GENETICS: D. F. JONES 
311 
from one generation to another in groups it would be practically impos- 
sible to recombine all the dominant characters in one individual and 
all the recessive in another. Hence the failure to obtain both the com- 
plete dominants and complete recessives which would breed true is ac- 
counted for. This view of the situation also explains why symmetrical 
distributions are obtained in F2. The development of each individual 
is assumed to be correlated with the number of different factors present. 
The individual with the greatest number of heterozygous chromosomes 
would have the greatest number of different factors present if the factors 
were distributed among all the chromosomes. The theoretical distri- 
bution of the F2 individuals according to the number of heterozygous 
chromosomes contained is in the ratio of the expanded binomial (a 
The expanded binomial is often used as an illustration of a normal fre- 
uency distribution. 
To account for the increase in growth in Fi it is necessary to have the 
favorable characters for the most part dominant over the unfavorable 
ones. This seems probable from the numerous cases of abnormalitites 
which are recessive to the normal condition. It is not necessary that 
there should be perfect dominance. It is necessary, however, to accept 
the conclusion that many factors in the In condition have more than one 
half the effect that they have in the 2n condition. 
Inbred strains of maize have been obtained by inbreeding which either 
lack chlorophyll entirely or are partially deficient in chlorophyll. Some 
strains are partially sterile. Some have fasciated ears. Some are sus- 
ceptible to a bacterial wilt disease. Some have contorted stems and 
still others have brace roots so poorly developed that they can not stand 
upright when the plants become heavy. Similar instances can be cited 
in many naturally cross pollinated species. Some of the strains may 
have more than one of these unfavorable characters. No one strain so 
far known has them all. 
Crossing these strains together gives perfectly normal Fi plants. They 
are able to grow better than their parents because the characters neces- 
sary for maximum development that one strain lacks are supplied by 
the other and conversely. This increased growth is heterosis. 
Dominance of characters gives a reason why heterozygosis should 
cause the Fi generation to grow more than the parents and not less. 
According to previous views it would have been just as reasonable to 
suppose that hybridization had a depressing or an indifferent rather than 
a stimulating effect. It also makes it easier to understand why hetero- 
zygosis should operate throughout the life of the individual even through 
innumerable generations of vegetative propagation. 
