Good, clear new policy on DNA evidence in immigration cases from the Home Office following the scandal last year:

The Home Office cannot require that DNA evidence is provided as part of an immigration application. This is reflected in the fact that the department has no specific statutory power to require DNA evidence. Officials can give applicants the opportunity to provide DNA evidence as one of a range of options to prove a relationship, but it is voluntary, and it is the applicant’s choice as to whether they wish to provide it in further support of their application. If an applicant chooses not to provide DNA evidence, no negative inferences can be drawn from this. In the absence of DNA evidence, an application must be determined on the basis of the available evidence.

The policy makes clear that where a relationship is in doubt, the applicant declines to produce DNA evidence and the Home Office ultimately concludes that the relationship is unproven (supposedly on the balance of probabilities standard of proof, although the policy rather unhelpfully does not actually say this) then the application will be refused.