Jack has been diagnosed with neurofibromatosis 1 (NF1) and presents with skin that is populated by flat, light-brown spots and neurofibromas under the skin. He also has a mild learning disorder. His sister Jill has also been diagnosed with NF1, however, her only symptom is the neurofibromas. The most likely cause of these differences is:

The most likely cause of this disease is that siblings have a variable expression of this disease.

Explanation:

Neurofibromatosis 1 is an autosomal dominant disorder, even though it is a very rare disease to occur. This dominant nature causes this disease to be expressed even in people who have heterozygous alleles for it, which makes it easier for siblings to have this disorder, since only those who are hom0zygous recessive would not present it. However, the expressiveness of the disease may vary between siblings, as is the case shown above, where although the two siblings have the same disease, they present it in different ways.

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the answer is:

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