212 



The Journal of Heredity 



delimit the characteristics of atoms and 

 molecules. Presumably there may be 

 other types of inheritance, but the 

 only one thus far described is an ex- 

 clusively maternal inheritance of cer- 

 tain plastid characters. And even in 

 this case, it is not absolutely certain 

 we are not dealing with symbiotic 

 organisms that are transferred from 

 host to host in some such way as the 

 numerous more or less yeast-like forms 

 being daily described in relation to 

 insects and other lower animals. The 

 mere fact that numerous dicotyledons 

 and monocotyledons on the one hand, 

 and mammals, birds, amphibians, rep- 

 tiles, fishes, arthropods, and molluscs 

 on the other, show essentially iden- 

 tical types of heredity, makes it 

 probable that the generalized mechan- 

 ism has been discovered. At the same 

 time, though the angiosperms, insects, 

 and mammals thus far studied inten- 

 sively, distribute their units of heredity 

 with a convincing similarity of detail, 

 it is altogether likely that special cases 

 of peculiar distributions will be found 

 later. And there is every reason to 

 believe that these odd or unique types 

 of inheritance will parallel a specializa- 

 tion in chromosome distribution de- 

 parting somewhat from the one we 

 have come to look upon as regular. 



THE LAWS OF HEREDITY 



This regular or common scheme of 

 chromosome distribution in sexual re- 

 production has been the basis of 

 practically all of our present genetic 

 knowledge. From past experience 

 with it one can pick out the following 

 inductions, each of which has been 

 tested with variable degrees of thor- 

 oughness. They are not set down 

 here with the idea they are necessarily 

 more inviolable than the laws of the 

 Medes and Persians, but merely with 

 the annotation that no experimental 

 data have thus far overstepped them 

 except those on the plastids. They 

 may be called Provisional Laws of 

 Heredity. 



The first five of these generalizations 

 describe the mechanism of heredity 



in the preparation of the germ cells for 

 fertilization. 



1. There is segregation of paternal 

 genes from maternal genes, each un- 

 changed by the association. The pedi- 

 gree culture evidence supplements the 

 cytological evidence in favour of the 

 idea that this segregation takes place 

 at the reduction division of the chro- 

 mosomes in the maturation of the germ 

 cells, when homologous paternal and 

 maternal genes pair and separate, one 

 of each pair passing to each of the two 

 daughter cells. 



2. There may be any combination of 

 the choice of one out of each pair of genes 

 in making up the genetic constitution of 

 the gametes. 



3. In transmission to the two daughter 

 cells, certain sets of genes are always 

 manipulated independently of all other 

 genes. This is a statement of inde- 

 pendent inheritance, or rather of inde- 

 pendent genetic recombination, with- 

 out reference to the chromosomes. It 

 is an unworthy piece of quibbling, 

 however, not to accept the simple 

 indication of cytology that this law 

 is the result of the operation of the 

 chromosomes acting as gene carriers. 

 It results from the fact that No. 1 

 pair of chromosomes, no matter how 

 it may be packed with genes, carts its 

 cargo independently of all other pairs. 



4. The manipulation of genes within a 

 given series is always dependent. This 

 is the phenomenon of linkage, or 

 association in inheritance. Concretely, 

 we assume that the genes packed with- 

 in each freight carrier pair, an homo- 

 logous pair of chromosomes, are 

 mutually dependent to various degrees 

 in their recombinations with each 

 other. 



5. The number of dependent series, or 

 linkage groups is limited. In the fruit 

 fly, the number corresponds to the 

 number of chromosomes, and the 

 presumption is that this is the case in 

 every species. 



Added to these five laws are four 

 more generalizations which refer par- 

 ticularly to the architecture of the germ 

 plasm within the carrier. 



