Answer: Gaining or losing a chromosome will result in major genetic changes for that horse, as either it will be missing important DNA or will have too much DNA. Either can cause mutations to occur in the animal's genome. Chromosomal changes are caused by an error during either meiotic or mitotic cell division. An error could be that a cell has too much copies of a chromosome, or not enough, which causes the mutations mentioned above. Hope this helps some, I know it's not a super meaty answer.
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