446 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
between 1 and s was 40.6, which falls short of the map distance by 
almost exactly the amount indicated by Haldane’s table. 
In the sweet pea the earliest discovered examples of linkage are 
found. Here are known two linkage groups containing each three 
pairs of characters as follows: 
Dominant Recessive 
1. Blue vs. red flower color 
Group 1 | 2. Long vs. round pollen 
3. Erect vs. hooded standard 
1. Dark 9s. light leaf-axil 
Group 2 2. Fertile vs. sterile anthers 
3. Normal vs. cretin lowers 
Results described by Bateson and by Punnett indicate that in Group 1 
the map relations of the three genes are: 
E—B L 
0.78 12.5 
The group is a compact one, with E and B very closely linked, cross- 
over percentage less than one, with B and L showing between 11 and 
12 per cent crossovers, and with E and L showing about 12.5 per cent 
of crossovers. 
In Group 2, the cross-over percentage between D and F is about 
6.2, between F and N about 25.0. Until the cross-over percentage 
between D and N has been experimentally determined, it cannot be 
stated whether the ‘“‘map” order is FDN or FND. In the former 
case, the total map distance will be 25, or about double the length of 
Group 1; in the latter case, it will be still longer, or about 31.2. 
In garden peas two independent pairs of linked characters are 
known and two more are suspected (White). In one of the established 
cases Close linkage is found between round starchy seeds and tendrils 
on the leaves, with about 1.5 per cent of crossing-over. In the other 
case a gene for late flowering is linked with red flower color with an 
estimated cross-over percentage of between 12 and 16. 
In the snapdragon, Antirrhinum, two factors for flower color were 
found by Baur to be linked, with about 20 per cent of cross-overs occur- 
ring. 
In maize three linkage groups are known, one of four factors and 
two of two factors each. Group 1 includes a factor for waxy endo- 
sperm and the factor C for aleurone color. These show a cross-over 
percentage of 26.7. Group 2 includes four linked factors, aleurone 
fa ctor R, chlorophyll factor G, chlorophyll factor L, and aleurone 
