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BRCA-1 is a tumor suppressor gene and on mutation can undergo a loss of function to become cancer-inducing.

Loss of function mutations are usually recessive and require the mutation of both alleles. However in this case only one allele needs to be mutated to cause loss of function. Why is this so?

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The answer depends on when and where you are identifying the phenotype. www.cancer.gov has much useful info, including a fact sheet: https://www.cancer.gov/about-cancer/causes-prevention/genetics/brca-fact-sheet

From that:

Even if someone has inherited a harmful variant in BRCA1 or BRCA2 from one parent, they would have inherited a normal copy of that gene from the other parent ... But the normal copy can be lost or change in some cells in the body during that person’s lifetime. Such a change is called a somatic alteration. Cells that don’t have any functioning BRCA1 or BRCA2 proteins can grow out of control and become cancer.

The pre-existing loss of one functional allele from conception makes any later somatic inactivation of the other allele a homozygous loss of function rather than just a heterozygous LOF.

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