The nonmutant allele of the BRCA1 gene helps to suppress tumor formation in women who are heterozygous for the mutation. Women homozygous for BRCA1 nevertheless have a 50% to 70% chance of developing breast cancer before age 70, and the usual reason is that the nonmutant allele is lost or inactivated in a lineage of cells. One possible mechanism for such "loss of heterozygosity" is:

Question

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Loss of heterozygosity occurs when one of the two copies of a gene is lost or inactivated in a lineage of cells. This can happen due to a somatic mutation, such as a mutation in a breast cell that inactivates the nonmutant BRCA1 allele. Germ cells in the affected individual developing a mutation in the nonmutant allele (A) or a silent mutation occurring in the nonmutant allele (C) would not result in loss of heterozygosity.

The Nonmutant Allele of the BRCA1 Gene Helps to Suppress