Final answer:
To establish biological parentage, the child's DNA fingerprint needs to match certain genetic markers present in both parents' genotypes.
Step-by-step explanation:
In order to establish that a child is related to the biological parents, the child's DNA fingerprint must match certain genetic markers present in both parents. These genetic markers are known as alleles and are specific variations of a gene. By comparing the child's genotype with the genotypes of the potential parents, it is possible to determine whether there is a match.
In this case, the child has the genotype 12, 15 at the tho1 STR locus. To find the consistent biological parents, we would need to compare this genotype with the genotypes of potential parents. If both the mother and father have alleles 12 and 15 at the tho1 STR locus, it would be consistent with the child's genotype. However, if either the mother or father has different alleles at this locus, it would suggest that they are not the biological parents.