Subqueries and the optimizer

So, I have a table with an index:

dev=# explain SELECT blast_id FROM genes WHERE blast_batch_id = 2 AND
locus_id IN (1,2);
QUERY PLAN
------------------------------------------------------------------------
------------------------------
Index Scan using idx_genes_locus_id, idx_genes_locus_id on genes
(cost=0.00..88.21 rows=14 width=4)
Index Cond: ((locus_id = 1) OR (locus_id = 2))
Filter: (blast_batch_id = 2)

So far so good, but when I try it with a subquery:

dev=# explain SELECT blast_id FROM genes WHERE blast_batch_id = 2 AND
locus_id IN (SELECT locus_id FROM ll_out_mm);
QUERY PLAN
------------------------------------------------------------------------
Seq Scan on genes (cost=0.00..21414353.48 rows=11003 width=4)
Filter: ((blast_batch_id = 2) AND (subplan))
SubPlan
-> Seq Scan on ll_out_mm (cost=0.00..1267.64 rows=59264 width=4)

How can I nudge the optimizer in the direction of using the index in the
second case as well? Or is it supposed to be doing this in this case.

Thanks,
Dmitri