Question

1. A woman is BRCA1⁺/BRCA1^-. If you could analyze 10 of her non-cancerous somatic cells, how many wild-type and how many mutant copies of BRCA1 would you expect to find in each cell?
2. If you could analyze 10 of her tumor cells, how many wild-type and how many mutant copies of BRCA1 would you expect to find in each cell?

1. A man is BRCA1⁺/BRCA1^- but does not have breast cancer. If you could analyze 10 of his sperm cells, how many wild-type and how many mutant copies of BRCA1 would you expect to find in each cell?

Answer 1

The woman is genotypically BRCA+/BRCA-. Hence, each of her somatic cell will have one copy of BRCA+ and one copy of BRCA-. Therefore, if we analyze 10 of her non-cancerous somatic cells, we will get 5 copies of BRCA+ and 5 copies of BRCA-. Hence, we shall find 1 mutant copy of BRCA in each cell.

A tumor cell is formed when a mutation has resulted in the genome that makes both the alleles mutant. And hence, in 10 tumorous cells, we will get 10*2 = 20 copies of mutant alleles. That is to say that each of the cells would have two copies of mutant alleles.

Now, the man is BRCA+/BRCA-. The sperms are haploid cells are formed via a process of meiosis. Each sperm will have one allele for each locus, with each of the two alleles going to half the cells. Therefore, BRCA+ will be present in half the sperms and BRCA- will be present in the other half. Therefore, in each cell we would find either BRCA+ or BRCA- with equal probability.