11(1 
R( search Bulletin No. 2 
THE MANIPULATION OF QUANTITATIVE CHARACTERS IN PLANT 
BREEDING. 
The principal use of hybridization in plant breeding is to 
secure new combinations of the characters present in the par- 
ents — to combine in one race the desirable characters of the 
parent races and to eliminate the undesirable characters. In 
some cases a condition intermediate between the parents with re- 
spect to a certain character may be desired. Crossing may also 
be of value for bringing out characters that were latent in the 
parents, tho, since such possibilities are usually unforeseen, this 
is seldom the main object of any cross. The increased vigor- 
induced by heterozygosis, tho often of value in plant breeding, 
is aside from the purpose of this discussion. 
The method of procedure to be followed in combining the 
more simple independent Mendelian characters is well known. 
All that is necessary to do is to cross appropriate types and to 
grow a sufficient number of P 2 individuals to be sure of getting 
the desired combination of characters in the homozygous condi 
tion. When this has been done it remains merely to propagate 
the new types. If it is known beforehand upon what factors the 
desired characters depend, it can even be told how many F 2 
individuals must be grown to afford an even chance of getting 
the combinations wanted. For instance, if it were desired to 
combine in one variety of corn the two characters sugary en- 
dosperm and a peculiar, erect leaf-habit, due to the absence of 
auricles from the base of the leaves (Emerson 1912), a type that 
was first found in cultures of dent corn, and if all the other 
characters of this non-auriculate dent corn were sufficiently like 
those of the sweet corn chosen as the other parent so that they 
could be disregarded, or if it were immaterial what these other 
characters were, then the two varieties could be crossed with con- 
siderable assurance that about one in every sixteen F 2 plants 
would be homozygous for both erect leaves and sugary endosperm. 
Or better yet, since segregation in endosperm characters occurs 
on Fj ears, only sugary grains need be planted, with the assur- 
ance that all the progeny will be sweet corn and about one in 
four will have erect, non-auriculate leaves. 
The procedure when dealing with quantitative characters 
differs in no way from that outlined above, except that, since 
more factors are concerned, more individuals must be grown in 
F 2 or else the F 3 and perhaps even the F 4 generation must be 
awaited for the desired combination. As a matter of fact even 
the more simple qualitative characters are rarely ever so easily 
handled in actual practice as is indicated by the example given 
above. It almost never happens that both of the types chosen 
