I read this story in an anthology in the late 80's or early 90's.
The story deals with a father and son at a zoo, and the father describing how radiation has mutated the animals over the years. Towards the end of the story, the boy gets attacked and loses an arm. When the father asks the doctor if his son will be okay the reply is that he'll be fine humans did great with only two arms.
I think it was probably in an Alfred Hitchcock anthology but I'm not sure. I checked it out from the school library.
On a side note, I found someone else asking about the story on another site in 2012, but they never got a response.