INDEPENDENT MENDELIAN INHERITANCE 13 
included tall plants of the genetic constitution TT and tall hybrids of 
the genetic constitution Tt. The distinction between the genotypes of a 
given phenotype is only possible by further breeding tests. In general 
a hybrid is best detected by crossing it to the recessive form in which 
case it will produce half dominants and half recessives, whereas the pure 
dominant will produce only dominants. Such a cross is known as a back 
cross or sesqui-hybrid. 
With respect to history an extracted dominant or recessive is one 
which has been derived from a hybrid form. The historical fact with re- 
gard to an extracted form that the parent or other known ancestor did not 
breed true for the character in question is the only distinguishing feature 
about it, the factors which it contains are the same as those in the parent 
races. 
The parents of a hybrid are generally called the P; generation. The 
progeny obtained by crossing two distinct races is the first filial genera- 
tion, conveniently designated the f;. The progeny of the F; are the F2 
generation and so on. 
The above terms are constantly employed in even the most simple 
cases and their application will soon become clear to the student. Other 
terms are used in connection with more complex cases, but these will be 
introduced only when their significance may be made clear from the 
manner in which they are employed. 
The Chromosome Interpretation—The chromosome interpretation 
of a case of monohybridism is very simple. It depends on the assumption 
for the case of tall vs. dwarf peas that the factor T is a chromomere 
occupying a definite position in each member of a certain pair of 
chromosomes of the tall race. The factor ¢ is correspondingly located 
in exactly the same pair of chromosomes in the dwarf race. Aside 
from this difference in one pair of chromomeres which occupy identical 
positions in corresponding pairs of chromosomes the chromosomes of 
the two races bear exactly the same set of factors. Accordingly, of the 
seven pairs of chromosomes in the cells of the garden pea, only that pair 
need be considered which bears the factor 7, or in the dwarf races its 
allelomorph, the factor ¢. In the hybrid produced by crossing a tall and 
a dwarf pea one member of the pair of chromosomes bears the factor T, 
and the other the factor ¢. In the reduction divisions the members of 
this pair of chromosomes are separated and distributed to different germ 
cells, consequently half the number of germ cells will receive that mem- 
ber which bears the factor 7, and half that member which bears the factor 
t. Recombination of these gametes gives the offspring in the ratio 3 
tall: 1 dwarf, which has been pointed out previously. If in Fig. 34 the 
rectangles containing the factors T and ¢t are taken to represent the mem- 
bers of this pair of chromosomes instead of entire gametes, this figure 
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