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.

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explanation:

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).