Meiosis 



27 



provide the physical basis for the segregation 

 of paired genes. What relationship exists at 

 the completion of meiosis between the two 

 haploid sets of chromosomes, one of which 

 was previously contributed by each parent 

 to the zygote? Is it a fact that the maternally- 

 derived and the paternally-derived members 

 of a chromosome pair segregate so that only 

 one or the other is present in a given gamete? 

 Is the haploid set of chromosomes in a gamete 

 composed of all maternally-derived or of all 

 paternally-derived chromosomes? 



For typical meiosis, the answers to these 

 two questions depend upon two events. The 

 first of these is the manner in which the 

 centromeres of the bivalents become arranged 

 at the equator of the spindle at metaphase I. 

 Relative to the poles of the spindle, each 

 bivalent arranges itself at the equator inde- 

 pendently of other bivalents, so that it is 

 purely a matter of chance whether the ma- 

 ternal chromosome will go to one specified 

 pole and the paternal chromosome to the 

 other, or vice versa. Consider the distribu- 

 tion of two bivalents only, for example. Of 

 the many cells undergoing meiosis at meta- 

 phase I, approximately half will have the two 

 paternal univalents going to one pole and the 

 two maternal univalents going to the other 

 pole at anaphase I, and approximately half 

 will have one maternal and one paternal 

 going to one pole and one paternal and one 

 maternal to the other. As a result, the 

 chromosomal content of all haploid nuclei 

 produced at the completion of meiosis will 

 be 25% paternal, paternal; 25% maternal, 

 maternal; 25% paternal, maternal; 25% 

 maternal, paternal. Because the centromeres 

 of each bivalent line up at metaphase I with 

 an equal frequency in the one direction 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 

 chromosomes. Note also that, when we 

 were considerins the fate of two bivalents. 



50% of haploid products had the same com- 

 binations of nonhomologous chromosomes 

 as entered the individual in the parental 

 gametes, therefore retaining the old or 

 parental combinations, whereas 50% of 

 haploid products carried new, nonparental 

 combinations or recombinations. Let us 

 defer considering the genetic implications of 

 this until we have discussed the second factor 

 in meiosis which bears upon the maternal- 

 paternal chromosome content of gametes. 

 This second factor will modify in some 

 respects the conclusions just reached. 



Recall the chiasmata which are seen at 

 prophase I during diplonema. On the basis 

 of certain evidence, it may be considered that 

 each chiasma represents a place where one 

 maternal chromatid of one univalent and one 

 paternal chromatid of the other have "broken" 

 at exactly corresponding positions and cross- 

 united. If such an exchange had taken place, 

 the continued close pairing between paternal 

 chromatid segments and between maternal 

 chromatid segments would produce the 

 chiasma configuration seen. Accordingly, a 

 tetrad containing one chiasma would have the 

 paternal (p) and maternal (m) hnear constitu- 

 tion, as shown in Figure 4-5 where the 

 centromere is represented by C. From this 

 Figure you can see that, following one 

 chiasma, one chromatid remains entirely 

 maternal, one remains entirely paternal, but 

 the other two are composed of segments of 

 both paternal and maternal origin. Note 

 again that in any one chiasma only two of the 

 four chromatids exchange parts. However, 

 since a tetrad normally contains several 

 chiasmata, usually each of the four chroma- 

 tids has exchanged at one place or another 

 with a chromatid derived from the other 

 parent and consequently has a biparental 

 composition along its length. 



We can now return to our questions re- 

 garding the maternal-paternal chromosome 

 content of the haploid products of meiosis. 

 Since the centromeres of the bivalents sepa- 



