Answer: If the cross is LMLM x LNLN, the probability is 100%; If the cross is LMLN x LMLN, the probability is 25% for LMLM and 25% for LNLN.
Explanation: In genetics, one gene is considered dominant when the gene manifests itself over its other counterpart. However, there are cases in which an individual with two different genes express them simultaneously, To it, it's called Codominance.
As cited above, the MN blood group is a example of codominance. Due to it, when crossing two species, there can be individual with 1, 2 or 3 differents genotypes. If crossing a homozygous LM individual with a homozygous LN individual, their offspring will have a 100% chance of having a blood group different from their parents.
If crossing two heterozygous individual, the probability of having a child with blood genotype different from his or hers parents is 25% for type M and 25% for type N, because the ratio in codominance is 1:2:1.