创世记 38:27-30

"And it came to pass in the time of her travail, that, behold, twins were in her womb. And it came to pass when she travailed, that one put out a hand; and the midwife took and bound upon his hand a scarlet thread, saying, This came out first. And it came to pass, as he drew back his hand, that, behold his brother came out; and she said, Wherefore hast thou made a breach for thyself?, therefore his name was called Perez. And afterward came out his brother, that had the scarlet thread upon his hand: and his name was called Zerah."

Such details were recorded because of the importance of the key link in Messiah's line. Perez, the firstborn, was the one through whom Jesus came. It was only an oddity that the firstborn was not Zerah; and perhaps the ancients saw in this a figure of how narrowly the Messianic line was spared the necessity of passing down from the daughter of Shua the Canaanite.

"This incident testifies to the importance and privileges attached to the firstborn."[29]

It is inconceivable that some scholars, vainly trying to make this event fit into the story of Joseph, which they erroneously suppose to be the subject of this section of Genesis, appear to be totally blind to the fundamental importance of this narrative regarding the posterity of Jacob, which, of course, is the true subject all the way to the end of Genesis.