510 Introduction to the Study of Science 



parents which had similar unit characters. So the color of the 

 eyes of an individual is pure when it is derived from a line of 

 unmixed unit characters, either brown or blue. An individual 

 is, on the other hand, said to be cross-bred or hybrid in respect to 

 a certain character, when this character is inherited from 

 parents having different unit characters of the kind and is 

 capable of being transmitted to offspring. Thus the eye color 

 of an individual is hybrid when it is derived from parents one 

 of whom possesses the unit character brown while the other 

 possesses the unit character blue. The offspring of parents, 

 one of whom has pure brown eyes and the other pure blue eyes, 

 will all have hybrid brown eyes, as brown color in eyes is domi- 

 nant. But they are capable of transmitting both characters to 

 their progeny. Compare this with the scheme, showing the 

 first and succeeding generations of cross-fertilized tall and short 

 peas. 



There are several possible combinations with fairly well- 

 known results, of which two may be suggested. The offspring 

 having hybrid brown eyes may mate with individuals having 

 pure blue eyes; their children will be about equally divided 

 into two groups, the one having hybrid brown eyes and the 

 other blue eyes. Or the offspring having hybrid brown eyes 

 may mate with individuals having hybrid brown eyes. In 

 this case the distribution of eye color of their offspring would 

 correspond with the distribution of the third generation of the 

 crossed tall and short peas, that is, there would be a ratio of 

 about three having brown eyes to one having blue eyes. Of 

 the three having brown eyes, one will have pure brown eyes, 

 and two will have hybrid brown eyes. The pure brown eyes 

 mated with pure brown eyes will transmit true to type, just as 

 the pure tall peas repeat in reproduction. The hybrid brown 

 eyes will follow the rule for combinations with pure or hybrid 

 brown, or with pure or hybrid blue. When opportunity offers, 

 observe this in the case of parents, children, and grandchildren. 

 It must be remembered, however, that human beings are very 



