East: As Genetics Comes of x'\e;e 



213 



6. There is a stable orderly arrange- 

 ment of genes. 



7. This arrangement is linear. The 

 genes appear to be strung together 

 much as a string of magnetized steel 

 balls. 



8. Rearrangement of genes after link- 

 age breaks is stable, orderly and linear. 

 By this it is meant to say that re- 

 arrangement of the package after 

 interchange of the contents of a pair 

 of carriers is also constant and of the 

 same linear order as in No. 6 and No. 7. 



9. A regularity of behavior has been 

 found in the interference of one crossover 

 or linkage break, with a second cross- 

 over in the same carrier, ivhich says 

 that the percent of crossovers varies 

 independently of the per cent of inter- 

 ference. The converse is also true. 



There is a fascination to this picture 

 of germplasm architecture, and the 

 type of investigation which has led to 

 these statements will undoubtedly lead 

 to still greater things in the future; 

 but we should do well to realize that 

 each of these generalizations, except 

 probably the sixth, may be a special 

 case; and that new and different special 

 cases may be found without necessitat- 

 ing a change in the first five conclusions. 



There is just one other generaliza- 

 tion to be mentioned. It is the only 

 one concerned with fertilization, and 

 upon its truth has depended the dis- 

 covery of all the others. 



10. There is no selective fertilization 

 between complementary, compatible, 

 functional gametes. 



In other words, if a series of male 

 gametes meets a series of female 

 gametes sufficiently alike to be com- 

 patible with them, fertilization takes 

 place by chance. Clearly chaos would 

 result if this were not so. Gametes 

 produced by a single hermaphroditic 

 organism may present hundreds of 

 hereditary differences. The slightest 

 tendency to selective fertilization, 

 therefore, would prevent genetic analy- 

 sis of the results. Happily, this is 

 not the case. Even in the flowering 

 plants where varying lengths of pistil 

 tissue must be traversed by the male 

 gametophyte before fertilization is 



possible, proof has been offered that 

 rapidity of passage is not affected by 

 differences in gametic constitution. 

 Genes evidently do not begin to func- 

 tion as such until the life cycle of the 

 new generation commences. 



OTHER FACTS OF HEREDITY 



There are other categories of facts, 

 several of them probably not flowing 

 out of the above conclusions, that are 

 as interesting to the general biologist 

 as the abstract laws. Without them 

 he cannot get a just idea of the actual 

 concrete working out of the heredity 

 mechanism. 



First let us speak of dominance. 

 Dominance was originally defined as 

 an observational phenomenon, the 

 appearance of the effect of one of a 

 pair of different homologous genes, as 

 opposed to the disappearance or ex- 

 clusion of the effect of the other. Later, 

 it was taken to be the presence of an 

 effect as the antithesis of its absence; 

 and this idea was carried to such 

 lengths that many geneticists came to 

 believe that dominance of "A" over 

 "a" was due to actual absence of any 

 function of "a," or even to the physical 

 absence of any gene whatsoever. Now 

 we have come to see that dominance 

 is a mere arbitrary measurement of 

 the approach of the result 'Aa" to 

 that of "AA" or "aa." This has been 

 brought about by finding cases in which 

 the effect "A" or "a" in the haploid 

 condition could be compared with the 

 effects of "AA," or "Aa" and of "aa." 

 These cases make it seem doubtful 

 whether the association of "A" with 

 "a" ever wholly inactivates the latter. 



With this conception of the function 

 of the genes in mind it is possible to 

 work out pretty definitely the actual 

 resultant ontogenetic characters after 

 different matings, both with and with- 

 out linkage, by applying the laws I 

 have just discussed. It is merely a 

 straight mathematical relationship fol- 

 lowing immediately after acceptance 

 of these basic conclusions. But there 

 are several difficulties involved in 

 identifying the concrete results of 

 breeding with the abstract results of 



