244 HEREDITY AND ENVIRONMENT 



and one pure recessive (the ones in the upper 

 left and lower right corners of Fig. 54) . 



When the parents differ in one character 

 only, the offspring formed by their crossing 

 are called monohybrids, when there are two 

 contrasting characters in the parents the off- 

 spring are dihybrids, when three, trihybrids, 

 and when the parents differ in more than three 

 characters the offspring are called polyhybrids. 

 There are certainly few cases in which parents 

 actually differ in only a single character, but 

 since each contrasting character may be dealt 

 with separately, as if it were the only one, and 

 since the number of types of offspring in- 

 creases greatly when more than one or two 

 characters are considered at the same time, it 

 is customary to deal simultaneously with only 

 one or two characters of hybrids, even though 

 the parents may have differed in many 

 characters. 



3. Inheritance Formulae. — Mendel repre- 

 sented the hereditary constitution of the plants 

 used in his experiments by letters employed 

 as symbols, dominant characters being repre- 

 sented by capitals and recessives by small let- 



