50 I The Process of Evolution 



tion of the banding patterns). Since synapsis will be unafiFected in 

 the homozygote, recombination will not be reduced, but the linear 

 arrangement and linkage relations of the genes will, of course, be 

 changed. 



Reciprocal translocations involve nonhomologous chromosomes. 

 Here a portion of one chromosome is transferred to another, and 

 vice versa, so that linkage groups are changed. The size of the seg- 

 ments exchanged may be the same or different, large or small. 

 Sometimes one chromosome will exchange a heterochromatic region 

 (with little specific genetic activity) for a euchromatic region (with 

 typical genetic behavior). Heterozygosity for large reciprocal 

 translocations is visible in meiosis, as well as in organisms with 

 somatically paired polytene chromosomes. Synapsis will result in the 

 association of four chromosomes, two standard and two with trans- 

 located regions. At metaphase the appearance of this quadrivalent 

 will depend, among other things, upon the distribution of chiasmata. 

 Usually the chromosomes separate to form a ring. Chromosome ends 

 are held together bv terminalized chiasmata. Disjunction may occur 

 so that adjacent centromeres go to the same pole. Examination of 

 Fig. 3.3 will show that there are two different possibilities for this 

 sort of disjunction but that either will lead to the production of un- 

 balanced gametes (i.e., those with duplications and deficiencies). 

 Only if alternate centromeres go to the same pole can balanced 

 gametes result. The fusion of such gametes, randomly, will produce 

 standard homozvgotes, translocation heterozvgotes, and transloca- 

 tion homozygotes in the ratio of 1:2:1. Organisms that are homozv- 

 gous for a reciprocal translocation exhibit no meiotic peculiarities 

 since svnapsis is undisturbed. The linkage groups are changed, how- 

 ever; this can be detected bv genetic analvsis. 



It is possible for more than one inversion to occur in a nucleus 

 or in a chromosome. Inversions in a chromosome may be in one arm 

 only or may include the centromere. If there are several, they may be 

 independent or overlapping, or one or more may be included within 

 another. Detailed studies of these conditions have been made in 

 Drosophila and are discussed in Chap. 9. With more than one trans- 

 location, the result depends upon which chromosomes are involved. 

 If a different pair exchanges segments after the first translocation, 

 the result will be the formation of two quadrixalents at metaphase of 

 meiosis. If one of the chromosomes experiencing the first transloca- 

 tion exchanges a segment with a third, a ring of three chromosomes 

 is found. Finally, in some organisms, all the chromosomes exchange 

 arms and all are attached in a ring at meiosis. Examples of this are 

 considered in Chap. 9. 



