TDT analysis when one parent is missing

Consider this trio, where the mother is untyped:

It is clear that the father has transmitted an A allele to the affected offspring, and has not transmitted the B.

Suppose that allele A is much commoner than allele B. Then most parents will be homozygous for A, but the following trios will sometimes occur:

If the disease and marker are not associated, these two types of trio will be equally common.

If the heterozygous parent is untyped, the trio will be uninformative. If the homozygous parent is untyped but the child is heterozygous, the trio will again be uninformative. The only informative trios will be like the first one shown, which appear to support preferential transmission of allele A.

For a multiallelic marker carrying out TDT with a missing parent will not produce bias if the offspring is heterozygous, but if a dominant disease causes excess mortality in the parent then the surviving parent may be uninformative.