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, 7.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 7 and 
t are allelomorphs. 
The genotype is the constitution of an organism with respect to the 
factors of which it is madeup. 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 tt. 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 
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