An organism with one extra chromosome (2n+1) instead of the normal diploid (2n) is known as:

• The correct answer is trisomic. Here's why:
● Monoploid: An organism with only one set of chromosomes (n) instead of the typical diploid number (2n).
● Monosomic: An organism missing one chromosome from a pair in an otherwise diploid set (2n-1).
● Trisomic: An organism with one extra chromosome, resulting in three copies of a particular chromosome instead of the usual two (2n+1).
● Tetrasomic: An organism with four copies of a particular chromosome instead of the usual two (2n+2).

🔑IMP Key Points:
• A normal human cell consists of 46 chromosomes (23 pairs), which is represented as 2n = 46
• So, a normal human cell is diploid.
• If by chance due to chromosomal mutation there is an addition or loss of one or a few chromosomes, this condition is called Aneuploidy.

✅Explanation:
The Correct option is 2 n – 1 = Monosomy, 2 n + 1 = Trisomy. This can be explained as

• Aneuploidy is of two types:
1. Hypoaneuploidy (loss)
• 2n – 1 = monosomy (loss of one chromosome only) i.e. the presence of a single copy of a particular chromosome. Eg – Turner's syndrome.
• 2n – 2 = Nullisomy (loss of a pair of the homologous chromosome)
2. Hyperaneuploidy (addition)
• 2n+1 = Trisomy: addition of one chromosome in one set. Eg – Down's syndrome
• 2n + 2 = Tetrasomy: addition of two chromosomes in one set.