416 



Aneuploids and Nondisjunction 



lents frequently behave abnormally at meiosis, often lagging 

 behind the other chromosomes and failing to be included in the 

 daughter nuclei. Because of such lagging, a higher percentage 

 of n — 1 gametes may be found than would be expected. Mc- 

 Clintock has found a 2n — 1 chromosomal chimera in a maize 



plant which clearly showed 

 nine bivalents and one uni- 

 valent at the metaphase of 

 the first meiotic division in- 

 stead of the expected ten bi- 

 valents (Fig. 113). This uni- 

 valent usually lagged at the 

 equator after the bivalents 

 had separated or it split into 

 two halves which were fre- 

 quently not included in the 

 daughter nuclei. Root-tip 

 .studies from the same plant 

 showed twenty chromosomes. 

 Apparently, early in ontogeny 

 a mitosis had been irregular, 

 and the upper part of the 

 plant became monosomic. 

 Since maize is presumably a 

 diploid, a monosomic would not be likely to occur unless it was 

 produced in this manner by an abnormal somatic mitosis. 



Trisomies 



If an organism has an extra chromosome, it is known as a 

 trisomic. Normally, this name implies that one complete chro- 

 mosome is present three times rather than twice, as it would be 

 in a normal diploid ; but in a few cases the extra chromosome is 

 not a normal complete chromosome but an interchange chromo- 

 some. 



One of the well-known trisomies is the triplo-IV Drosophila. 

 Unlike the haplo-IV, in which one of the fourth chromosomes is 

 missing, this type contains one extra member of chromosome IV 

 so that it has in all nine chromosomes instead of eight. Also 

 unlike the haplo-IV type, triplo-IV flies cannot be identified 



Fig. 113. Metaphase of the first 

 meiotic division in the developing 

 microspores of a 2n — 1 maize plant. 

 Nine bivalents and one univalent are 

 present. (Courtesy of Dr. B. McClin- 

 tock in the Journal of Heredity.) 



