Linked and unlinked genes

You want to test whether a particular allele of the β-interferon gene (the a3 allele) is responsible for an immune deficiency disease.  Using an assay for a restriction fragment polymorphism present in the a3 allele and absent in other β-interferon alleles, you have mapped the inheritance of the a3 allele and the disease phenotype in a large number of affected families.  You find that the map distance between the β-interferon gene and the disease phenotype is 2 cM.  (In other words, 2% of children who inherit the disease from affected parents with the a3 allele do not inherit the a3 allele.)  Which one of the following conclusions can you make from these results?

Choices:
   1)The disease is caused by mutations in the β-interferon gene, but not the a3 allele.
   2)The disease does not have a genetic basis.
   3)The disease is caused by mutations in a gene unlinked to the β-interferon gene.
   4)The disease is caused by mutations in a gene that is linked to the β-interferon gene.
   5)The disease is caused by the a3 allele of β-interferon.