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CHAPTER 1! 



individuals that are haploid or triploid with 

 respect to either one of the two large auto- 

 somes, such individuals dying during the egg 

 stage before they can hatch as a larva. In 

 these cases, death is attributable to the genetic 

 imbalance of having the numerous genes pres- 

 ent in a long autosome in excess in individ- 

 uals triploid in this respect, or to the de- 

 ficiency of these genes in individuals haploid 

 in this respect. 



We have seen (Chapter 14) how the diploid 

 individual contains, in its two sets of chro- 

 mosomes, a proper balance of genes for the 

 determination of male and female sex. There 

 is, therefore, every reason to expect that the 

 production of other phenotypic traits is also 

 dependent upon proper genie balance. It is 

 not surprising, then, that a haploid animal 

 mated to a diploid produces very few progeny, 

 since after fertilization most zygotes are chro- 

 mosomally unbalanced by the absence of one 

 or more chromosomes needed to make two 

 complete genomes. The triploid animal 

 mated to a diploid also produces zygotes that 

 are imbalanced, but in the opposite direction, 

 having one or more chromosomes in excess 

 of two genomes. 



However, in matings with diploids, the 

 triploid animal usually produces more off- 

 spring than the haploid one. This can be 

 explained as the result of the lesser imbalance 

 wrought by the addition of chromosomes to 

 the diploid condition than by the subtraction 

 of chromosomes from it. You can visualize 

 this by comparing how far from normality 

 (diploidy) each of the two abnormal condi- 

 tions is. In the former case, in which one 

 chromosome is in excess, the abnormal chro- 

 mosome number of three is one and a half 

 times larger than the normal number of two. 

 In the latter case, in which one chromosome 

 is missing, the abnormal chromosome num- 

 ber of one is two times smaller than the nor- 

 mal number. Thus, the addition of a chro- 

 mosome makes for a less drastic change in 

 balance than does its subtraction. (Accord- 



ingly, knowing in Drosophila that the triple 

 dose of a large autosome is lethal, we would 

 have predicted the single dose would be also.) 

 Chromosome addition and subtraction can 

 also be studied in Datura^ The haploid 

 number of chromosomes here is 12. It is 

 ^ Based upon work of A. F. Blakeslee and J. Belling. 



FIGURE 18-7. Silhouettes of the capsules formed 

 by the twelve kinds of trisomic Datura. 



II 



Normal 



f f f 



Rolled Glossy Buckling 



f f f 



Elongate Echinus Cocklebur 



fit 



Microcarpic Reduced Poinsettia 

 Spinach Globe Ilex 



