NATURE OF THE GENETIC EFFECTS 371 



their size and contained genes, they too tend to cause morphological and 

 physiological abnormalities. 



Deficiencies and duplications can be formed similarly as a result of 

 breakage of two "sister" (mother and daughter) chromatids in different 

 positions, when this is followed by eucentric interchange of segments 

 between them. In this case, however, the immediately following mitosis 

 will cause one daughter cell to receive the deficient and the other the 

 duplicated chromatid, instead of both complementary combinations. 



It has been explained on p. 365, in connection with the formation and 

 loss of acentric and dicentric isochromosomes following single chromosome 

 breakage, that plant nuclei which are deficient for a portion of chromatin 

 tend to die out in passing through the stage of the male gametophyte 

 generation, since this is haploid, and recjuires the active functioning of its 

 whole set of genes, and that to a lesser extent there is a similar elimination 

 in the metabolically less active female gametophyte stage. This same 

 principle also operates to eliminate in the gametophyte stage the nuclei, 

 deficient in one or more chromosome regions, which are formed as a 

 result of translocation, or as a result of meiosis in individuals carrying 

 translocations. Duplication of regions is also more likely to be fatal in 

 this stage than in the diploid stage, since it occasions more pronounced 

 genie imbalance when in a combination which is mainly haploid than in a 

 diploid. Similarly, the aneuploid combinations resulting from other 

 types of structural changes, to be described in the next two sections, tend 

 to be eliminated in the gametophytes of plants. In animals, on the con- 

 trary, since the genes are not functioning in the gametes that carry them, 

 all aneuploid combinations succeed in being transmitted to the zygotes. 

 They may or may not then cause death or abnormality of the offspring, 

 depending upon the drasticity of the genetic departure of the zygote from 

 the normal diploid combination. So, for example, fully viable zygotes 

 can readily be formed in animals, when a mating occurs between indi- 

 viduals both of which carry the same type of translocation, by the union 

 of one gamete having one aneuploid combination with another gamete 

 having the complementary combination, whereas this could hardly 

 happen in plants. 



6. CONSEQUENCES OF TWO BREAKS IN THE SAME CHROMOSOME 



When two different breaks occur in the same chromosome, two end 

 fragments and a middle fragment are formed. Which one of these bears 

 the centromere depends on the morphology of the original chromosome 

 and the location of the breaks. If one of the end pieces bears the centro- 

 mere, and this piece unites by its broken end with the broken end of the 

 other end piece, a process called deletion, a deleted chromosome, lacking 

 the middle section, is produced. This deficient chromosome will be 



