LINKAGE 85 



mother) . Here the son gets his sex-linked characters from 

 his mother, since his only X is derived from her. Experi- 

 ments have shown that this holds for any number of 

 sex-linked characters that are present in the mother.* 



Linkage has been demonstrated in a number of animals 

 and plants. The first case discovered was in sweet peas. 

 Bateson and Punnett (1905) found that when purple 

 flowers and long pollen grains went in from one parent, 

 and red flowers and round pollen went in from the other 

 parent, they tended to come out together more frequently 

 than would be expected on the two-factor ratio, 9 : 3 r 3 : 1. 

 In the case of these sweet peas the linkage is not com- 

 plete, apparently not in either sex. At present two dif- 

 ferent linkage groups are known in sweet peas, one made 

 up of three linked characters, and the other of three, pos- 

 sibly four. In the edible or garden pea there are two 

 linked characters, and two that are doubtful (Bateson 

 and Vilmorin, White). Mendel did not happen to make 

 any combinations of linked characters in this form, hence 

 he got free assortment. In the primrose (P. sinen- 

 sis) there is a group of five linked characters (Gregory, 

 Altenburg) ; in the snapdragon a group of five (Baur) ; 

 ia stocks a group of three or four (Saunders). In the 

 groundsel {Senecio vulgaris) there are two linked charac- 

 ters known; other cases occur in corn (Lindstrom), 

 tomatoes (Jones), wheat (Engledow), oats (Surface), 

 Oenothera (DeVries, MuUer ) . In animals the largest num- 

 ber of linked characters is found in the vinegar fly, Droso- 

 phila melanogaster, in which there are four groups— a 

 sex-linked group containing about 100 characters, a second 

 group containing 75 characters, and a third group contain- 

 ing about 60 characters, and a fourth group of two charac- 

 ters. In other species of DrosopMla, linked characters 

 (other than sex-linked) are beginning to be reported as 

 more characters are studied (Metz in D. virilis, Warren 



*A reservation for crossing over in a heterozygous mother must be 

 added to this statement. 



