322 WINGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS 
The next question was how the spread type might be kept con- 
stant, for if it was heterozygotic spread, it might be expected that both 
homozyégotic and heterozygotic spread individuals were segregated 
and that homozygotic recessive normal individuals constantly were 
segregated in great number, i.e. 1 Spr Spr: 2 Spr spr: 1 spr spr. 
The only satisfactory explanation I have been able to find is, 
that the spread factor in double dose has a lethal effect already 
on a very early stage, and that furthermore a recessive lethal 
factor 1) is found closely linked to spr, so that the spr spr indivi- 
duals generally also are lost. But a theory of two closely linked 
lethal factors which together have such an effect that a double 
heterozygote seems to be a homozygotic line is on the other hand 
of such a nature that one would refuse to accept it as long as 
possible, for when may we then on the whole be sure to have 
homozygotes and pure lines before-us ? 
However, it is hardly possible getting round the fact that these 
lethal factors are present, as I shall further mention in the following, 
but at the same time we learn how prudent we must be when 
drawing a conclusion from constancy to homozygoty, and how an 
apparent mutation may be due to crossing-over. 
We assume that two mutually closely linked pairs of factors 
A-a and B-b are found in a plant, and that A causes red petals 
(the recessive allelomorph: a white petal colour), while B is a 
necessary vital factor, so that bb acts lethally. It is furthermore 
assumed that À also in double portion acts lethally. — An indi- 
AB 
vidual of the formula ETA is then capable of living and has red 
flowers and it will appear homozygotic, for of the three types of 
ACD Mat AB 5 
offspring Fu 5 and FRA the two former, which both have lethal 
factors in double dose, will be ruined, and only the third type which has 
the genetic formula of the parent plant will appear generation after ge- 
neration, as if we had a pure line before us. If now and then 
crossing-over between A and B takes place, so that the gametes 
ab 
zB 
with white petals will be possible, which by self-fertilization will 
Ab and aB are formed, a sudden occurrence of an individual 
1) The allelomorph of a necessary vital factor. 

