30 



CHAPTER 4 



m 



N^ 



m 



FIGURE 4-5. Chiusiua showing paternal (/?) and nuiternal (/?/) composition of strands. 



rate at anaphase I, segregation of maternal 

 from paternal centromeres occurs at the first 

 meiotic division. Then, depending upon the 

 location and number of chiasmata at diplo- 

 nema, the dyad having the maternal centro- 

 meres will contain different paternal seg- 

 ments along its two chromatids, while the 

 dyad with the two paternal centromeres will 

 have the complementary maternal sections 

 along its two chromatids. Accordingly, 

 segregation of maternally-derived chromatid 

 material from paternally-derived chromatid 

 material occurs for centromeric and some 

 other regions of the chromatids at anaphase I 

 and is accomplished for other regions of the 

 chromatids at anaphase II. 



What genetic meaning can we give to the 

 meiotic processes described? It has already 

 been established that when the members of a 

 pair of genes segregate they do so cleanly, 

 entering the gametes of an individual just the 

 same as they were when they originally 

 entered that individual at fertilization. What 



chromosomal constitution can we correlate 

 with the properties of the gene? A whole 

 haploid set of chromosomes (a genome) can- 

 not be the physical basis of a gene since, 

 though the gametes produced by an indi- 

 vidual contain a single genome, this is usually 

 constituted (ignoring chiasmata, for the 

 moment) of som.e maternal and some paternal 

 chromosomes. Moreover, a pair of genes 

 cannot be identified even with an entire pair 

 of chromosomes, since a haploid chromosome 

 in a gamete is, because of chiasmata, typically 

 constituted of parts some of which were 

 originally maternal and other parts originally 

 paternal in derivation. The gene may only 

 be physically associated, therefore, with a 

 particular small segment of a chromosome 

 within which a chiasma cannot be formed 

 with a corresponding segment on a homol- 

 ogous chromosome. Such a segment would 

 always retain its pure maternal or pure pater- 

 nal constitution after segregation; its maximal 

 size would equal the gene's maximal size. 



SUMMARY AND CONCLUSIONS 



Meiosis involves two essentially mitotic divisions modified by the occurrence, during pro- 

 phase I, of synapsis and of chiasmata formation. As a consequence of meiosis the original 

 pair of genomes becomes single in the gametes. Any particular chromosome in a genome 

 of a gamete has an equal chance of having a maternally- or a paternally-derived centromere 

 (because of the random manner in which ditTerent bivalents align themselves on the spindle 

 at metaphase I) and usually contains segments originally derived from the other parent (as 

 a consequence of chiasmata). 



The hypothesis that the gene has a physical basis in chromosomal material may now be 

 made more specific — a gene is physically associated with a short segment of a chromosome 

 within which a chiasma cannot be formed. 



