3i6 CHROMOSOMES IN HEREDITY: PHYSIOLOGICAL 



He therefore properly inferred that the chromosomes of the com- 

 plement were qualitatively different. 



The same conclusion may be derived from recent experiments of 

 various kinds, some of which carry us much further. 



(a) MoNOSOMics. The first kind of evidence is derived from 

 organisms which lack either a part or a whole of one chromosome 

 of their diploid complement. The first are said to have a sectional 

 deficiency ; they are deficiency-heterozygotes. The second are said 

 to be " monosomic ; they are numerical hybrids." 



Such organisms have arisen in all the conceivable ways, as 

 follows : — 



1. Through loss of one chromosome in a somatic mitosis of a 

 diploid, e.g., Datura Stramonium, Blakeslee and Belling, 1924 ; 

 Zea Mays, McClintock, 1929 b ; GEnothera, Michaelis, 1926. 



2. Through parthenogenetic development of an " unreduced " egg 

 (Ch. XI) which has lost one chromosome at meiosis, e.g., GEnothera 

 Lamarckiana in a cross with CE. longiflora. Stomps, 1931. 



3. Through functioning of a deficient egg cell (x — 1) in an 

 animal [e.g., Drosophila melanogaster giving the haplo-IV type, 

 Fig. 121) or in a plant [e.g., Nicotiana alata, Avery, 1929 ; Goodspeed 

 and Avery, 1929 a). This is also possible in regard to a segment, 

 as in Matthiola heterozygous for lack of a trabant (Philp and 

 Huskins, 1931) which transmits the deficient chromosome through 

 the eggs but not through the pollen, and consequently, when selfed, 

 yields 50 per cent, of offspring heterozygous for the deficiency and 

 none homozygous. 



4. Through functioning of a deficient male gamete (;t: — i) in an 

 animal, e.g., Drosophila melanogaster, whose sperm will function 

 lacking an X chromosome (Kuhn, 1929 h). In special circumstances 

 this can occur in plants. Thus Stadler (1931) obtained 2x — 1 

 plants by X-raying pollen of Zea Mays. The x — i pollen functioned 

 because no normal uninjured pollen grains were left to compete 

 with it. Even pollen deficient for a small segment of a chromosome 

 fails in competition with normal pollen in Zea, although embryo- 

 sacs of this kind will develop (Stadler, 1932). As a rule, where it is 

 known to be regularly formed at meiosis (as in ring-forming Rhoeo 

 and GEnothera ; cf. D., 1929 c, 1931 h), x — i pollen never reaches 



