132 GENETICS 



although not necessarily so, that is, they may have 

 the same phenotypical constitution, but their geno- 

 typical composition is always different. 



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 

 possible to distinguish one from the other by inspec- 

 tion, although it may frequently be done, as will be 

 pointed out later. The only sure way to identify a 

 heterozygote is by breeding to a recessive and observing 

 the kind of offspring produced. 



Peas of the formulae TT and T(), 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 

 follows : 



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

 Case II. T + t X t + t = 50 per cent T(t) + 50 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. 



9. THE PRESENCE AND ABSENCE HYPOTHESIS 



Mendel's conception that every dominant character 

 is paired with a recessive alternative is now being 



