HEREDITY AND VARIATION 



behavior of the genes can be understood best if considered in connection with 

 that of the chromosomes. 



The Method of Cytology 



Chromosomes as' Carriers of the Genes. When Mendel's work was 

 brought to Hght in 1900, it was well known that new individuals developed 

 from zygotes formed by the union of ova and spermatozoa. These gametes, 

 or mature germ cells, carry the haploid number of chromosomes that is char- 

 acteristic of the species; the diploid number is restored in the zygote (pp. 

 136 and 143). As mitosis occurs during development of the zygote, the chromo- 

 somes are distributed equally to all the cells of the new animal (p. 43). 

 The primordial germ cells of any individual contain chromosomes that can be 

 grouped in pairs of similar size and shape. One member of each pair is of 

 paternal and the other of maternal origin (see Fig. 5.7, p. 135). When the 

 first meiotic division occurs, homologous chromosomes disjoin and pass into 

 different cells. This disjunction does not involve the separation of the sets 

 of chromosomes that came from the two parents at the time of fertilization. 

 On the contrary, the distribution of homologous chromosomes is random, with 

 the members of each pair of chromosomes separating independently. These 

 facts were discovered by the microscopical examination of germ cells by the 

 methods of cytology. In 1902 W. S. Sutton called attention to the behavior 

 of the chromosomes as furnishing a cellular mechanism for the explanation of 

 Mendel's results. Since that time the theory of the chromosomes as carriers 

 of the genes has been greatly extended, and chromosomes are now considered 

 to be the physical basis of heredity. The researches of E. B. Wilson (Fig. 

 6.8) and Nettie M. Stevens in 1905-1906 were important in the analysis of 

 the numbers and types of chromosomes in male and female animals; the work 

 of Eleanor E. Carothers in 1917 furnished evidence for the independent assort- 

 ment of homologous chromosomes during the meiotic divisions. 



If the example of monohybridization between a gray and a white mouse is 

 analyzed according to the concept that the genes are located in the 

 chromosomes, the assumption is made that the zygote from which the gray 

 mouse developed contained two genes for grayness, one from each of its 

 parents, whereas the zygote from which the white mouse developed contained 

 two genes for whiteness, one from each of its parents (Fig. 6.11). Each of 

 these genes is regarded as being located in a separate chromosome, but the 

 two genes of each animal are present in homologous chromosomes. When 

 separation of the homologous chromosomes occurs at the disjunctional divi- 

 sion during meiosis, the genes are carried into diflferent gametes. All the 

 gametes of the gray parent possess a single gene for gray coat color, and 

 those of the white parent a single gene for white coat color. When fertiliza- 

 tion occurs, each zygote obtains two genes for coat color, but one is for 

 whiteness and one for grayness; the allelomorphic genes are now together in 



185 



