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 S.;^ 

 (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 F2 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 



01 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 (in a 

 large series of backcrosses of Fi 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, Apotettix, 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 oogenesis 

 of Drosophila and in the spermatogenesis of the silkworm would seem 

 to imply unusual cytological conditions in those cases. 



