430 



Aneuploids and Nondisjunction 



will then appear on the otherwise red petals and will be wedge- 

 shaped. The size of these white patches will depend upon the 

 stage in the formation of the flower at which the nondisjunc- 

 tion occurred. If the flower was young, the white sectorial 



chimera will be large, but if it was nearly 

 developed at the time the nondisjunction took 

 place, the white area will be small. Lawrence 

 has reported several such chimeras in Dahlia 

 variabilis. Wedge-shaped sectorial chimeras 

 have been observed in Nemesia strwnosa 

 (Fig. 120) and probably result from somatic 

 nondisjunction. In Nemesia, some purple 

 flowers have white sectors as do some orange- 

 colored flowers, showing that apparently both 

 the C and the gene are on chromosomes 

 that can undergo somatic nondisjunction. 

 In a very few white flowers of plants hetero- 

 zygous for bm, the gene for blue-margin, a 

 blue chimera was found on the upper lips 

 of the flower. 



Fig. 120. A purple 

 flower of Nemesia 

 strumosa showing 

 two relatively large 

 and one small sec- 

 torial chimeras re- 

 sulting probably 

 from somatic non- 

 disjunction. The 

 small chimera re- 

 sulted probably from 

 a nondisjunction 

 which occurred at a 

 later stage in the 

 ontogeny of the 

 flower than the 

 aberrations that pro- 

 duced the other 

 chimeras. Compare 

 with Fig. 10. 



Organisms with More Than One Extra Chro- 

 mosome 



Since a plant or animal might have one 

 extra chromosome, we might wonder whether 

 it could have more than one. Theoretically, 

 if a plant could be trisomic for one pair of 

 chromosomes, every chromosome pair might 

 be in the trisomic condition. In Datura 

 plants have been found which are trisomic 

 for two pairs of chromosomes. Such double 

 trisomies have the formula 2n -{- 1 -{- 1, and 

 they are generally less viable than simple 

 trisomies. Plants trisomic for three chro- 

 mosomes are 2n -f- 1 -|- 1 + 1, but are practically nonexistent. 

 Plants with 2n + 2 chromosomes are tetrasomic for one of the 

 chromosomes. They show a greater exaggeration of the char- 

 acters determined by their genes and a greater unbalance than 

 the corresponding trisomic, and they are generally weaker and 

 less viable. In Datura four primary chromosomes can be trans- 



