The sentence I'm thinking about is:
Trisomy is an aneuploidy (numerical chromosomal mutation) in which a chromosome is over-represented once.
Organisms like human beings have a pair of each type of chromosome; two Chr-1 two Chr-2 ..... two Chr-23; it has a pair of 23 chromosomes. So they are called diploids(2n, n=total types of chromosome, 23 in humans) (di means two). Now, Trisomy is a condition in which a diploid organism happens to have an extra copy of a particular chromosome (2n + 1), i.e. they have three copies of that chromosome and I'm trying to bring out the presence of a single extra copy of chromosome in the definition.
I want to use a similar phrase to define tetrasomy too in which there are four copies of a particular chromosome- ....over-represented twice.
Can once be used with over-represented/under-represented in this way? If not how can it be framed?