NOTES AND LITERATURE 



The Nature of "Unit" Characters One of the clearest pre- 

 stations of Mendelian principles that has appeared recently 

 i that of Dr. E. Baur in Beihefte zur Medizinischen KIM, 

 r ol. 4, 1908, pp. 266 et seq. He has given special at- 

 sntion to inheritance in crosses between the various varieties 

 f Aiifinlmnnn nni pis. Me states that he knows 250 dis- 



lo not obey Mendel's law. He has 

 characters for the species, which 

 is more than the number of chromosomes present, from which 

 fact he concludes that the chromosome 'as a whole can not be 

 considered as the basis of Mendelian unit characters. Others 

 have cited as a basis for the same belief that more character 

 pairs are known in Pisum than there are chromosomes present 

 in the cells. This conclusion is not a necessary one, as is seen in 

 the following. Speaking in a general way, the chromosomes are 

 present in pairs of homologues. For each pair in the cells of a 

 given individual there is a homologous pair in the cells of other 

 individuals of the species. For convenience we may designate 

 one of these pairs as A chromosomes, a second as B chromosomes, 

 etc., the same pairs as a rule being found in different individuals 

 of the same species. 



If we consider the species as a whole, the number of pairs of A 

 chromosomes is equal to the whole number of cells in all the 

 individuals of the species. It is conceivable that, since certain 

 of these A chromosomes may trace back thousands of generations 

 before their ancestral lines unite, it is possible that there may be 

 an indefinite number of subgroups of A chromosomes in the 

 species, and that each subgroup may represent a heritable dif- 

 ference from other groups. But each of these subgroups would 

 represent a Mendelian character. Hence, there might possibly 

 be an indefinite number of Mendelian character pairs in a species 

 having only a single pair of chromosomes. 



But these characters could not all exist in one individual. 

 Only two of the subgroups could be present together. Hence, 

 in such a species, only two independent (not correlated) domi- 

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