IN THE TROPICS. 
307 
In Mendel’s cases such heterozygotes exhibit only one of 
the pair of allelomorphic characters, and this is said to be 
dominant , the other allelomorph being recessive. A dominant 
allelomorph is thus one which is “so preponderant that it is 
difficult or quite impossible to detect the other in the 
hybrid (from its external appearance). 
By self-fertilization of the hybrid generation F x is obtained 
F a , the first generation bred from the hybrids. In F 2 the 
recessive character appears in one out of every four of the 
offspring, the remaining three still exhibiting the dominant 
character. In F 3 and in all subsequent generations these 
“ extracted récessives ” breed true, and the dominant 
character, so far as is known, never reappears among their 
self-fertilized offspring. The dominant forms of F 2 are 
proved on examination of F 3 to have been of two types. One 
out of every three of them (the extracted dominant) breeds 
true, and continues to do so in subsequent generations, whilst 
the two remaining dominants give in F 3 a mixed offspring 
consisting of dominants and récessives in the same propor¬ 
tion as before 3 :1. And these on examination show the 
same constitution as in the previous generation. 
We thus have, for the zygotes in each generation : —- 
P (dominant character) A x a (recessive character) 
F, Aa 
(heterozygote looking like A) 
F 2 
AA 
Aa 
aA 
aa 
(extracted 
dominant) 
(heterozygotes, but looking like AA : (extracted 
on self-fertilization they each give) recessive) 
_ _ _ K- 
f 3 
AA 
AA 
Aa aA 
- T 
aa 
aa 
(extracted 
dominant) 
'- y - 
(extracted 
recessive) 
f 4 
AA 
AA 
and so on 
aa 
aa 
and so on 
and so on 
and so on 
and 
so on 
breeding 
true 
breeding 
true 
breeding 
true 
breeding 
true 
* The term hybrid was used by Mendel to denote the offspring of cross¬ 
fertilization between members of different groups of plants, no matter 
whether the latter are described by systematists as species or varieties. De 
Vries points out (70) that this usage is historically correct. 
