Somatic Nondisjunction 



429 



Other causes for the origin of trisomies are also possible. 

 Rhoades, for example, found a strain of maize in which an extra 

 chromosome was present consisting of the short arm of chromo- 

 some V, broken in such a fashion that the centromere was at the 

 end of this centric fragment chromosome. A terminal centro- 

 mere is unstable and produces a number of abnormalities. Oc- 



g3 gS g4 



V2 V3 V, V* ^' v« 



Fig. 119. Two of the trisomies of Oenothera Lamarckiana. The desig- 

 nation of chromosomal ends by letters rather than numbers was a tem- 

 porary expedient of the British geneticists before their material and the 

 American material could be compared and similar numbers adopted for 

 both. Above, pairing in plant resulting from union of a gamete with 

 seven velans and one gaudens chromosomes with a normal gaudens gamete. 

 Below, pairing in a plant from the same eight-chromosome gamete and a 

 velans gamete. (Redrawn from Catcheside in the Journal of Genetics.) 



casionally, it divides transversely instead of longitudinally, and 

 in this way it can produce a secondary trisomic with two iden- 

 tical arms. 



Somatic Nondisjunction 



If a red-flowered plant is heterozygous for the gene C, non- 

 disjunction of the chromosome that carries the C gene during 

 somatic divisions of the cells of the epidermis of the developing 

 petals may produce cells with the constitution CCc and others 

 with only the c gene. All cells that develop from the first cell 

 that lacks the C gene will be white instead of red; white patches 



