680 General and Applied Biology 



In chromosomal deficiency a segment of a chromosome is missing, 

 while in chromosomal duplication an extra segment is present. The 

 duplicated segment may be inserted within a chromosome or it may be 

 attached to the exterior. Because of chromosomal aberrations, the genes 

 within the invohed chromosome can produce disturbed genetic phe- 

 nomena and ratios. In inversion a chromosome segment becomes in- 

 verted in position (changed end for end), and this may occur spon- 

 taneously and naturally or may be produced by various types of irradia- 

 tions. If a portion of one chromosome is transferred to another position 

 on the same chromosome or to another chromosome by a process that is 

 not normal crossing over (to be discussed later), this aberration is called 

 translocation. Commonly a segment of one chromosome may become 

 exchanged for a segment of a nonhomologous chromosome (not one of 

 the pair) . 



When aberrations of entire chromosomes are considered, there may be 

 one or more entire chromosomes missins^ or one or more extra chromo- 

 somes present. In either case the genes involved may have effects of 

 greater or lesser importance, depending on the type of involvement. 

 These phenomena appear to originate from an irregular cell division so 

 that two homologous chromosomes (members of a pair) become included 

 in one daughter nucleus instead of one to each daughter nucleus. This 

 abnormal behavior in which homologous chromosomes fail to separate 

 normally is called nondisjunction. This phenomenon usually occurs at 

 meiosis (maturation of germ cells) but may occur during mitosis. Many 

 plants and some animals have been found which differ from the normal 

 diploid set of chromosomes, some having only one set (haploid), while 

 others possess three or more sets (polyploidy). All of these phenomena 

 may have a corresponding effect on the traits which may develop. 



4. Genes and Genie Action. — Genes are thought to be minute, in- 

 visible, molecules of highly specific giant nucleoproteins with enzymatic 

 or catalytic properties capable of influencing structural, functional, and 

 developmental processes in cells and consequently in organisms which 

 are composed of these cells (Figs. 334 and 335). 



They are thought to act as autocatalysts, because they generate or 

 increase their own substance prior to each mitosis (cell division). In 

 other words, each gene reproduces itself during mitosis, thus forming 

 two genes which are identical. One of each pair of genes is placed in 

 each new daughter cell. The characteristics of adult organisms are 

 inherited from their parents through the medium of one or more specific 

 genes for each trait in each of the gametes (sex cells). Each gene has 



