Knight —126— Dictionary 



Yzil - p)Ab, Yzil - p)aB, Yzpab, where p represents the 

 recombination fraction. Where there is no linkage, p = 0.5. 



Recombination Index. — A measure of the mean number of 

 independently segregating chromosome segments in the species, 

 individual, etc. The recombination index is calculated by adding 

 the gametic chromosome number to the average total number 

 of chiasmata in the mother-cell. 



Recombination Value. — Recombination Fraction, q.v. 



Recurrent Parent. — That parent of a hybrid with which it 

 is again crossed or with which it is repeatedly crossed; the 

 backcross parent. 



Reduced Apogamy. — Haploid parthenogenesis. See under 

 Parthenogenesis. 



Reduced Fertilization. — Replacement of normal fertilization 

 by the non-sexual union of cells, e.g. by the union of two 

 gametes both of the same sex (usually female). 



Reduction. — (i) The halving of the chromosome number at 

 meiosis. (ii) The relegation of a character to the immature 

 stage of an animal (it being vestigial in the adult) although 

 the character was present in both the young and adult stages 

 of the animal's ancestors. 



Reduction Division. — Reductional Division, q.v. 



Reductional Division. — A separation of homologous parts 

 of chromosomes derived from opposite parents at anaphase of 

 a first or second division (Darlington). Heterotypic Divi- 

 sion. 



Reduplication. — The state of having one segment of a 

 chromosome present twice in the one complement, the com- 

 plement being otherwise undoubled. 



Regression, Filial. — The tendency for the offspring of ex- 

 ceptional parents to revert towards the norm of the species. 



Relationship Coefficient. — See Appendix 1. 



Relaxin. — A sex hormone produced in the female reproduc- 

 tive glands of animals (including humans), which causes the 

 symphysis pubis to relax thereby facilitating parturition. 



Rensch's Rules (Birds). — (i) The races of a species which 

 live in the cooler parts of the range of that species lay more 

 eggs per clutch than the races in the warmer parts of the 

 range, (ii) Stomach, intestines and caeca of birds that live 

 on a mixed diet are relatively smaller in the tropical than in 

 the temperate zone races, (iii) The wings of races that live 

 in a cold climate or in the high mountains are relatively longer 

 than those of the races which live in the lowlands or in a 

 warm climate (Mayr, 1942). 



