MARKER-BASED ESTIMATES OF IDENTITY BY DESCENT AND ALIKENESS IN STATE AMONG MAIZE INBREDS

Molecular markers are useful for determining relationships and similar ity among inbreds, especially if the proportion of marker loci with al leles common to inbreds i and j is partitioned into: (1) the probabili ty that marker alleles are identical by descent ((M)f(ij)); and (2) th e conditional probability that marker alleles sere alike in state, giv en that they are not identical by descent (theta(ij)). Our objectives were to: develop a method, based on tabular analysis of restriction fr agment length polymorphism marker data, for estimating (M)f(ij), theta (ij), and the parental contribution to inbred progeny; validate the ac curacy of the method with a simulated data set: and compare the pedigr ee-based coefficient of coancestry (f(ij)) and (M)f(ij) among a set of maize (Zea mays L.) inbreds. Banding patterns for 73 probe-enzyme com binations were determined among 13 inbreds. Iterative estimation of (M )f(ij), theta(ij), and the parental contribution to progeny was perfor med with procedures similar to a tabular analysis of pedigree data. De viations of (M)f(ij) from pedigree-based f(ij) ranged from 0.002 to 0. 285, indicating large effects of selection and/or drift during inbreed ing for some inbreds. Differences between marker-based estimates and e xpected values of parental contribution to inbred progeny were as larg e as 0.205. Results for a simulated set of inbreds indicated that tabu lar analysis of marker data provides more accurate estimates of (M)f(i j) and theta(ij) than other methods described in the literature. Tabul ar analysis requires the availability of marker data for all the proge nitors of each inbred. When marker data are not available for the pare nts of a given inbred, Mf(ij) and theta(ij) may still be calculated if parental contributions to the inbred are assumed equal to their expec tations.