respective mutant alleles, a and b, the coupling or cis heterozygote, a+b+/ab, appears wild type, while the repulsion or trans heterozygote, u+b/ab+, exhibits the mutant phenotype. Considerable speculation, disagreement, and confusion exists concerning the interpretations to be given to these observations. The confusion may be avoided if it is recognized that the disagreement is concerned with two quite distinct problems: 1) The observations associated with pseudoallelism have led to interpretations regarding the structural organization of the hereditary material. Three views have been offered: (a) The chromosome is the fundamental unit of heredity. This interpretation proposes that the chromosome is a physical continuum whose intact organization is necessary to exhibit the normal phenotype. Mutations are believed to be due to rearrangements within this organization. Such rearrangements exhibit the separable behavior of genes when subjected to a breeding experiment (GOLDSCHMIDT 1938, 1951). (b) The chromosome consists of a linear arrangement of separable segments. Each segment consists of a grouping of subsegments exhibiting a low order of recombination (PONTECORVO 1953). (c) The genetic material consists of discrete units which are separable on the basis of recombination. This view suggests that pseudoalleles are duplicates which may have arisen by unequal crossing over (LEWIS 1951; GREEN 1955). 2) The second area of disagreement concerns the nature of the physiological mechanisms underlying the phenotypic and position effects associated with pseudoallelic loci. (a) One hypothesis proposes that the entire segment acts as a physiological unit. Such regions may exist in alternative forms, subject to both internal