THE CHROMOSOME MECHANISM 



somes intervenes between these two divisions of the nucleus. If either 

 of the divisions fails to separate two daughter nuclei effectively, an 

 unreduced or diploid germ cell is formed. And if both fail, the germ 

 cell or spore is tetraploid. 



Now the four germ cells produced by a regular meiosis are alike 

 in being haploid. But in all other respects their nuclei are different. 

 The arrangement of the bivalents at first metaphase has been, as we 

 can show, at random, so that maternal and paternal chromosomes 

 will have been assorted at random. At the same time we have to 

 realize that we can no longer speak of chromosomes as the units they 

 were at mitosis. Their parts have also been reassorted and exchanged 

 or recomhined by crossing-over. Those parts which separated rcduc- 

 tionally at the first division, separate equationally at the second, and 

 vice versa. Further, since every bivalent has been held together by 

 at least one chiasma, and its chromatids have undergone at least 

 one crossing-over, all its four chromatids distributed to the four 

 nuclei are different combinations. And finally each bivalent in each 

 mother cell differs from all others almost without limit, since each 

 crossing-over can occur at hundreds of different positions. Hence 

 each mother cell will have given rise to four spores or sperms dif- 

 ferent from one another and different from those produced by any 

 of the other mother cells. It was the consideration that the universal 

 production of four cells at meiosis implied that the four must be 

 universally different that led Janssens in 1909 to suppose that crossing- 

 over was a universal concomitant of meiosis; that it was associated 

 with the formation of chiasmata; and that it led to the recombination 

 of hereditary differences. 



In short, by virtue of crossing-over, no two of the products of 

 one meiosis will be alike. And since crossing-over can probably take 

 place between any two chromomeres, it will be rare indeed for two 

 identical haploid nuclei to be produced from different meioses— 

 unless the pairing chromosomes are themselves identical. Still rarer 

 will it be that two haploid gametes will combine in fertilization 

 to restore the type of either of their parents. Here then we sec how 

 variation, the occurrence of differences between individuals, can be 

 maintained from generation to generation in sexual reproduction; 

 and not only maintained, but rearranged and redistributed in such 

 a way as to make every varying individual unique. 



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