After DNA fragments are sequenced and mapped to a reference, various hypotheses for the sequences in a variant region can be scored to find which sequence hypotheses are more likely. A hypothesis can include a specific variable fraction for the plurality of alleles that comprise the sequence hypothesis in the region. A likelihood of each hypothesis can be determined using a probability that accounts for the fraction of the alleles specified in the respective sequence hypothesis. Thus, other hypotheses besides standard homozygous and equal heterozygous (i.e., one chromosome with A and one with B in a cell) can be explored by explicitly including the variable fractions of the alleles as a parameter in the optimization. Also, a variant score can be determined for a variant relative to a reference. The variant score can be used to determine a variant calibrated score indicating a likelihood that the variant call is correct.