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 site 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, in 2% of cases, children inherit the disease allele without inheriting the a3 allele.) Which one of the following conclusions can you make from these results?

The disease is caused by mutations in the β-interferon gene, but not the a3 allele.

The disease does not have a genetic basis.

The disease is caused by the a3 allele of β-interferon.

The disease is caused by mutations in a gene unlinked to the β-interferon gene.

The disease is caused by mutations in a gene that is linked to the β-interferon gene.