Prior to Jesus conception, the angel Gabriel told Mary: "Holy spirit will come upon you, and power of the Most High will overshadow you. For that reason also what is born will be called holy, Gods Son." (Luke 1:35) Evidently, Gods holy spirit caused an ovum in Marys womb to become fertile, transferring the life of Gods firstborn Son from the spirit realm to earth. The holy spirit no doubt saw to it that this Sons perfect male life force canceled out any Adamic imperfection in Marys ovum, while retaining certain genetic traits. Likely, Jesus resembled Mary.
You have to remember that Jesus did not have an earthly father, only an earthly mother, therefore he was 50% perfect at worst. However, since a perfect life-force (causing the conception) canceled out any imperfection existent in Marys ovum, thereby producing a genetic pattern (and embryonic development) that was perfect from its start. Whatever the case, the operation of Gods holy spirit at the time guaranteed the success of Gods purpose.
Ask yourself this question: If god, the person who created the universe, is able to fertilize Mary through Holy Spirit without a physical father, do you think he is uncapable of overcoming the tiny hurdle of Jesus being without sin?