Question

How can one set of parents produce 19 children who all look different and unique but yet have some similar characteristics or traits

Answer 1

Polygenic inheritance. The characteristics of a person aren't controlled completely by a single gene, but rather several genes that generally only make a small contribution to how the child turns out in the outcome. At one point there was a study that showed over 400 genes were linked to height, so imagine any other characteristics of a person. Environmental effects also serve a large purpose in how a child is to turn out. These children may all be growing up in the same environment, however that's not to say they all grow up the same. They could all be treated the same way and react differently to it, this counts both for physical and mental traits. All human's features such as eye colors, hair color and height are determined by several different genes. As an example, there are around 2 major eye color genes, however there are also around 14 different genes that actually contribute in determining a person's exact eye color. Some are bound to look the same, as they have the same parents, however, just because they have the same parents doesn't mean they all will look exactly alike.