BREEDING SYSTEMS 



outbreeding confer on their possessors? Indeed, what is the 

 advantage of the normal sexual system ? These questions we shall 

 examine in the next two chapters. 



KINDS OF "TWINS 



2x-2x. 





2oz-, 



2.x-S,x 2.X-OX. 2nc-3oc 2oc-cc 



Zoc-2.x 

 C I or\al 



Fig. 69. — Twins produced in flowering plants classified according to their genetic 

 relationship. One body in the nucleus stands for one haploid set. Arrows represent 

 sperm which are assumed to be haploid (but can be diploid). Apomictic nuclei or 

 embryos (which will develop without fertilization) are stippled. A, the two types 

 formed in animals except that the type giving identical twins is not clonal but the 

 result of fertilization. The lower group are fraternal or semi-fraternal, i.e. identical 

 on the female side only (based on Linum, Kappert, 1933; Poa, Secalc, etc., Miintzing, 

 1933; Gossypium, TrifoUum, etc., cf. Skovsted, 1939). 



REFERENCES 



BARMGOZZI, c. 1 946. Uber die geographische Verbreitung der Mutanten von 



Artemia salina Leach. Arcbiv. d. Julius Klaus-Stift., 21: 479-482. 

 BRiEGER, F. 1930. Selhsterihtdt und Kreuzungsterilitat in Pflanzenreich und Tierreich. 



Berlin. 

 COOPER, K. w. 1937. Reproductive behaviour and haploid parthenogenesis in the 



grass mite, Pediculopsis ^raminum. Proc. Nat. Acad. Sci. Wash., 23 : 41-44. 

 CRANE, M. B., and LAWRENCE, w. J. c. 1947. The Genetics of Garden Plants. 3rd ed. 



London. 

 CRANE, M. B., and THOMAS, P. T. 1940. Reproductive versatility in Rubus.J. Genet., 



40: 109-128. 

 DARLINGTON, c. D. 193 1. The cytological theory of inheritance in Oettothcra. 



J. Genet., 24: 405-474- 



270 



