196 



FUNDAMENTALS OF CYTOLOGY 



The meiotic behavior typical of plants in which reciprocal transloca- 

 tion of nonhomologous chromosomes has occurred is shown in Figs. 

 143 145. An individual carrying the two translocated chromosomes and 

 the two normal chromosomes with which they are homologous is called a 

 structural hybrid: it is "heterozygous for the translocation." In its 

 sporocytes these chromosomes form a cross-shaped configuration by the 

 synapsis of homologous parts and then open out into a ring-of-4 at dia- 

 kinesis. Such ring or chain formation is known as catenation. While 



Alternate distributionj give 

 functional spores and gametes 



Adjacent distributions give 

 non -functional spores and gametes 



Unions of functional gametes give individuals of three types: 



Stanolarcl type 



Structural hybrid 



Modified type 



Fig. 143. — Diagram sliowing some of the effects of reciprocal translocation between 

 nonhomologous chromosomes (distinguislied by heavy and light lines and unUke knobs.) 



the other chromosomes of the complement form bivalents and disjoin 

 as usual, the chromosomes in the ring disjoin in two ways. In some 

 sporocytes, alternate members go to the same pole, giving spores and 

 later gametes of two kinds. Since both kinds contain all the chromo- 

 somal elements, although in unlike arrangements, they are both func- 

 tional. In other sporocytes, adjacent members of the ring go to the same 

 pole, yielding spores and gametes all of which lack certain elements of the 

 genome and are nonfunctional. Hence such plants are said to be semi- 

 sterile, although the percentage of sterility differs considerably in different 

 cases. When such a plant is selfed, male gametes of the two functional 

 kinds meet female gametes of the same two kinds and j^roduce offspring 



