Why two-headed animals are so common and why were the born this way?

Question

is mutated genes common now lol

Answer 1

It's probably mutations in Hox genes, which control development of major limbs, etc. Hox genes basically tell the cells in the body when to form new parts, and when you have problems with them, you can get pretty crazy mutations.
Also, if you transplant Hox genes between species, you can force them to grow different body parts.
However, two headed animals aren't terribly common. I think they are most common in heavily polluted environments where heavy metals, etc, can mess up developmental pathways.

Answer 2

I assume you mean real 2 headed, single- bodied and not fused twins. They are usually from an injury to the cephaic (front) of the embryo after the neural tube has formed but before brain formation is completed. It seems to be more successful in lower organisms. Mammals usually don't survive. You can try it yourself with a hot needle and frog, turtle, or chicken embryos. Michael

Answer 3

Genetic Modification

Answer 4

Most likely in-breeding !!