474 GENETICS 



usually called factors, or genes. For each character of an indi- 

 vidual, such as height, and color of hair or eyes, there are at least 

 two factors present as determiners. These pairs of factors are 

 known as allelomorphs. In many well-known experiments more 

 than two factors are concerned in the production of a character, 

 but for the purposes of this discussion the examples are selected 

 from the simplest cases. 



The Method of Cytology 



Chromosomes as Carriers of the Genes. — When Mendel's 

 work was brought to hght in 1900, it was, of course, well known 

 that, in sexually reproducing organisms, a new individual develops 

 from a zygote formed by the union of an ovum and a spermatozoon. 

 These gametes, or germ cells, carry half the number of chromo- 

 somes characteristic of the species, and the full number is restored 

 in the zygote. As the zygote divides by mitosis the chromo- 

 somes are distributed equally, both as to quantity and quality, 

 by the longitudinal halving, to the cells of the new individual. 

 The primordial germ cells of any individual contain a number of 

 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. When the reduction division of maturation occurs, the 

 chromosomes are reduced to one-half the number present in the 

 other cells of the body. This reduction does not involve the sep- 

 aration of the chromosomes that came originally from one parent 

 from those that came from the other, and the placing of the two 

 groups in separate gametes. On the contrary, the distribution is a 

 random one, with the members of each pair of chromosomes sep- 

 arating independently of other pairs. The details of the behavior 

 of the chromosomes during mitosis and maturation have been 

 stated in previous chapters (cf. pp. 137, 227, and 229). These facts 

 have been discovered by the microscopical examination of germ 

 cells, or by the methods of Cytology. Sutton, in 1902, called 

 attention to the behavior of the chromosomes as furnishing a 

 cellular mechanism for the theoretical explanation of Mendelian 

 results. Since that time the theory of the chromosomes as carriers 

 of the genes has been greatly extended, and they may be regarded 

 as the physical basis of heredity. The research of Professor Wilson 

 (Fig. 249) was important in the analysis of the numbers and types 



