INDEPENDENT MENDELIAN INHERITANCE 73 



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 PI generation. The 

 progeny obtained by crossing two distinct races is the first filial genera- 

 tion, conveniently designated the FI. The progeny of the FI are the F z 

 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 t 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 T, or in the dwarf races its 

 allelomorph, the factor t. 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 t. 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 T, 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 



