v] MENDELIAN HEREDITY 69 



the pure short race produces only t, so the offspring 

 of the hybrid and the original short should give equal 

 numbers of hybrid tails and pure shorts. Similarly 

 the hybrid zygote crossed with the pure tall should 

 give equal numbers of pure tails {TT) and hybrid 

 tails {Tt), Mendel found that this expectation was 

 in fact verified by experiment. The whole series 

 may be made clearer by a diagram (p. 60), in which 

 the zygotes are represented by squares, the gametes 

 by circles. 



The middle part of this diagram represents .the 

 production of the Fi zygote and its ofispring when 

 self-fertilised, producing equal numbers of T and t 

 gametes (four of each being represented) and thus 

 giving offspring in the ratio of \TT, 2Tt, lit; the 

 sides of the diagram represent the results of crossing 

 back the Fi zygote with the parental types TT and tt. 



At this point it is necessary to explain certain 

 convenient technical terms introduced by Bateson. 

 It has already been mentioned that a pair of alter- 

 native characters which segregate in the gametes, as 

 described, are called allelomorphs. When an indi- 

 vidual is produced by two gametes bearing different 

 allelomorphs, so that it contains both members of 

 a pair, it is called a ' heterozygote,' or is said to be 

 * heterozygous ' in respect of the character considered, 

 e.g., an individual of constitution Tt is heterozygous 

 in respect of talluess. If it contains only one kind 



