5<0 THE PROBLEMS OF HEREDITY AND THEIR .SOLITTTON. 



properties unlike in respect of a given character are borne ny 

 different individuals. 



In the simplest case, suppose^ a gamete from an individual present- 

 ing any character in intensity .1 unite in fertilization with another 

 from an individual presenting the same character in intensity </. For 

 brevity's sake we may call the parent individuals A and ^/ and the 

 resulting zygote Ao. What will the structure of ^\(f be in rc^gai'd to 

 the character we are considering!' 



l^p to Mendel no one proposed to answer this question in any other 

 way than by reference to the intensity of the character in the pi'ogen- 

 itors and primarily in the parents, A and c/, in whose l)odies the 

 gametes had been developed. It was well known that such a refer- 

 ence gave a very poor indication of what Aa would l)e. Both A and 

 a may come from a population consisting of individuals manifesting 

 the same character in various intensities. In the pedigree of either A 

 or a these various intensities may have occurred few or many times. 

 Common experience leads us to expect the probability in regard to 

 Aa to be influenced by this histor3^ The next step is that which 

 Galton took. He extended the reference beyond the immediate par- 

 ents of Aa to its grandparents, great-grandparents, and so on, and in 

 the cases he studied he found that from a knowledge of the intensity 

 in which the given character was manifested in each progenitor, even 

 for some few generations back, a fairly accurate prediction could be 

 made, not as to the character of any individual Aa, but as to the aver- 

 age character of ^l^'s of similar parentage in general. 



I^ut suppose that instead of individuals presenting one character in 

 differing intensities, two individuals breed together, distinguished by 

 characters which we know to be mutually exclusive, such as ^1 and Jj. 

 Here again we may speak of the individuals producing the gametes as 

 A and B and the resulting zygote as ^i/>. What will AB be like? 

 The population here again may consist of many like ^1 and like £. 

 These two forms ma}' have been breeding together indiscriminately, 

 and there may have been many or few of either type in the pedigree 

 of either A or I). 



Here again (lalton applied iiis method with remarkalile success. 

 Referring to the progenitors of A and />. detiM-niining how many of 

 each type there were in the direct pedigree of ^1 and of B, he arrived 

 at the same formula as ))efore, with the simple difference that instead 

 of expressing the ^irobable average intensity of one character in sev- 

 eral individuals, the prediction is given in terms of the prol)able mmi- 

 ))er of ^Ts and ^'s that would result on an average Avhen partieuhir 

 ^I's and B'h of known pedigree breed together. 



The law as Galton gives it is as follows: 



"It is that tiie two parents conti'ibute l)etweeii iheiii on tlie average 



