GENERAL ZOOLOGY 



Scale 



Fig. 6.19. Chromosomes of Drosopkda. A, equatorial plate from an oogonium; chromosome 

 pair IV is seen in the center of the group. B, chromosome pair IV from a salivary gland cell; 

 members of each pair of chromosomes are closely associated, greatly increased in size, and exhibit 

 conspicuous banding. (From C. B. Bridges, 1935, Journal of Heredity, vol. 26.) 



hereditary factors, or genes. It is known that many genes are found in a 

 single chromosome and that they are arranged in a row along the length of 

 the gene string. In the cells of the salivary gland of Drosophila the chromo- 

 somes are very large because of several reduplications of the genonemata with- 

 out nuclear division (Fig. 6.19). The bands on these chromosomes are 

 characteristic and constant in position on the members of a pair which are 

 closely associated side by side. When certain genes are found to be missing 

 in breeding experiments, a cytological examination of these banded chromo- 

 somes reveals that certain bands are absent (Fig. 6.20); that is, the genes 

 of a linkage group can be considered to occupy definite positions in a specific 

 linear order on a pair of homologous chromosomes. 



Genes are distributed equally by the mechanism of mitosis to all the germ 

 cells derived from a zygote. The mechanism of disjunction and independent 

 assortment of the pairs of homologous chromosomes, during maturation of the 

 germ cells, and the possibilities of random combination of the germ cells 

 furnish a cellular basis by means of which Mendelian ratios can be explained. 



The Method of Experimental Embryology 



Experimental breeding is usually carried on under the conditions most 

 favorable for the organisms concerned. The results obtained, therefore, are 

 comparable with what might be expected to occur in uncontrolled breeding 



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