Unbalance. — See Balance. 



Unconditioned Reflex. — Hereditarily determined reflex ac- 

 tions which appear in all normal members of the species and 

 which do not result from individual experience or habit, cf. 

 Conditioned Reflex. 



Unequal Bivalent. — A bivalent in which one of the consti- 

 tuent chromosomes is longer than the other, so that it has an 

 unpaired region at one end (White). See Univalent. 



Uniovular Twins. — Monozygotic twins; twins arising from 

 a single ^gg. 



Uniparous. — Producing only one offspring at a birth. 



Unipolar Spindle. — A type of spindle with only one pole 

 which occurs during meiosis in some insects; a 'half-spindle.' 



Unisexual. — Of one sex, i.e. male or female, as opposed to 

 hermaphrodite. 



Unisexualism. — The state of having the two sexes separated 

 by reason of their being confined to separate individuals. 



Unit Character. — In mendelian inheritance, a character or 

 alternative difference of any kind, which is either present or 

 absent, as a whole, in each individual, and which is capable 

 of becoming associated in new combinations with other unit 

 characters (Hayes & Garber). 



Univalent. — A body at the first meiotic division corresponding 

 with a single chromosome in the complement; especially when 

 unpaired. Bivalent, Trivalent, Quadrivalent, Quinquevalent, 

 Sexivalent, Septivalent, Octavalent (for Octovalent), etc. are 

 associations of chromosomes held together between diplotene 

 and metaphase of the first division by chiasmata. Similar as- 

 sociations of more than two including non-homologous chromo- 

 somes in structural hybrids should be described as "unequal 

 bivalents", "associations" or "rings" of three, four, etc. {see 

 Hexad). The hybrid forms, "monovalent", "tetravalent", "pen- 

 tavalent", "hexavalent", etc., used in chemistry should be 

 avoided (Darlington). 



