A DJ U S T M r. N T AND I! A I A N CE 



sexually ditfcrentiated animals, the plants show segregatioiwl 

 sterility. 



hi Raphano-hrassica, pairing of the chromosomes is almost absent 

 from meiosis, so that gametes may receive any number, as well as 



TABLE 20 



AVERAGE FREQUENCIES OF CHIASMATA IN THE BI- 

 VALENT CHROMOSOMES IN RELATION TO THE 

 FAILURE OF METAPHASE PAIRING AND THE POLLEN 

 FERTILITY OF DIPLOID (;c = 7) GRASS SPECIES AND 

 THEIR DIPLOID HYBRIDS (PETO, 1933) 



Note: Xta - chiasmata. The percentage figures are univalents where these occur. 

 * The LoUum parent in this cross was itself a varietal cross, hence the low pollen 

 fertility. 

 I Used for backcrossing. 



any combination, of chromosomes. In Primula kewensis pairing is 

 sufficiently regular for the great majority of the gametes to receive 

 the customary 9 chromosomes, one from each pair, but these 9 

 may represent any combination of the maternal and paternal sets. 

 Evidently the property of harmonious working, which is shown 

 by the complete paternal and maternal sets, is not shared by their 

 individual parts. Owing to structural changes in their ancestry these 



232 



