176 GENETICS AND EUGENICS 



*' constricted" fruit shape show about 20 per cent of crossing- 

 over. In another linkage group, no crossovers have been 

 observed between green foHage color and two-celled fruit, as 

 opposed to yellow foliage color and many-celled fruit, in a 

 total of 24 r2 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-e}^e (r) and pink-eye (p), which may be 

 mapped, thus 



c_i._______ _____-- p 



1 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 back- 

 crosses of Fi hybrid male with double recessive female, pro- 

 ducing 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 dis- 

 covered by Nabours, which have this curious feature, that 

 crossing-over seems to occur much more frequently in fe- 

 males 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 spermato- 

 genesis of the silkworm would seem to imply unusual 

 cytological conditions in those cases. 



