Expert: Dana Krempels, Ph.D. - 12/8/2011

Question
what is the reason for the constant number of chromosomes in allium cepa *onion* from generation to generation?

Answer
Dear Samson,

I am not sure exactly what you are asking.  But if you mean why a daughter Allium cepa has the same number of chromosomes as the parent plants, it's for the same reason that all diploid, sexually reproducing organisms have the same number of chromsomes from generation to generation (at least if things proceed as normal).

It is because during meiosis, one copy of the genome (in the form of half of the chromosomes) is distributed into each gamete.  Upon fertilization, the zygote gains two full copies of the genome again (as all cells in an adult also have), and proceeds to grow into a diploid adult.

Errors in meiosis can cause aneuploidies (incorrect number of any given homologous set of chromosomes) or polyploidy (multiple chromosome sets; much more common in plants than animals).  But in normal circumstances, a sperm and egg each donate one set of chromosomes, restoring the normal condition (diploid) to the zygote.

Hope that helps.

Dana