# Why do we abbreviate trisomy as n + 1?

Oct 10, 2017

Here's the reason.

#### Explanation:

Firstly I would like to throw some light that $n + 1$ is not an abbreviation of trisomy.

General abbreviation of trisomy is $2 n + 1$.

$n$ is the haploid number of chromosomes at a certain sequence.

Now, it's diploid is $2 n$.

Now, haploidy & diploidy are common intermediates of chromosomes.

But, an abnormal count of chromosomes is called aneuploidy.

Some aneuploid conditions are $\rightarrow$

(1). 2n − 1$\rightarrow$ monosomy.
(because only one copy of same specific chromosome is present).

(2). $n + 1$ $\rightarrow$ disomy.
(because two copies of same specific chromosome is present).

(3). $2 n + 1$$\rightarrow$trisomy.
(because three copies of same specific chromosome is present).

(4). 2n − 2 $\rightarrow$nullisomy.
(because no copy of a specific chromosome is present).

Hope it Helps:).