THE FUNDAMENTALS OF MENDELISM 3I 



be three tails (of which one will be pure breeding TT and two Tt like 

 the Fi) to one pure breeding tt short. (Fig. i.) 



Faaors of the kind postulated above are called MendeUan factors or 

 genes. A collection of genes alternative to one another, so that normally 

 a gamete contains only one of the set, is spoken of as a series of allelo- 

 morphic factors (the position they occupy is their "locus"); there may be 

 more than two members of such a series, for instance, there might have 

 been another alternative "dwarf" for Mendel's peas. Zygotes which 

 contain two similar allelomorphs are said to be homozygous for the 

 gene in question (e.g. the pure breeding TT or tt\ while if the two 



P1. TALL n X SHORT tt 



(germ cells all T) 



(germ cells all t) 



FI. TALL It 



(germ cells equal numbers of T and t) 



I (seifed) 



F2. 1 TALL 77 : 2 TALL Tt 1 SHORT tt 



(germ cells 7) (germ cells 7 and t) (germ cells t) 



(seifed) I (seifed) 



(seifed) 



I I I 



F3. All 77 1 77 : 2 7t : 1 tt All tt 



Fig. 1. Mendel's experiment of crossing tall and short peas. 



allelomorphs are dissimilar, as in the Fi plants, the organism is hetero- 

 zygous. If, in a heterozygote, only one of the two allelomorphs has an 

 effect on the character of the organism, that allelomorph is dominant 

 over the other, which is recessive; dominance and recessiveness may be 

 partial, when the heterozygote will show some effect of both genes, 

 neither of which completely suppresses the other. 



The fundamental points of the hypothesis developed above, which 

 is known as Mendel's first law, are (i) that there are factors which 

 affect development and that these factors or genes retain their indivi- 

 duaUty from generation to generation and do not become contaminated 

 when they are mixed in a hybrid, and (2) that they become sorted out 

 from one another when the gametes are formed. 



A Note on Symbols. — There are several different systems in use for sym- 

 bolizing genes. The simplest refers to a locus by a certain letter, and indicates the 

 dominant by the capital, the recessive by the lower case letter (e.g. Aa for the 

 heterozygote). If there are more than two allelomorphs, they can be indicated 

 by small superscript letters (e.g. c*, cr, ck, etc., for the albino series in rodents). 



