68 M. M. RHOADES 



sporogenesis in plants heterozygous for a normal and an abnormal type of 

 chromosome 10. Approximately 70 per cent of the functioning mega spores 

 possessed the abnormal 10 instead of the usual 50 per cent. The excess of 

 female gametes with the abnormal 10 was not due to lethal factors or to 

 megaspore competition. The disjunction of the two dyads comprising the 

 heteromorphic bivalent at anaphase I, and of the two monads of each dyad 

 at anaphase II, was such that an abnormal 10 chromosome tended to pass 

 with a high frequency to the basal spore of the linear set of four. 



The factor or factors responsible for this preferential segregation reside 

 in the chromatin segments which differentiate the two kinds of chromosome 

 10. Whether the distal one-sixth of the long arm or the large heterochromatic 

 piece of extra chromatin carries the causative genes for preferential segrega- 

 tion has not yet been determined — since these two regions of the abnormal 

 chromosome 10 have never been separated by crossing over. The locus of 

 the gene R is in the long arm of chromosome 10. There is approximately 1 per 

 cent recombination between R and the end of the long arm in plants hetero- 

 zygous for the two kinds of chromosome 10; but every crossover distal to R 

 occurred to the left of the dissimilar chromomeres in the distal one-sixth of 

 the long arm. Apparently little or no crossing over takes place here, although 

 pairing at pachytene is intimate. 



Strictly terminal chiasmata in the long arm have not been observed at 

 diakinesis in heterozygous plants. The close linkage of the R locus with the 

 extra segment of abnormal 10 is due to a suppression of crossing over in the 

 end regions of the long arm. E. G. Anderson (unpublished) has studied a re- 

 ciprocal translocation involving normal 10 with the break distal to R, and 

 found 5 per cent recombination between R and the translocation point. 

 There is an undetermined amount of crossing over between the translocation 

 point and the end of the chromosome. It should be possible to locate the re- 

 gion or regions in abnormal 10 responsible for preferential segregation by ob- 

 taining successively larger terminal deficiencies, but this has not been at- 

 tempted. 



The dissimilarity in chromomere pattern in the distal portion of the long 

 arms of the abnormal and normal chromosomes 10, together with the lack of 

 crossing over in this region, suggest the possibility that the gene content may 

 not be identical in the two kinds of chromosome 10. Inasmuch as plants 

 homozygous for the abnormal chromosome 10 are not noticeably different in 

 growth habit and general appearance from sibs carrying only the normal 10, 

 it would appear that some kind of structural modification was responsible for 

 the suppression of crossing over. To assume that this distal region consists 

 of non-homologous loci in the two types of chromosome would mean that 

 plants with two abnormal 10 chromosomes would be homozygous deficient 

 for certain loci found in the comparable region of normal 10. This appears 

 unlikely. 



