Midrash sur L’Exode 21:19

2) "He has cursed his father and his mother": What is the intent of this? Because it is written (Shemoth 21:19) "And he who curses his father and his mother shall be put to death," I might think that he is not liable until he curses both of them; it is, therefore, written "He has cursed his father; ("He has cursed) his mother" — either one. A proselyte, then, is liable for (cursing) his mother, but not for (cursing) his father, (there being no "fatherhood" for a proselyte.) These are the words of R. Yossi Haglili. R. Akiva says: "He has cursed his father and his mother": One who is liable for (cursing) his father is liable for (cursing) his mother. One who is not liable for his father is not liable for his mother. R. Akiva concedes that a shtuki (an illegitimate child of unknown fatherhood) is liable for (cursing) his mother even though he is not liable for (cursing) his father.