BASES OF CHANGE 



the diploids is close and they are consequently common in horti- 

 culture. 



In a great many plants and a few animals triploid (sx) types occur. 

 They arise by failure of meiosis in one of their parents. Gametes 

 containing the diploid instead of the haploid number of chromo- 

 somes are produced, and by uniting with haploid mates give triploid 

 progeny (Fig. 19). A cross between diploid and tetraploid will, of 

 course, give the same result. Triploids are again more robust than 

 their parents which they otherwise resemble closely except in being, 

 as a rule, highly infertile. Triploid apples and pears, however, set 

 just enough of the ten seeds in each fruit to provide a satisfactory 

 crop without overburdening the tree and this places them amongst 

 the most valuable varieties. 



The immediate cause of the infertility of triploids is seen when we 

 examine such of their progeny as survive. For the triploid rarely, if 

 ever, produces triploid offspring. Its gametes contain all the possible 

 combinations of the extra or odd chromosomes (Fig. 28); but most 

 of these die either in the pollen or, if carried by the eggs, in the 

 embryo-sac or young embryo, and the survivors have, as a rule, only 

 one or two beyond the diploid number. These survivors are always 

 of reduced vigour and abnormal form, but their abnormalities are 

 different from those of triploids or tetraploids. They are no longer 

 general in character, but rather specific. They affect different parts 

 of the plant or animal, which consequently seems unbalanced. The 

 type of its unbalance, we find, goes with the particular extra chromo- 

 somes which it gets from the extra chromosome set of the parent. 

 In Datura stramonium and the tomato (2x = 24) there are 12 types 

 of trisomic (with 24 -|- i chromosomes) corresponding to each of 

 the 12 chromosomes in the haploid set {cf. Fig. 67). The trisomies 

 stand between the diploid and the triploid in fertility. 



Similar off-types also come directly from the diploid in many 

 plants and animals. In Drosophila melanogaster, triploid flies 

 occasionally appear amongst the diploid males and females. And also 

 some with the small fourth chromosome represented once or three 

 times instead of twice. Such monosomic and trisomic flies are again 

 distinct from the disomic type. They result, presumably, from the 

 failure of the fourth chromosome to pair at meiosis, in which case 

 the two partners behave independently and may pass to the same 



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