5()4 THE TEOBLEMS OF HEREDITY ANJ) THEIR SOLUTION. 



could not he at once referred to one or otlierol' th(^ parental tonus were 

 not found. 



In the case of each pair of characters there is thus one which in the 

 first cross prevails to the exchision of the other. This prevailing 

 character Mendel calls the dominant cliaiactiM-, the other heino- the 

 recessive character." 



That the existence of such •"dominant" and *' r(>cessiv(> " characters 

 is a frequent i^henomenon in cross hi-eedin*^-, is well known to all who 

 have attended to these subjects. 



By letting the crossbreds fertilize themselves Mendel next raised 

 another generation. In this generation were individuals which showed 

 the dominant character, but also individuals which presented the 

 recessiv(> character. Such ii fact also was known in a good many 

 instances. Rut Mendel discovered that in this generation the numer- 

 ical pi'opoi'tion of dominants to recessives is on an average of cases 

 a])proximately constant, l)eing in fact as three to one. AN'ith very 

 considerable regularit}' these numbers were ap})r()ached in the case of 

 each of his pairs of chai'acters. 



There are thus in the first generation raised from the crosshi'cds 75 

 per cent dominants and 25 per cent recessives. 



These plants were again self-fertilized, and the offspring of each 

 plant separately sown. It next appeai'cd that the ofl'sjning of the 

 recessives remained pu,re recessive, and in subse(|uent genei-ations 

 never produced the dominant again. 



But Avh(Mi the seeds obtained ])y self-f(M-tilizing the dominants were 

 examinetl and sown it was found that the dominants were not ail alike, 

 but consisted of two classes, (1) those which gav(> rise to pui(> domi- 

 nants, and (2) others which gave a mixed otfspring, compos(>d ])artly 

 of recessives, ])artly of dominants. Here also it was found that the 

 average mun(M-ical proportions were* constant, those with pui-e domi- 

 nant oH'spring being to those with mixed oll'spring as one to two. 

 Hence it is seen that the 75 per cent dominants ar(» not really of simi- 

 lar constitution, but consist of twenty-tive which nvv \m\v dominants 

 and fifty which are really crossbreds, though, like the cr<)ssl)r(Hls raised 

 by crossing th(^ two original varieties, they only exhibit the dominant 

 character. 



To resume, then, it was found that by self-fertilizing the original 

 crossbreds the same proportion was always ai)i)roached, namely, 25 

 dominants, 50 crossbreds, 25 recessives, or l/> : 2/>7? : 1^. 



Like the pure recessives, the pure dominants are thencefortli i)ui'e, 

 and only give rise to dominants in all succeeding generations studied. 



On the contrary the fifty crossbreds, as stated al)ove, ha\-e mixed 

 oflspring. But these otl'spring, again, in their muuerical proportions, 



« Note that by these novel terms the complications involved by use of the expres- 

 sion " j)r(>potent" are avoided. 



