Studies of genetics and ecology often require estimates of relatedness coefficients based on genetic marker data. However, with the presence of null alleles, an observed genotype can represent one of several possible true genotypes. This results in biased estimates of relatedness. As the numbers of marker loci are often limited, loci with null alleles cannot be abandoned without substantial loss of statistical power. Here, we show how loci with null alleles can be incorporated into six estimators of relatedness (two novel). We evaluate the performance of various estimators before and after correction for null alleles. If the frequency of a null allele is ,0.1, some estimators can be used directly without adjustment; if it is .0.5, the potency of estimation is too low and such a locus should be excluded. We make available a software package entitled PolyRelatedness V1.6, which enables researchers to optimize these estimators to best fit a particular data set.KEYWORDS relatedness coefficient; null alleles; method-of-moment; maximum likelihood P AIRWISE relatedness is central to studies of population genetics, quantitative genetics, behavioral ecology, and sociobiology (e.g., Mattila et al. 2012). The relatedness coefficient between individuals can be calculated from a known pedigree (Karigl 1981). In the absence of pedigree information, this coefficient can be estimated by using genetic markers.When codominant genetic markers are used to estimate relatedness, some genotyping errors may occur. For example, in polymerase chain reaction-based markers, allelic dropout, false allele, and null allele errors are common, especially with microsatellites. In allelic dropout, the low quality of the DNA template inhibits the amplification of one of the two alleles in a heterozygote, causing the heterozygote to be observed as a homozygote (Taberlet et al. 1996). False alleles are artificial alleles produced during amplification, resulting in homozygotes mistyped as heterozygotes (Taberlet et al. 1996). Null alleles are alleles that cannot be detected because of mutation, often within the primer site (Brookfield 1996). These incorrect genotypes can cause serious problems in the estimation of genetic relationships. For instance, in parentage analysis, genotyping error can mistakenly reject the true father due to an observed lack of shared alleles with the offspring (Blouin 2003). Similarly, when pairwise genetic relatedness is estimated, genotyping error can cause true relatives to be omitted. This is the subject of this article.The first two error types can be eliminated by multiple-tube methods because the errors appear at random (Taberlet et al. 1996;He et al. 2011). Genotyping errors due to null alleles cannot be resolved in this manner. Null alleles are pervasive in microsatellite markers and many studies report null alleles (e.g., Kokita et al. 2013). Solutions for dealing with null alleles are limited, but when null alleles are detected, they can be eliminated by redesigning the primers or by developing new micro...