Question #10

Some possibilities:

    Autosomal recessive diseases usually have no previous history (consanguinity may be present, but usually is not for relatively common autosomal recessive disorders).

    New mutation transmitted by one of the parents.

    Germline mosaicism in one of the parents.

   Most chromosome abnormalities (aneuploidies, translocations, deletions, duplications) have no previous family history.

    Reduced penetrance can produce no previous history, although the penetrance would have to be very low.

    Anticipation is a possibility, although all diseases in which anticipation has been observed are dominant and thus they usually do have a previous family history.