The first person in a family to exhibit Leber hereditary optic neuropathy (LHON) was II-3 in the pedigree shown below, and all of her children also exhibited the disease. Provide two possible explanations as to why II-3's mother (I-1) did not exhibit symptoms of LHON. 

Verified step by step guidance

1. Identify that Leber hereditary optic neuropathy (LHON) is a mitochondrial disorder, which means it is passed through the maternal line.
2. Observe that individual II-3 is affected and all her children (III-1, III-2, III-3) are also affected, indicating maternal inheritance.
3. Consider that I-1, the mother of II-3, might not exhibit symptoms due to heteroplasmy, where a mixture of normal and mutated mitochondria exists, and the proportion of mutated mitochondria is not sufficient to cause symptoms.
4. Another possibility is that I-1 has a lower penetrance of the disease, meaning she carries the mutation but does not express the symptoms due to other genetic or environmental factors.
5. Conclude that the absence of symptoms in I-1 does not preclude her from being a carrier of the mitochondrial mutation responsible for LHON.