DEFICIENCY AND BALANCE 



deficient chromosome passes through meiosis and reaches the haploid 

 generation it may still prove fatal. The danger is greater when this 

 generation has to undergo several cell divisions: thus, as we shall see 

 in detail in the next chapter, the pollen is more sensitive than the 

 embryo-sacs, and both are more sensitive than the sperm in animals. 

 The haploid egg nucleus in animals, since it is immediately fertiHzed, 

 is never expo'sed to any trial of sensitivity. Sperm of Drosophila 

 lacking whole chromosomes, even the large autosomes, can fertilize 

 eggs. But pollen grains of lilies so lacking will never even pass 

 through their vegetative mitosis. And, even where deficient gametes 

 survive, a diploid homozygous for their deficiency cannot do so. 

 Evidently, as we might expect, nearly all the genes are indispensable 

 for the life of the cell in one dose and are most efficient in tw^o doses. 

 The loss of a small part of a chromosome, and even of a whole 

 chromosome, must be of some importance in the origin of natural 

 variations as the sex chromosome differences show. The Y chromo- 

 some of mammals is little more than a deficient X. The male is 

 therefore a deficiency heterozygote. The situation is even clearer 

 in those insects and spiders with no Y chromosome at all. With 

 them the male is heterozygous for the deficiency of the whole X. 

 It is monosomic or XO. 



Heterozygous deficiencies and duplications are, obviously, 

 for segments what monosomies and trisomies are for w4iole 

 chromosomes. Corresponding to the homozygous deficiencies 

 and duplications are nullisomics (zx — 2) and tetrasomics 

 (2X+ 2). Nullisomics cannot survive in diploids, but they appear 

 in analogous forms in polyploids {e.g. 6x — 2). Tetrasomics in 

 Datura can survive, but are even poorer and more abnormal than the 

 corresponding trisomies. The homozygous duplication, in Drosophila 

 and maize, bears a similar relation to the heterozygote. 



Two laws of gene balance emerge from these considerations. 

 First, the larger the proportion of the chromosome outfit, up to 

 a half, which departs from the numerical proportions of the rest, 

 the greater the disturbance of development. A trisomic for two 

 chromosomes, or double trisomic, is more abnormal than a single 

 trisomic — except in Crepis capillaris (x = 3) where 2x+ i + i = 

 3x— I. Secondly, the larger the departure in proportion of the 

 unbalanced element the greater the disturbance. A monosomic or 



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