448 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
spotting factor, S. No crossovers have been observed between R and 
L which behave as if they were allelomorphs, or “completely linked.” 
The cross-over percentage between L and G has been determined as 23, 
that between R and G has been determined less accurately as 19, and 
that between R and S as 12.5. The order of the genes is accordingly 
R—L S G. 
Group 3 includes the two characters, starchy endosperm and tunicate 
(“‘podded”’) seeds. The cross-over percentage in this case is 8.3 
(Jones and Gallastegui). 
In the cultivated tomato two cases of linkage have been reported. 
A gene for ‘‘standard”’ vine habit and a gene for “constricted” fruit 
shape show about 20 per cent of crossing over. In another linkage 
group, no crossovers have been observed between green foliage color 
and two-celled fruit, as opposed to yellow foliage color and many- 
celled fruit, in a total of 24 F. plants. It seems probable that the 
linkage in this latter case is close, though the number of observations 
is too small to do more than establish a probability. 
In rats a group of three linked characters has been found, albinism 
(c), red-eye (r) and pink-eye (p), which may be mapped, thus 
c—r p 
or 20 
In mice albinism (c) and pink-eye (p) are linked, as they are in rats, 
but the cross-over percentage is less, viz., 14.3. 
In the silkworm, linkage occurs between a factor, Q, which gives 
to the larva characteristic pattern markings, and a factor, Y, which 
gives to the blood of the larva and the silk of the cocoon a yellow color. 
Crossing-over occurs only in males, and in a percentage of 26.1 (ina 
large series of backcrosses of F, hybrid male with double recessive 
female, producing 24,918 individuals). In Drosophila crossing-over 
occurs only in the female parent, that is in the maturation of the eggs. 
This is true of all linkage groups, whether they involve sex-linkage 
or not. In the grouse-locust, A potettix, a linkage group of seven or 
more characters has been discovered by Nabours, which have this 
curious feature, that crossing-over seems to occur much more fre- 
quently in females than in males. In all other known cases of linkage, 
crossing-over occurs with about the same frequency in the gametes 
formed by both sexes. This accordingly is to be regarded as the 
normal condition. Failure of crossing-over to occur in the odgenesis 
of Drosophila and in the spermatogenesis of the silkworm would seem 
to imply unusual cytological conditions in those cases. 
