Although today we know that the shared DNA from an organism’s parents determines biological characteristics such as appearance, the ancient Israelites had no such conceptions, nor any way to know that. Because of this, we find that the Israelites of the Old Testament believed that the appearance of the offspring was based on what the mother was looking at when she conceived.
When Jacob and Laban were involved in their dispute over Jacob’s wages, Laban agreed to give Jacob all the livestock that had unusual markings, such as stripes and spots. He then gave any livestock with those unusual markings to his sons so that they would not be passed on to Jacob. Jacob was responsible for the breeding of the livestock, so he placed them in pens where the livestock would see mottled patterns and so would breed babies with those patterns. He did this with the strongest of the livestock so that only the healthiest livestock would inherit the patterns of those that were to be Jacob’s.
Although this is not our scientific view of inheritance, it makes sense in the confines of the ancient worldview and helps explain an otherwise unusual story in Genesis.