HEREDITY 155 



times be in the direction of less valuable characters. 

 Eventually no breed of animals can be most useful until 

 the desirable characters are firmly fixed and reasonably 

 certain to be transmitted by inheritance. 



146. Mono-hybrids and di-hybrids. In all of the 

 examples so far used to illustrate the principles of segrega- 

 tion and dominance as developed by Mendel, only such 

 simple unit characters have been employed as indicate 

 clearly the law of Mendel. Such simple contrasting 

 characters are known as mono-hybrid. It is easy to 

 demonstrate the mendelian hypothesis in the case of 

 mono-hybrids. 



But among domestic animals the qualities which have 

 made organic beings useful to man are in most part a 

 combination of two or more unit characters. In such 

 cases it is far more difficult to use the mendelian formula. 

 Mendel himself determined the probable application of 

 the principle to cases where two different unit characters 

 were present. He crossed the wrinkled green peas with 

 smooth yellow peas. Manifestly, the number of com- 

 binations of characters was greater than when mono- 

 hybrids were examined. The proportion of 3 plus 1 is 

 the universal result in the FI generation where single unit 

 characters are involved. Mendel found in the case of 

 combinations of two unit characters that the mathemat- 

 ical statement (3 plus I) 2 (16) represented the true 

 result. There would appear sixteen possible zygotes as 

 a result of crossing individuals containing two unit 

 characters in each of the parents. In the offspring result- 

 ing from such crosses the characters would be combined 

 in such a manner as to produce sixteen kinds of indi- 

 viduals. 



It is apparent that if the number of combinations of 



