biallelic

adjective

bi·​al·​le·​lic (ˈ)bī-ə-ˈlē-lik, -ˈle- 

: of, relating to, or affecting both alleles of a gene

The risk of colorectal cancer among carriers of biallelic mutations appears to be close to 100% by 60 years of age …

—William D. Foulkes, The New England Journal of Medicine

This mode of oncogenesis differs from the inactivation of most tumor-suppressor genes, which are biallelic—that is, one allele is inactivated in the germ line, followed by mutation of the second allele at the somatic-cell level.

—Roel Nusse, Science