§4 AN INTRODUCTION TO MODERN GENETICS 



12 and all 12 trisomic types are known. ^ In the tomato, also, with a 

 haploid number of 12, 11 primary trisomies have been described,^ and 

 many other nearly as complete series are known, e.g. in Matthiola, 

 Crepis, Zea. The simplest reduplication of a chromosome gives a 

 primary trisomic; redupUcations combined with various types of 

 fragmentation and interchange give the secondary and tertiary types 

 (p. 108). The phenotypic appearance of each trisomic is characteristic 

 and shows the effect of an extra dose of the assemblage of genes in the 

 reduplicated chromosomes (p. 170). 



4. Segregation in Trisomies: Secondary Non-disjunction 



The three similar chromosomes in a trisomic may all be associated 

 as a trivalent at meiotic metaphase, as is usually the case in Datura, or 

 in a certain percentage of cases the association may be only of two 

 members, the other chromosome being free. In this respect the tri- 

 somic behaves exactly like a triploid, and follows the general rule for 

 polyploid pairing (p. 68). Segregation will give various proportions of 

 gametes with and without a dupHcated chromosome. The gamete 

 ratio for a factor A present "simplex" Aaa will be lA : 2Aa : 2a : laa 

 if there is no preferential pairing; the ratio may also be modified by 

 chromatid segregation (p. 103). 



The zygotic ratios are not immediately calculable from the gametic 

 ratios because the presence of the extra chromosome usually has a very 

 depressing effect on the viabiHty both of the gametes and of the zygotes 

 in which it is contained. In animals the effect is mainly on the viability 

 of the trisomic zygotes, the gametes being apparently little affected, but 

 in plants the trisomic chromosomes, although transmitted almost in the 

 expected frequency through the ovules, may be eliminated nearly 

 completely in the pollen, which even if viable may grow too slowly to 

 be effective in competition with pollen tubes containing normal 

 nuclei. 



5. Chromosome Mosaics 



Non-disjunction in the strict sense can only take place at meiosis, 

 but somewhat similar abnormaUties of chromosome separation can 

 occur in mitoses, one of the sister halves of a chromosome failing to 

 move away from the metaphase plate and thus not being included in 

 either of the daughter nuclei. This elimination of a chromosome gives 

 rise to a patch of tissue which has less than the normal quantity of (is 



^ Rev. Blakeslee 1928, 1934. . - Lesley 1928, 1932. 



