Meiosis and Chromosomal Segregation 



19 



other bivalents, so that it is purely a matter 

 of chance whether the copy of the ma- 

 ternally-derived chromosome will go to one 

 specified pole and the copy of the paternally- 

 derived chromosome to the other, or vice 

 versa. Consider the distribution of two bi- 

 valents, for example. Since there are many 

 cells undergoing meiosis in any sex organ, 

 or gonad, at metaphase I, approximately half 

 of these will have the two paternal univalents 

 going to one pole and the two maternal uni- 

 valents going to the other pole at anaphase 

 I, and approximately half will have one ma- 

 ternal and one paternal going to one pole 

 and one paternal and one maternal to the 

 other. As a result, the chromosomal con- 

 tent of a pool of all the haploid nuclei pres- 

 ent at the completion of meiosis will be 25% 

 paternal + paternal; 25% maternal + ma- 

 ternal; 25% paternal -f- maternal; 25% ma- 

 ternal + paternal. Because the centromeres 

 of each bivalent line up at metaphase I in 

 one direction with a frequency equal to that 

 in the other and because each bivalent does 

 so independently of all other bivalents, we 

 see that the segregation which follows occurs 

 independently for different pairs of chromo- 

 somes. 2 Note also, from the fate of two bi- 

 valents, that 50% of haploid products have 

 the same combinations of nonhomologous 

 chromosomes as entered the individual in 

 the parental gametes, therefore retaining the 

 old or parental combinations, whereas 50% 

 of haploid products carry new, nonparental 

 combinations or recombinations . 



Let us defer considering the genetic im- 

 plications of these conclusions until we have 

 considered the second question, which also 

 bears upon the maternal-paternal chromo- 

 some content of gametes: our answer may 

 modify the conclusions just reached. Is a 

 chromosome in a gamete, in fact, a com- 

 pletely uniparental replica, or has it a bi- 



2 As shown by E. E. Carothers (1921). 



B 



figure 2—3. Lily diplonema showing chro- 

 matids (1-4) with different synaptic partners 

 on different sides of a chiasma. (Courtesy of 

 R. E. Cleland.) 



parental derivation? The latter situation 

 would obtain if one segment of a gametic 

 chromosome were a copy of a portion of 

 one homolog and another segment a copy 

 of a portion of the other homolog. 



Considerable evidence exists that some 

 time between the onset of meiosis and diplo- 

 nema a cytologically undetected event occurs 

 which results in two of the four chromatids 

 in a tetrad having segments which are bi- 

 parental copies, exactly reciprocal in con- 

 tent. Thus, if one biparental segment of a 

 chromatid has a linear sequence that is ma- 

 ternal-paternal, the other is paternal-mater- 

 nal in composition. The other two chro- 



