I08 AN INTRODUCTION TO MODERN GENETICS 



B. TRANSLOCATION 



I. Datura Secondaries and Tertiaries^ 



In Datura, the tomato, and some other plants, translocation and 

 duplication is often found combined with trisomy. The trisomic 

 chromosome is in these plants not simply an extra member of the 



Fig. 51. Chromosome Constitution and Association in Trisomic Types in 

 Datura. — The column on the left shows the chromosome constitution; in 

 primary trisomies the reduplicated chromosome is one of the normal set, in the 

 secondaries it has two similar ends, in the tertiaries it has an end of one chromo- 

 some united with part of a different chromosome. The ends are distinguished by 

 numbers. The main data about the behaviour of the chromosomes relate to their 

 associations at metaphase of meiosis, when the chiasmata are completely ter- 

 minalized (right-hand column). In the primaries, the characteristic configuration 

 is a ring of two with a third attached; in the secondaries, rings of three may be 

 formed, and in the tertiaries the reduplicated chromosome may unite two other 

 pairs, giving chains of five. (Based on Belling.) 



normal set, but involves a translocation. In one type, called secondary 

 trisomies, the trisomic chromosome consists of a reduplicated half: 

 from a chromosome pair abed, abed, the translocations abba, deed are 

 formed, and one of these compounds is present in the trisomic, giving, 

 for instance, a form with the normal abed, abed, plus abba. In the other 

 type (tertiaries) the trisomic body is a translocation between two non- 

 homologous chromosomes, e.g. abpq. 



^ Revs. Blakeslee 1930, 1934. 



