72 GENETICS IN RELATION TO AGRICULTURE 



character rather than on the factor as is today the case; and we now have 

 numerous examples of characters which behave as units in certain con- 

 trasts, but in others behave as compound characters. It is, therefore, 

 questionable whether in a rigid sense there are any such things as unit 

 characters, but the term has been much used in Mendelian literature, 

 and the conception to which it gives rise, namely that particular indi- 

 viduals or races possess a number of unit characters which may be dis- 

 sociated from them and recombined in various fashions with the unit 

 characters of related individuals or races, is a useful one and is strictly 

 in accordance with experimental results. 



Allelomorphs are contrasted factors or characters. More rigidly as 

 applied to characters, an allelomorph is one of a pair of characters which 

 display alternative inheritance, i.e., inheritance in which one or both of 

 the contrasted characters, although obscured, retain their identity and 

 emerge unchanged from the hybrid. With respect to factors allelo- 

 morphism is a relation between two factors such that they are sepa- 

 rated into sister gametes in germ-cell formation; they never both enter 

 the same gamete. The allelomorphic characters in our sample are 

 characters tallness and dwarfness, and correspondingly the factors T and 

 t are allelomorphs. 



The genotype is the constitution of an organism with respect to the 

 factors of which it is made up. Rigidly the genotype is the sum total of 

 genes or factors of an individual, but it is customary to speak of the sum 

 total of analyzed factors which are under immediate consideration as the 

 genotype. The genotype of the tall race of peas in the above experiment 

 was TT, of the dwarf race it. The factor arrangement of an individual 

 is also called its genetic constitution when a particular set of factors are 

 concerned and this term is also employed to designate a particular set 

 of factors carried by a gamete. Genotypes of the constitution TT or tt, 

 or in general those which receive the same factors from both gametes 

 are homozygous, whereas those which receive different factors from the 

 two germ cells or gametes are heterozygous, as for example plants of 

 the genetic constitution Tt. Similarly an individual contains a duplex 

 dose of a given factor when it receives that factor from both parents, or 

 a simplex dose if the factor comes in in only one of the germ cells. The 

 substantives corresponding to the adjectives homozygous and hetero- 

 zygous are homozygote and heterozygote, respectively. 



The phenotype is the aggregate of the externally obvious characters of 

 an individual or a group of individuals. Thus in the second generation of 

 the above experiment there were two phenotypes, tall and dwarf, and all 

 the second generation plants belonged to one or the other of these classes. 

 Moreover all members of a phenotype do not necessarily possess the 

 same genetic constitution. In the above example the tall phenotype 



