Glossary | 329 



Reciprocal translocation transposition of two segments between non- 

 homologous chromosomes. 



Recombination the formation of gene combinations not present in the 

 parental tvpes. 



Recombination index a measure of the number of new gene combina- 

 tions that can be produced in a given time; of Darlington: the average 

 number of chiasmata plus the gametic chromosome number. 



Recon the smallest unit whose interchange between chromosomes may 

 be detected by the techniques of the fine-structure geneticist. 



Reduction a reaction in which hydrogen is acquired or oxygen is lost 

 by a compound or in which the valence of the metallic element is 

 lowered. 



Regulator gene the DNA transmitter of specific cytoplasmic signals in 

 the form of repressor-substance molecules. 



Replicate to duplicate repeatedly. 



Repressors molecules with the property of combining with a particular 

 metabolite (which may either activate or inhibit it) and a particular 

 operator whose action is then blocked. 



Reproductive cells the gametes and their immediate predecessors from 

 which thev are produced by division. 



Reticulnte evolution evolution in which lines combine as well as split. 



Ribonucleic acid (RNA) giant molecules of various types, of which, in 

 most organisms, some copy the genetic code from the DNA and carry 

 it to the sites of protein synthesis (messenger RNA) and some aid in 

 the organization of the synthesis (transfer RNA); see nucleic acid. 



Ribosomes granules associated with the boundary of the endoplasmic 

 reticulum; the presumptive sites of protein synthesis; see microsome. 



RNA see ribonucleic acid. 



Sample a subset of a population; a group of items drawn by some 

 procedure from a population and from which one hopes to learn cer- 

 tain things about the population. 



Sampling error variation due to random elements in the sampling 

 process. 



Segmental allopolyploid an allopolyploid in which the combined 

 genomes are homologous in many small segments throughout the com- 

 plement; crossing-over may recombine material from different genomes. 



Selection differential reproduction of genotypes. 



Selection coefficient the measure of the disadvantage of a given geno- 

 tvpe relative to other genotypes in the population. 



Sex chromosome usually a largely heterochromatic chromosome func- 

 tioning at least in part in sex determination. 



Sexual reproduction reproduction involving the fusion of gametes pro- 

 duced by a prior meiotic process. 



Specialized applying to an organism or character judged to be more 

 changed from a presumed common ancestral state than another with 

 which it is compared. 



