Nondisjunction 423 



need not necessarily be so. In fact, Blakeslee has found three 

 types of trisomies in Datura. The type we have just described 

 has one normal chromosome in excess so that one pair of homo- 

 logues is represented by three members. This type is frequently 

 called a primary trisomic. In Datura, the primary trisomic 

 Rolled has the 1-2 chromosome present three times instead of 

 twice. If two of these homologous chromosomes were to undergo 

 reciprocal translocation at about the centromere, two new chro- 

 mosomes would be formed which would be 1-1 and 2-2. If now 

 a trisomic was formed which had two normal 1-2 chromosomes 

 plus this new 1-1 chromosome, this trisomic would be different 

 in its pairing and in the resulting phenotype from the normal 

 primary trisomic. These new trisomic types are known as sec- 

 ondary trisomies. In Datura, the primary trisomic Rolled (1-2) 

 has two corresponding secondaries, Polycarpic (1*1) and Sugar- 

 loaf (2-2). If we remember our rule of part-by-part pairing, 

 metaphase configurations of a secondary trisomic with com- 

 plete terminalization of chiasmata will include some circles of 

 three chromosomes. 



In tertiary trisomies the extra chromosome arose by a recipro- 

 cal translocation between two nonhomologous chromosomes. 



Nondisjunction 



Now that we have discussed both monosomic and trisomic 

 types, it may well be asked how they originate. They appear 

 to arise from an irregularity at a cell division such that the 

 two homologous chromosomes become included in the same 

 daughter nucleus instead of being distributed one to each daugh- 

 ter nucleus. This distribution usually occurs at meiosis but may, 

 as we have pointed out, occur at a somatic mitosis, producing 

 chromosomal chimeras. 



Should the chromosomes fail to pair at the first meiotic di- 

 vision, should they not form chiasmata, or should the chiasmata 

 slip completely off the ends, the two chromosomes will not move 

 on to the equator as a bivalent, but as two separate univalents. 

 As univalents, each homologue is independent of the other, and 

 can go to either pole, and it is purely a matter of chance whether 

 one goes to one pole and the other to the opposite pole or whether 

 both go to one pole, leaving the other pole without any member 

 of that homologous pair. Thus two of the spores or gametes will 



