Knowledge of relationships among lines is essential to the maize (Zea
mays L.) breeder because it directs the exploitation of germplasm in h
ybrid production. Restriction fragment length polymorphism (RFLP) data
should be useful for estimating relationships among inbreds because t
hey reflect differences in the DNA sequence. Cluster analysis, based o
n RFLPs resulting from 46 probe-enzyme combinations, was used to revea
l associations among 148 U.S. maize inbreds and assign inbreds to hete
rotic groups. Estimates of genetic relationship between all pairs of i
nbreds were computed by Cower's Coefficient of Similarity and expresse
d as Euclidean distances. Then, these estimates were used to assign in
breds to groups by the average linkage method (UPGMA). Differences in
the hybridization fragments exhibited by lines within and between grou
ps were evaluated to explore the basis for partitioning the lines. In
the cluster analysis, inbreds were ordered into two major groups gener
ally coinciding with distinctions between breeding groups derived from
'Lancaster Sure Crop' open pollinated variety and from Iowa Stiff Sta
lk Synthetic. Within these two groups, 11 subgroups were formed, each
containing lines associated with an elite inbred (WF9, MO17, C103, PA9
1, OH43, T8, B14, B73, N28, B37, or OH07). The classification generall
y agreed with pedigree information and was supported by principal comp
onent analysis. Groups differed markedly in the proportions of lines e
xhibiting particular fragments. However, differences between groups re
flected differences in frequencies rather than distinctions between th
e presence or absence of fragments by the vast majority of the lines i
n the respective groups.