Eye color is a polygenic trait. This means that the pigmentation of the iris is determined by more than one .

gene

Explanation:

The two main themes reflected in this excerpt are:

the importance of family commitmentthe imortance of following one's dreamsThe longest part of the text consists of a description of how much Andrew and his parents are going to miss Robert as he leaves, and vice versa. Andrew even explains why it will be particularly difficult in this case: the two brothers have a special bond ("you and I ain’t like most brothers ... we’ve always been together"). The stage directions also indicate how moved the characters are about this departure ("with feeling"), and so does the punctuation, with exclamation marks and short sentences which usually express emotion ("Andy—believe that!").It is precisely this emphasis on the family bond that shows how much Robert's dream is important to him, because he is willing to suffer through separation to fulfill his desire to travel the world: "I hate to leave you and the old folks—but—I feel I’ve got to. There’s something calling me."