204 A TEXTBOOK OF GENERAL BOTANY 
linkage of factors in the same chromosome prevents the accumu- 
lation in a homozygous condition of all the dominant factors 
present in the hybrid. Thus, if a chromosome from one parent 
of the hybrid contained the dominant factors 4 and B and the 
recessive factors ¢ and d, and the corresponding chromosome 
from the other parent carried the recessive factors a and 6 and 
the dominant factors C and D, the hybrid would possess all four 
dominant factors; but it is not to be expected that self-fertiliza- 
tion would result in an individual homozygous for the four 
factors, A, B, C, and D. 
Inbreeding and outbreeding. Self-fertilization, or the crossing of 
closely related individuals such as brothers and sisters or parent 
and offspring, is known as inbreeding, while the crossing of unre- 
lated individuals is known as outbreeding. 
Inbreeding. The majority of flowering plants are provided 
with some device for promoting cross-pollination. On the other 
hand, some vigorous plants, such as wheat, rice, barley, oats, 
tobacco, beans, and tomatoes, are characterized by very nearly 
continuous self-fertilization. Self-fertilization in these cases cer- 
tainly does not appear to be harmful. Therefore we may conclude 
not only that cross-fertilization is not always necessary, but also 
that self-fertilization is not necessarily harmful. 
It would seem that continuous vegetative propagation without 
any sexual reproduction is also without deleterious effects. Ordi- 
nary varieties of bananas and pineapples are examples of very 
vigorous plants that are always reproduced vegetatively. More- 
over, they have reproduced in this manner throughout the whole 
period during which we have knowledge of them. As far as hered- 
itary composition is concerned, vegetative reproduction and self- 
fertilization of homozygous strains should have the same effect. 
Self-fertilization in corn. Vigorous varieties of corn have 
been found to be complex hybrids. Self-fertilization of such 
varieties results in rapid deterioration. This deterioration is 
most marked in the /, generation. The deterioration continues 
for several succeeding generations, but the amount of deteriora- 
tion becomes less and less. The final result is the production 
