What type of a mutation leads a creature to be born with an extra chromosome?

What type of a mutation leads a creature to be born with an extra chromosome? - an example is Downs Syndrome (one extra chromosome number 21) - results from a failure of some chromosomes to separate during meiotic production of the gametes (non-disjunction).

An extra chromosome is not a mutation, it is a failure during gamete production and is a cell malfunction.

A mutation is a change in the DNA sequence and affects genes, not chromosomes. The chromosome is only a package of genes and comes in different numbers in different species. Evolution of traits depends on changes in genes alone.

Some plants have fewer than 46 chromosomes, but have far more genes contained within them than human chromosomes. Domestic dogs have 78 chromosomes. Domestic horses and wild horses have different numbers of chromosomes, yet the same genetic information - the extra one in the wild horse is a small part of a large chromosome in the domestic horse.

The increase in genetic material originates in part from duplications of small sections of DNA, not whole chromosomes - a whole chromosome duplication would simply upset the complex interactions between genes and their products.

So for evolution think genes not chromosomes

Check out polyploidy - common in plants and less derived animal groups - even a few vertebrates. Rather than a gene mutation, this is a copy-mistake involving a string of dozens or hundreds of genes. Sometimes the entire genome is duplicated.

http://www.britannica.com/EBchecked/topic/469046/polyploidy