Recombination Index — 127 — Reverse Mutation 



Rensch's Rules (Mammals). — (i) The races in the warmer 

 dimates have less under fur and shorter contour hairs, (ii) 

 The number of young in a litter averages higher in the cooler 

 climates (Mayr, 1942). 



Rensch's Rules (Snails). — (i) Land snails reach their 

 greatest size in the area of optimum climate v^^ithin the range 

 of the species, (ii) The relative weight of the shell is highest 

 in the forms exposed to the highest radiation of the sun (in- 

 solation) or to the greatest aridity, (iii) Land snails tend to 

 have smooth glassy brown shells in cold climates, to have white 

 or strongly sculptured shells in hot dry climates (Mayr, 1942). 



Reproduction. — Thomas (1940) classifies the various types 

 of reproductive mechanism as shown in the table on the next 

 page. 



Reproductive Cell. — Gamete. 



Repulsion. — Signifies that one recessive and one dominant 

 allele are carried in one chromosome and that the two alterna- 

 tive dominant and recessive alleles are carried in the other 

 chromosome. See under Recombination Fraction. 



Residual Homology. — A degree of affinity still existing be- 

 tween once truly homologous chromosomes, which will now 

 only rarely permit crossing-over between them (Stephens). 



Resting Nucleus. — Any nucleus which is not undergoing divi- 

 sion. 



Resting Phase. — Interphase; the phase in the nuclear cycle 

 when it is at rest in the sense of not dividing. 



Restitution. — An induced chromosome break which has re- 

 joined in the original way. 



Restitution Nucleus. — A single nucleus formed through 

 failure of the first division (Rosenberg; Darlington). 



Restriction Factor. — A special form of modifying factor 

 which reduces the effect of a particular major gene by limiting 

 its phenotypic action to certain portions of the individual. 



Retardation. — The slowing-down of the rate of action of 

 a gene so that the character it controls is not affected till the 

 late adult stage and thus becomes vestigial. 



Reticulate. — Of evolution, species-formation, etc.: depend- 

 ing on repeated crossing between a number of lines. Reticu- 

 late evolution is characterized by the continual isolation of 

 groups which are then again brought together so that diversity 

 is increased by segregation and recombination. 



Reverse Mutation. — The mutation of a mtuant gene back to 

 its original state. 



