HEREDITY 



somes which we may designate by the letters A and a and 

 B and b, the resulting daughter cells may be AB and ah, or 

 they may be Ah and aB. 



It is not difficult to see that this arrangement provides 

 the physical basis of heredity which the pedigree culture 

 investigations found to be necessary. It must be assumed 

 that each chromosome carries numerous genes which act 

 as determiners of the various traits which will characterize 

 the mature individual. Each organism possesses two com- 

 plete sets of these determiners because it possesses two 

 complete sets of chromosomes. We know that each chromo- 

 some set is complete because certain plants and animals, 

 with proper treatment, can be induced to develop when 

 but one set is present. One set comes from the father and 

 the other set from the mother, as can be demonstrated by 

 crossing two species where the chromosomes of each set 

 differ in size. At maturity, this creature will produce 

 either eggs or sperms, and its germ cells will possess but one 

 complete set of chromosomes. Any given egg or sperm will 

 have one representative of the first pair of chromosomes, 

 and it will be a matter of mere chance whether it came from 

 the father or from the mother; similarly, it will have one 

 representative of the second pair of chromosomes, which 

 also may have originated with either the father or the 

 mother; and so on throughout the whole series. There is, 

 then, an orderly means by which the genes from the two 

 parents pass to the germ cells which are to function in 

 producing the new generation; and, generally speaking, 

 this process is one by which any given germ cell receives 

 one, and only one, gene-packed chromosome from pairs of 

 such chromosomes which have come from the maternal 

 and from the paternal side of the house. 



Obviously with such a mechanism for passing genes 

 from one generation to another, the number of chromosome 

 pairs possessed bv the species in question must represent 



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