MENDEL'S LAW 



23 



during the maturation divisions. The theory is usually known as the chromosome theory 

 of development and heredity. 



It is further of interest that when the process as defined by this theory is analysed, it has sug- 

 gestive relations to Mendel's ' law of heredity.' This law may be very briefly alluded to here 

 because of its relation to the cytological data. It may be represented in terms of a single pair 

 df hereditary qualities thus : 



A + B 



in which A and B represent single and distinctive qualities one being dominant, the other 

 recessive of two individuals uniting in a cross, AB. In the cross one quality, A, alone 

 manifests itself (the dominant] ; the other, 

 B, is latent (the recessive], so that the off- 

 spring of the cross appears as pure A. In 

 the next generation, no new cross in 

 respect of these qualities being effected, 

 the offspring appear in the proportion 

 of three individuals with the A quality 

 to one with the B quality. The B indi- 

 viduals now breed pure in all succeeding 

 generations, and some A individuals do 

 the same ; but a second set of A's are 

 really AB'S, and they in the next genera- 

 tion split up again into pure A's, AB'S 

 (appearing as A's) and pure B's, and so on. 

 The theory involves as a corollary 

 the purity of the gametes in respect of 

 the qualities, and this purity would be 

 attained by just such a process as is 

 assumed by the chromosome theory to 

 take place in maturation. Thus, if two 

 AB's cross, the gametes being pure in 

 respect of the qualities will be either A 1 

 or B 1 , A 2 or B 2 . Four combinations are 

 possible between these gametes viz. 



A'A-, A'B 2 , A-B 1 , B'B 2 , giving rise to three classes of individuals, pure A's, mixed AB'S, and pure 

 B's. The mixed individuals, however, always appear as A's, that quality being dominant and B 

 recessive, so that there are three A's to one B, as expressed in the ' law.' In higher forms, 

 all the individual gametes of the four groups are not functional, the three polar cells being 

 abortive ova, so that the formula requires a slightly different statement. The ovum in respect 

 of two qualities may be either A or B 



(1) A + A' or B' = AA' or B'A; or 



(2) B + A' or B' = BA' or BB'. 



In a sufficiently large progeny from a single pair the expectation would still be the same 

 viz. one pure A, two mixed AB'S appearing as A's and one pure B. ' 



1 For some interesting human cases see Batesoii, Brit. Med. Journal, July 14, 1906. The reader 

 must be referred for further information and for criticism of this theory, as well as for a statement of 

 other doctrines of heredity, to special treatises on the subject. For the cytological data, see more 

 especially Boveri, Ergebnisse iiber die Konstitution der chromatischen Kernsubstanz, Jena, 1903 ; 

 Sutton, ' Chromosomes in Heredity,' Biol. Bull., April 1903. For a statement of Mendel's law, see 

 Bateson, Mendel's Principles of Heredity, &c. (Cambridge University Press, 1902). The literature is 

 fully reviewed in Schwalbe's Jahresberichte of recent years. For a criticism from the cytological side, 

 a paper by R. Fick, Arch. Anat. und Physiol. Anat. Abt. 1905, may be mentioned. 



FlG. 32. FlBST POLAB SPINDLE (METAPHASE OF 



HETEBOTYPE), EGG OF MOUSE. (Sobotta.) 



