To clarify the issue of autosomal recessive:  If two parents who are both
carriers of a recessive gene have children, there is a 25% chance of an
affected child (both recessive genes), 50% of a carrier(one dominant and
one recessive gene), and 25% of an unaffected child (both dominant genes).
In this mother's case, however, she is already affected, so the
probabilities for her offspring depend on the genes of her partner:  If
she has a partner with no recessive genes for PKU, all their children will
be carriers but none will have the condition; if her patner is a carrier,
50% of their children will have the condition and 50% will be carriers; if
her partner has PKU, all their children will have PKU.

All this, however, is irrelevant to the issue of whether or not she can
nurse this baby.  That should be no different in this mom than in any
other, i.e. she should be able to bf along with some phenylalanine-free
formula and the baby should be monitored.  Hope this helps.  Alicia.