Mary Singleton is the mother of Rebecca Singleton.
Mary's daughter, Bex, had an enlarged pelvic lymph node that they had to biopsy. Before her biopsy, they ran labs that showed that her hormone levels were unbalanced, indicating the use of birth control pills. George asked her about it and she confessed that she'd been taking them to try to grow breasts.
Addison took her into biopsy, but they discovered that the tumor was compressing what they believed was an ovary. The pathology report revealed that it was actually a testes, meaning Bex is intersex. They talked to Bex's parents and called for a psych consult. The parents were told that physically, Bex would be fine as the tumor was benign, but the doctors encouraged the whole family to get therapy.
Bex's parents came to Addison and said they didn't want to tell Bex and asked Addison to remove the male sex organs when she went in to remove the tumor. Addison declined and said they'd have a hard time finding a doctor who would agree to it.
Before Bex's surgery, unable to lie to her, George forced Bex's parents to tell her about her condition. They were upset and George was taken off the case, but he later went to check on Bex and told her that she didn't have to be a boy, but she could if she wanted to. He then started to cut her hair at her request. Her parents came in and her mother took over the haircut.
She is married to Mr. Singleton. They have one child.
When Mary and her husband found out their child is intersex, they tried have all of Bex's male sex organs removed without telling her. However, when Addison refused to do it and George refused to lie to Bex, they were pushed into telling Bex, who then asked for a haircut, which George started and Mary finished.