Three individuals carry the same disease-causing mutation; two suffer from the disease but exhibit different symptoms, while the third is completely unaffected. Why?

"In cases of abnormal development, the brain fails to divide and remains fused, a condition known as holoprosencephaly. “In the second image, holoprosencephaly occurred in one out of every 8,000 ...

An abnormal development of the forebrain, holoprosencephaly, is the most common cranial abnormality in infants with Trisomy 13. The embryology and implications of holoprosencephaly are described.