24 BULLETIN 1489, U. S. DEPARTMENT OF AGRICULTURE 
number of unfavorable recessive characters that find expression in 
the ordinary corn field, however, may be quite large. 
The chances are that two unrelated varieties of corn will not con- 
tain the same assortment of unfavorable recessives. Consequently, if 
these varieties are crossed, the cross will tend to develop according to 
the dominant (better) allelomorph of each pair of factors contrib- 
uted by the parents. If, on the other hand, a corn plant is self- 
fertilized, any recessive factors for which it is heterozygous are car- 
ried in half of the eggs and half of the sperms. Many of the result- 
ing kernels will be homozygous for one or more of the unfavorable 
factors carried by the parent plant. Many of the plants from inbred 
seeds therefore are poorly developed as a result of the less favorable 
characters that come into expression in this way. 
A better picture of this effect with reference to a single factor pair 
may be had by assuming that a specific character, japonica striping, 
for example, is unfavorable to growth. Referring to Figure 5, it is 
seen that japonica striping does not appear in the cross (F t genera- 
tion), as it is recessive. Consequently there would be no reduction in 
yield due to japonica in this generation. There are three normal 
plants to one japonica plant in the F 2 generation, and the reduction 
in vigor would begin. If all classes are propagated equally, the 
proportion of japonica plants becomes greater and greater in the suc- 
ceeding generations. In the F 6 generation there are 496 japonica 
plants against 528 normal plants, or practically half of the entire 
population consists of inferior plants. The proportion of japonica 
plants increases quite rapidly at first, but the rate decreases in the 
successive generations. 
The characters responsible for the more gradual decline in vigor 
resulting from inbreeding are less obvious in their individual effects 
than characters such as japonica striping and the like. They are 
numerous, however, and the reduction in vigor is due to the cumula- 
tive interaction of the many slightly unfavorable controlling factors. 
Just as the proportion of japonica plants increased most rapidly 
between the F x and F 2 generations, the rate of decrease in vigor is 
greatest between the first and second years of inbreeding and becomes 
'less as stability is reached. 
The facts of a decrease in vigor accompanying inbreeding and of 
increased productiveness following crossing are of the utmost im- 
portance in practical corn improvement. Any system of breeding 
that tends to bring about a narrowing of the hereditary lines gives 
opportunity for the expression of deleterious recessives, with a re- 
sulting tendency toward a decrease in yield. Conversely, crossing 
two strains or varieties tends to suppress any unfavorable recessive 
characters that are not common to both strains. 
That crosses between varieties, strains, or selfed lines of corn 
usually yield more than the average of the parents and frequently 
yield more than the better parent is interesting evidence of the 
phenomenon of hybrid vigor. It is of little practical importance 
whenever the cross is not more productive than the better varieties 
already existing. The corn breeder is interested in bringing together 
the largest number of desirable characters into a single stock. So 
far it has been necessary to resort to hybridization in order to accom- 
plish this. Hybrid vigor accordingly has been an important tool 
