Occasionally a child is born with an extra chromosome 13, but never do you see a child with an extra chromosome 12, why?

This is because some of the trisomies (one extra chromosome, three instead of two) are fatal, births cannot survive.

Theoretically, trisomies can occur with any chromosome, but often result in miscarriage.

The types of autosomal trisomy that survive to birth in humans are:

Trisomy 8 or Warkany syndrome 2

Trisomy 9

Trisomy 13 or Patau syndrome

Trisomy 18 or Edwards syndrom

Trisomy 21 or Down syndrome.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.