As many as 10-15% of breast cancers in the us are believed to be hereditary.
These are linked to mutations of a particular group of genes called the brca genes. There is a simple blood test to check to see if someone has a brca mutation. There is a 50:50 chance of inheriting this from either parent. If a woman inherits this gene, there's a 50-75% chance of breast ca; men will have <10% risk.