232 GENETICS IN RELATION TO AGRICULTURE 
1. The phenomenon of increased growth derived from crossing both plants and 
animals has long been known but never accounted for in a comprehensible manner by 
any hypothesis free from serious objections. 
2. The conception of dominance, as outlined by Keeble and Pellew in 1910 and 
illustrated by them in height of peas, has had two objections which were: a. If hetero- 
sis were due to dominance of factors it was thought possible to recombine in genera- 
tions subsequent to the F» all of the dominant characters in some individuals and all 
of the recessive characters in others in a homozygous condition. These individuals 
could not be changed by inbreeding. b. If dominance were concerned it was con- 
sidered that the /, population would show an asymmetrical distribution. 
3. All hypotheses attempting to account for heterosis have failed to take into 
consideration the fact of linkage. 
4. It is shown that, on account of linked factors, the complete dominant or com- 
plete recessive can never or rarely be obtained, and why the distributions in F2 are 
symmetrical. 
5. From the fact that partial dominance of qualitative characters is a universal 
phenomenon and that abnormalities are nearly always recessive to the normal condi- 
tions, it is possible to account for the increased growth in F, because the greatest 
number of different factors are combined at that time. 
6. It is not necessary to assume perfect dominance. It is only necessary to accept 
the conclusion that many factors in the 1n condition have more than one-half the 
effect that they have in the 2n condition. 
7. This view of dominance of linked factors as a means of accounting for heterosis 
makes it easier to understand: a, why heterozygosis should have a stimulating rather 
than a depressing or neutral effect; and b, why the effects of heterozygosis should 
operate throughout the lifetime of the individual, even through many generations 
of asexual propagation. 
In order to extend their investigations to normally self-fertilized 
species, East and Hayes made numerous crosses between different 
species of Nicotiana. As a result they found, as indeed had previous 
investigators on this the favorite genus for hybridization studies, that 
the vigor of hybrids varied all the way from a condition so weak as to 
give embryos incapable of germination to a condition greatly exceeding 
in vigor that exhibited by either parent. Thus Nicotiana tabacum, 
the commonly cultivated tobacco, when crossed with N. sylvestris gives 
hybrids which exceed by 35 per cent. the average height of the parents 
and are estimated to be 20 per cent. more vigorous. Similarly N. 
tabacum, when crossed with N. rustica, gives hybrids exceeding by 80 
per cent. the average height of the parents, but when crossed with N. 
alata grandiflora, the hybrids are only about 10 per cent. of the average 
of the parents, both in height and vigor. About fifty species crosses 
within the genus were made by these investigators with results in re- 
spect to vigor which bore out those already known for various specific 
crosses. 
It is obvious from these results that stimulation in the hybrid is a 
result of certain specific interactions. East and Hayes regard those 
hybrids which show decreased vigor as evidence of such great differences 
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