The following is from Jimmy Akin’s “Internet Question Box”:
(see link below)
“In most editions of the Douay-Rheims Bible, Genesis 3:15, in which God is addressing the serpent, reads like this:
“I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel.”
In the New American Bible, as in all other modern Bibles, it reads like this:
“I will put enmity between you and the woman, and between your offspring and hers; He will strike at your head, while you strike at his heel.”
The essential difference between these two renderings – or at least the one people always ask about – concerning who will crush the serpent’s head and who the serpent is trying to strike. The Douay-Rheims uses feminine pronouns – she and her – implying that the woman is the person being spoken of in this part of the verse. All modern translations use masculine pronouns – he and his – implying that the seed of the woman is the of that part of the verse.
The reason for the difference in the renderings is a manuscript difference. Modern translations follow what the original Hebrew of the passage says. The Douay-Rheims, however, is following a manuscript variant found in many early Fathers and some editions of the Vulgate (but not the original; Jerome followed the Hebrew text in his edition of the Vulgate). The variant probably originated as a copyist error when a scribe failed to take note that the subject of the verse had shifted from the woman to the seed of the woman.
People notice this variant today because the expression found in the Douay-Rheims has been the basis of some popular Catholic art, showing a serene Mary standing over a crushed serpent.
This is because Christians have recognized (all the way back to the first century) that the woman and her seed mentioned in Genesis 3:15 do not simply stand for Eve and one of her righteous sons (either Abel or Seth). They prophetically foreshadow Mary and Jesus. Thus, just as the first half of the verse, speaking of the enmity between the serpent and the woman, has been applied to Mary, the second half, speaking of the head crushing and heel striking, has also been applied to Mary due to the manuscript variant, though it properly applies to Jesus, given the original Hebrew.
This does not mean that the idea cannot be validly applied to Mary as well. Through her cooperation in the incarnation of Christ, so that the Son of God (who, from the cross, directly crushed the head of the serpent) became her seed, Mary did crush the head of the serpent. In the same way, the serpent struck at Christ on the cross, and indirectly struck at Mary’s heart as well, who had to witness the death of her own Son (cf. John 19:25-27). As the holy priest Simeon had told her years before:
“Behold, this child is set for the fall and rising of many in Israel, and for a sign that is spoken against – and a sword will pierce through your own soul also – that thoughts out of many hearts may be revealed” (Luke 2:34b-35).
Thus Jesus crushed the serpent directly and was directly struck by the serpent; Mary, through her cooperation in the incarnation and her witnessing the sufferings and death of her Son, indirectly crushed the serpent and was indirectly struck by the serpent.
This has long been recognized by Catholics. The footnotes provided a couple of hundred years ago by Bishop Challoner in his revision of the Douay state, “The sense [of these two readings] is the same: for it is by her seed, Jesus Christ, that the woman crushes the serpent’s head.”
For more information, see A Catholic Commentary on Holy Scripture, edited by Bernard Orchard [New York: Thomas Nelson & Sons, 1953], p. 186.”
Click on Mary and Genesis 3:15.