282 FOUNDATIONS OF BIOLOGY 



the two genes are not identical (e.g., S and s), then one, the 

 dominant (S), is expressed in the soma (the plant is tall), 

 while the other, the recessive (s), is not expressed. The indi- 

 vidual is heterozygous with respect to the character in ques- 

 tion (e.g., size). 



At the maturation of the germ cells of the individual, an in- 

 dependent assortment, or segregation, of the genes occurs so 

 that the gametes bear only one gene (e.g., either S or s) for 

 each unit character. Thus the gametes of homozygous indi- 

 viduals are all alike with respect to the gene in question (e.g., 

 all bear S or s) , while the gametes of heterozygous individuals 

 are of two numerically equal classes (e.g., half bear S and the 

 other half bear s). 



UNIT CHARACTERS. From the standpoint of heredity an 

 individual organism may be regarded as comprising a com- 

 plex of single characters, each of which, broadly speaking, 

 behaves essentially as a unit. 



DOMINANCE. When the determining genes (allelomorphs) 

 for each of a pair of alternative characters are present in the 

 zygote, one (the dominant) is expressed in the resulting indi- 

 vidual; although the other (the recessive) is also present in all 

 of its somatic and in one half of its mature germ cells. In 

 other words, the recessive is not expressed unless it is present 

 in duplicate. 



SEGREGATION. The genes for each of a pair of alternative 

 characters are never both present in the same gamete. There- 

 fore the ripe germ cells of hybrids fall into two numerically 

 equal classes : in one the gene of the dominant character and 

 in the other the gene of the recessive character is segregated. 

 This is the so-called purity of the germ cells. 



D. NEO-MENDELISM 

 It so happens that, as data accumulate, it becomes more 



