I think that the parenting gene is present--or not--from birth. There are men and women who just seem to take parenting in stride. And there are those who will never be a parent no matter how many biological children they have. Most of us know that kind. They're the people who just can't seem to care what their children need, whether it's affection, vegetables, or clothes.
I believe those type of people have an inherent selfishness that prevents the parenting gene from working. That's too bad because there are tons of people out there with a functioning parenting gene who would love to have a child and can't.
There are a lot of single people who have the parenting gene in full force. These people often step in and take responsibility for a child who needs an extra boost or a child that's neglected by those "other kind" of parents. Biology is not an issue. Love and commitment are the important facets in the relationship.
There are a lot of throw away children out there in the world because the adults responsible have a defective parenting gene. Part of that defect is their wish to have a child so that they have someone who loves them unconditionally. Unfortunately, much like the people who get puppies and kittens because they're cute, people who have babies because they're cute usually find that the cuteness wears off pretty quickly.
If you're one of those singles with the parenting gene who's stepped up to the plate, thank you. If you're one of the thousands of parents with a clutch of children of your own who's also taken a few more under your wing, thank you.
If you one of those people without the parenting gene... please don't have any more. We already have more uncared for children than we can handle.