If the chromosome number was not reduced during meiosis what would happen during fertilization?

Then the number of chromosomes in each generation would be double that of the parents. This would not be possible as the number of chromosomes in the nucleus is specific to the species of animal. For example, having just one extra chromosome in a human baby results in Downs Syndrome. Sometimes mutations such as changes in genes or the number of chromosomes can make the resulting zygote non viable and therefore the body can reject it.