When I was growing up, my father would plant a summer garden. And as many are wont to do, when he planted his vegetables he would carefully place the tiny placard - the picture of what the vegetable should look like printed on a slip of cardboard - at the head of each row. Perhaps uniquely, however, my father would place placards with the pictures facing the plants, so that the casual observer would only have a view of the blank backsides; I asked him about this.
He informed me that it was for the benefit of the plants, so that when they first poked their heads out of the soil they would have a reference to guide them as to what they should aspire to look like.
Jacob tries something similar with Laban's flocks of sheep and goats. To Jacob would belong the dark, the speckled, and the spotted progeny of Laban's herd and - to "help" him along - Laban had removed all of those from the breeding pool. So what does Jacob do? Using shoots of poplar, almond, and plane, Jacob places reminders in front of the bovidae to remind them of what their offspring should look like, and it works: almost every lamb and kid is spotted, speckled, or dark, quite unlike its pure white progenitors.
Even the Rabbis placed some faith in the theory that what the mother (or sprout) sees will influence the eventual appearance: Rabbi Yochanan was so beautiful of face that he used to go and sit by the entrance to the local mikveh (ritual bathhouse), thinking:
When Jewish women come up from their immersion for the sake of a mitzva, after their menstruation, they should encounter me first, so that they have beautiful children like me, and sons learned in Torah like me (Bava Metzia 84a).
Science tells us that while there are environmental factors that act upon children's (and plants') development, to borrow from The Fantasticks, if you plant a radish, you get a radish. So what is the reason for the Bible's depiction of Jacob's attempts to divert what would be the normal expectations of genetic inheritance?
Perhaps it's to open up the possibility of doubt: maybe it was God's will, a miracle, maybe it was an accident, maybe it was the equivalent of Bronze Age science. Without his chicanery, would the result have been the same? In any event, it makes for an interesting story to tell the children, which I suspect was what my father was doing when he gave me his explanation for putting the placards in backwards.