Question

Which chromosomal mutation requires the presence of another chromosome?

Answer 1

The only one I am familiar with that includes the presence of another (extra) chromosome is klinefelts syndrome: http://en.wikipedia.org/wiki/Klinefelter_syndrome

Answer 2

The first thing I thought of was more along the lines of trisomy ( http://en.wikipedia.org/wiki/Trisomy). This would not only include Klinefelter Syndrome, but also include Down's Syndrome (AKA Trisomy 21) and a number of other conditions with an exra chromosome.