The theta factor or co-ancestry coefficient is a measurement of co-ancestry within the population. In forensic genetics, it is used in statistical analysis to correct the degree of relatedness of alleles with common ancestry in populations.

The value of theta depends on the level of inbreeding in the population. For the majority population for which there is no evidence of inbreeding, the value of theta can be set as 0.001. in some isolated populations for which we can expect inbreeding to happen or populations where social norms promote inbreeding, the theta factor can be 0.003.