KFAR SABA, Israel — Ever since she tested positive for a defective gene that causes breast cancer, Tamar Modiano has harbored a mother’s fear: that she had passed it on to her two daughters. Ms. Modiano had her breasts removed at 47 to prevent the disease and said that the day she found out her older daughter tested negative was one of the happiest of her life.

Now she wants her younger daughter, Hadas, 24, to be tested so she can start a family early if she is positive and then have a double mastectomy too. Ms. Modiano’s elder daughter, Suzi Gattegno, 29, disagrees.

“You’re keeping her from living her life,” Ms. Gattegno told her mother. “You want to marry her off early.”

“If she’s a carrier, she should marry early,” her mother countered.

“She doesn’t even have a boyfriend,” the daughter said. “You need to stop pressuring her.”