Why would extra chromosomes result to abnormal phenotypes?

Question

Why would extra chromosomes result to abnormal phenotypes?

Answer 1

The condition of having extra chromosomes (polysomy) can be thought of as a sort of genetic information overload.  An extra copy of a chromosome means there's an extra copy of all the genes carried on that chromosome.  Genes encode proteins; too many genes encode too many proteins.

This can cause any number of problems - if the protein is structural (that is, a building block for a part of your body), that part of the body could develop abnormally.  If the protein is an enzyme, or otherwise part of a cellular reaction, that reaction may not proceed properly, if there is too much of that protein.  If the protein is part of a cell signaling network, signals may be sent incorrectly (or at too high an intensity).

Answer 2

Think of all the chromosomes in your cell as chapters in a book.

If I take some chapters and add them in the middle of other chapters and perhaps stick some too in the end and beginning....the story you would read would be pretty messed up....just as you would be if you came out to this world having extra chromosomes (chapters)..

Answer 3

It may cause over-expression of a protein.  Chromosome 21 is the smallest (with the exception of a male Y) and the addition of one extra leads to Down Syndrome which has causes a large number of phenotypic problems