Laban asks Jacob what he would like as a parting gift and Jacob magnanimously asks only that he can take the sheep and goats that are “spotted”, “speckled” or “streaked”. Laban agrees but then secretly orders his men to remove any such sheep and goats from the herd and take them a full three days journey away. Laban, it seems, is just as deceitful as Jacob.
But poor Laban does not understand the breeding knowledge Jacob has acquired. Jacob has learned a method of creating striped and speckled offspring which would amaze today’s geneticists. That's assuming, of course, that this literally happened.
We are told in detail how Jacob manages to alter the physical appearance of Laban’s flock. He takes branches from three types of tree, poplar, almond, and plane, and carves off strips of bark revealing the white underneath and thereby giving the branches a striped appearance (Gen. 30: 37). Then, and this is seemingly crucial, he places the striped branches in the troughs where the flocks come to drink. Also crucial is the fact that the animals happen to mate in front of the troughs. So, according to Jacob’s logic, if the animals mate in front of these striped branches and drink from water that the branches have been placed in, they will give birth to striped or speckled offspring: “And since they bred when they came to drink, the flocks bred in front of the sticks and so the flocks brought forth striped, speckled, and spotted (Gen. 30: 38-39 ESV).”
The power of the striped branches is made clear in the way Jacob goes on to multiply the size of his flock. Whenever the stronger animals were mating, he placed the branches in the trough, but when the weaker animals were mating he took them out. In this way, he was able to make his flock larger than Laban’s: “Whenever the stronger females were ready to mate, Jacob would place the peeled branches in the watering troughs in front of them. Then they would mate in front of the branches. But he didn’t do this with the weaker ones, so the weaker lambs belonged to Laban, and the stronger ones were Jacob’s (Gen. 30: 41-42).”
Since this seems like obvious superstitious nonsense, apologists have had to look hard for a way of making sense of it. They typically point to a subsequent discussion between Jacob and his wives:
"However, God has not allowed him to harm me. If he said, 'The speckled ones will be your wages,' then all the flocks gave birth to speckled young; and if he said, 'The streaked ones will be your wages,' then all the flocks bore streaked young. So God has taken away your father's livestock and has given them to me" (Gen. 31: 7-9).
Therefore, they claim, Jacob knew all along that Yahweh was doing it. But then, why would Jacob go to all the trouble of carving stripes in the branches and putting them in the trough? And if the striped branches had no actual effect, why did he have to put the branches back in the trough each time the strong animals mated and take them out each time the weak animals mated? This makes no sense if he believes Yahweh is doing it all with no help from the branches. The narrator appears to believe that the branches are at least contributing to the change. Even Christian commentators are willing to admit this.
We can start with a rather quaint example from the popular Bible commentary of Adam Clarke. His commentary was written in the 19th Century without the benefit of modern genetic knowledge, but his explanation shows that this idea was quite acceptable until actual science demonstrated otherwise:
“It has long been an opinion that whatever makes a strong impression on the mind of a female in the time of conception and gestation, will have a corresponding influence on the mind or body of the fetus…It is not necessary to look for a miracle here; for though the fact has not been accounted for, it is nevertheless sufficiently plain that the effect does not exceed the powers of nature; and I have no doubt that the same modes of trial used by Jacob would produce the same results in similar cases”
So right up to the 19th Century, the belief was quite acceptable among the faithful. The Catholic New American Bible explicitly acknowledges this in its notes: “Jacob's stratagem was based on the widespread notion among simple people that visual stimuli can have prenatal effects on the offspring of breeding animals.” And the American Bible Society’s Contemporary English Version agrees. In its notes it states, “It was believed by some that what sheep and goats saw at the time of breeding would determine the color of their young.”
Even some Bible teachers who have the benefit of modern genetic knowledge feel the need to defend Jacob's strategy. Renowned creationist Henry Morris wrote, “It is possible that certain chemicals in the wood of these trees – peeled rods of which were actually in the water when the flocks came to drink – were capable somehow of affecting the animals. If nothing else, water treated in this way may have served as an aphrodisiac and fertility promoter.” (The Genesis Record, Baker Books, 1976, 474-477)
And the writer of Genesis clearly thought the placement of striped branches in the troughs at the time the animals mated was a factor in the animals appearance. He also knew his audience would accept this idea as plausible. Whether you think this magical form of biological manipulation actually happened will depend, I suppose, on how deep your faith is and, perhaps, on how little you know about genetic engineering.