If you have 13 important chromosomes, one of them is getting left behind when your cells divide to produce gametes.

There are insects that use haplo-diplody (spl?) where the males can have an odd number of chromosomes (n) and females have the regular diploid number (2n)

If you reproduce asexually, I can’t think of a reason why having an odd number of chromosomes would be a problem.

There are some animals where different sexes have a different number of chromosomes, and one of them is an odd number.

I can’t find examples of particular diseases, but there are situations where heterozygous individuals have lower fitness than homozygous individuals. Try searching for disruptive selection or underdominance. Or causes for low hybrid fitness.

In plants there is “hybrid necrosis” as a result of incompatible immune systems. Mammals have “hybrid sterility”.