

MENDELISM 105 



The word "genotype" was suggested by Jbhannsen 

 in honor of Darwin and his theory of pangenesis, al- 

 though there are certain objections to its use in this 

 connection for the reason that systematists have 

 already appropriated it in a different sense. As here 

 used it signifies "the fundamental hereditary constitu- 

 tion or combination of genes of an organism" (Shull). 



8. THE IDENTIFICATION OF A HETEROZYGOTE 



"Homozygote" and "heterozygote" are terms then 

 descriptive solely of the genotypical constitution of 

 organisms, and, as has been said, it is not always pos- 

 sible to distinguish one from the other by inspection. 

 The only sure way to identify a heterozygote is by 

 breeding to a recessive and observing the kind of off- 

 spring produced. 



Peas of the formulae TT and T(t) 9 for example, 

 both look alike, since a single determiner for the tall 

 character, T, is sufficient to produce complete tallness. 

 When, however, these two kinds of tall peas are each 

 bred to a recessive dwarf pea, of the formula tt, the 

 progeny will differ distinctly in the two cases as fol- 

 lows : 



Case I. T + T X t + t = 100 per cent T(t). 



Case II. T t X * + * = 50 per cent T(t) + $0 per cent tt. - 



That is, if the dominant to be tested is homozygous 

 (Case I), the entire progeny will exhibit the dominant 

 character, but if the dominant to be tested is heterozy- 

 gous (Case II), then only one half of the progeny will 

 show the character in question. 



