It is true that some diseases in humans require only 1 "mistake" in 1 copy of the gene and other diseases require that the same mistake occur in both copies of the gene before the disease state becomes evident or damaging.
For example, in sickle cell anemia
, if one of your 2 copies of chromosome 11 has a mistake at Amino Acid number 6 in the globin gene, you have sickle cell trait, a condition with almost no symptoms or noticeable abnormalities, not the full blown disease. But, if both copies of that chromosome 11 have the exact same mutation, the individual will have sickle cell anemia, a life-long disease characterized by recurring painful episodes, severe anemia, jaundice
, and high risk of strokes, heart failure
, kidney failure
and chronic lung disease. On the other hand, many cases of classic hemophilia
are caused by a single break in the dna of the x chromosome followed by a repair error - the broken off piece gets stuck back in the chromosome where it is supposed to be, but it is "upside down" or inverted.