PART I. INTRODUCTORY. 

 MENDEL'S LAW OF SEGREGATION. 



Although the ratio of 3 to I in which contrasted characters reappear 

 in the second or F 2 generation is sometimes referred to as Mendel's Law 

 of Heredity, the really significant discovery of Mendel was not the 

 3 to i ratio, but the segregation of the characters (or rather, of the 

 germinal representatives of the characters) which is the underlying 

 cause of the appearance of the ratio. Mendel saw that the characters 

 with which he worked must be represented in the germ-cells by specific 

 producers (which we may call factors), and that in the fertilization of 

 an individual showing one member of a pair of contrasting characters 

 by an individual showing the other member, the factors for the two 

 characters meet in the hybrid, and that when the hybrid forms germ-cells 

 the factors segregate from each other without having been contaminated one 

 by the other. In consequence, half the germ-cells contain one member 

 of the pair and the other half the other member. When two such 

 hybrid individuals are bred together the combinations of the pure germ- 

 cells give three classes of offspring, namely, two hybrids to one of each 

 of the pure forms. Since the hybrids usually can not be distinguished 

 from one of the pure forms, the observed ratio is 3 of one kind (the 

 dominant) to I of the other kind (the recessive). 



There is another discovery that is generally included as a part of 

 Mendel's Law. We may refer to this as the assortment in the germ-cells 

 of the products of the segregation of two or more pairs of factors. If 

 assortment takes place according to chance, then definite F 2 ratios 

 result, such as 9:3:3:1 (for two pairs) and 27:9:9:9:3:3:3:1 

 (for three pairs), etc. Mendel obtained such ratios in peas, and until 

 quite recently it has been generally supposed that free assortment is 

 the rule when several pairs of characters are involved. But, as we 

 shall try to show, the emphasis that has been laid on these ratios has 

 obscured the really important part of Mendel's discovery, namely, 

 segregation; for with the discovery in 1906 of the fact of linkage the 

 ratios based on free assortment were seen to hold only for combinations 

 of certain pairs of characters, not for other combinations. But the 

 principle of segregation still holds for each pair of characters. Hence 

 segregation remains the cardinal point of Mendelism. Segregation is 

 to-day Mendel's Law. 



LINKAGE AND CHROMOSOMES. 



It has been found that when certain characters enter a cross together 

 (i. <?., from the same parent) their factors tend to pass into the same 

 gamete of the hybrid, with the result that other ratios than the 

 chance ratios described by Mendel are found in the F 2 generation. 



