Early in the book, David discovers that Sophie has six toes. During his narration of the encounter, he tells the reader that the commandments have been drilled into his head over and over again. One of those commands is as follows:
'Then God created woman, also, and in the same image, but with these differences, according to her nature: her voice should be of higher pitch than man's: she should grow no beard: she should have two breasts ...'
That quote by itself is meaningless in comparison to your question. What makes that commandment important is David's response to it after seeing Sophie's toes. The following is his response.
I knew it all, word for word — and yet the sight of Sophie's six toes stirred nothing in my memory. I saw the foot resting in her mother's lap. Watched her mother pause to look down at it for a still moment, lift it, bend to kiss it gently, and then look up with tears in her eyes. I felt sorry for her distress, and for Sophie, and for the hurt foot — but nothing more.
According to Waknuk society and custom, David should be mortified by the sight of Sophie. He should be sick to his stomach and be running to report the abomination that Sophie supposedly is, but he doesn't do that. Not even close. The commandment simply doesn't matter to David. Instead David's response shows that he views Sophie as a completely normal human being. He doesn't see her as more or less capable than the next person. He sees Sophie for who she is regardless of her appearance.
A bit later, in chapter 6, the reader finds this quote:
But what worried them most — and worried the people they talked to when they got back — was to see how the things which are against God's laws of nature flourish there, just as if they had a right to.
The narration describes what the world is like out in the Badlands and the Fringes. Waknuk society banishes and gets rid of anything that is genetically abnormal. They claim that the genetic variation is no good for life, but the above quote clearly shows that thinking is flawed. Things that are not supposed to be living well are doing just the opposite of that. They are not only living, but flourishing. That includes the Fringes people as well. The Fringes are considered less than human by the Waknuk, but it's clear that the Fringes are quite capable humans despite their genetic variations.