400 



BREAKDOWN OF GENETIC SYSTEMS 



Crepis, Matthiola) or is segregated by inbreeding (Zea, Randolph, 

 1928 ; Secale, Lamm, 1936). Secondly, there are organisms which 

 differ from the normal merely in having one chromosome or 

 part of a chromosome too few or too many {Avena, Allium, Primula 

 kewensis). In these, abnormal meiosis is determined by unbalance. 

 Thirdly, there are hybrids in which one sex (or kind of mother-cell) 

 has normal pairing of chromosomes which do not pair in the other 

 sex and do not pair even when the chromosome number is doubled 



Table 59 



Chromosome Pairing in Two Strains of Crepis capillaris and in Their 

 Hybrids by C. tedorum (HoUingshead, 1930 h, cf. Tables 22 and 26). 



(e.g., Viola Orphanidis, Drosophila pseudo-obscura, Smerinthus and 

 Pygcera hybrids). Similarly sister progeny in back-crosses or 

 FgS may show sharp differences in regard to chiasma-frequency and 

 chromosome pairing which must be due to segregation of specific 

 genetic factors and not to particular structural differences, since all 

 the chromosomes are equally affected. Thus Peto (1934) found the 

 following chiasma- frequencies : Lolium perenne, i-8i ; Festuca 

 pratensis, 1-88 ; Fj, 171 ; Fg, 5 plants 1-57 to i-8o ; 2 plants with 

 defective pairing 0-80 and 0-62. Again in a tetraploid backcross 



