Match Column I with Column II and select the correct answer using the codes given below :
Column I - Column II
(A) Monosomic - (i) 2n+2
(B) Nullisomic - (ii) 2n+1
(C) Trisomic - (iii) 2n-1
(D) Tetrasomic - (iv) 2n-2

Question

Match Column I with Column II and select the correct answer using the codes given below : 
Column I - Column II
(A) Monosomic - (i) 2n+2 
(B) Nullisomic - (ii) 2n+1
(C) Trisomic - (iii) 2n-1 
(D) Tetrasomic - (iv) 2n-2

Accepted Answer

The correct option is 4: A(iii) B(iv) C(ii) D(i)

This question tests your fundamental knowledge of aneuploidy, which is the condition of having an abnormal number of chromosomes in a cell, often due to the gain or loss of one or more whole chromosomes. The base somatic chromosome number for a diploid organism is represented by $2\text{n}$. All four terms in Column I describe variations on this $2\text{n}$ state. **Monosomy** is the genetic condition where an organism has lost one chromosome from a single homologous pair, resulting in the formula $2\text{n}-1$. An example is Turner syndrome in humans ($45, \text{X0}$). **Nullisomy** involves the loss of an entire homologous pair of chromosomes, making the cell $2\text{n}-2$. This is generally lethal in diploid organisms but is occasionally studied in polyploids. **Trisomy** is the presence of an extra chromosome in one homologous pair, yielding the formula $2\text{n}+1$; the classic human example is Down syndrome (Trisomy 21). Finally, **Tetrasomy** is the gain of two chromosomes, meaning an entire extra homologous pair is present, resulting in the formula $2\text{n}+2$. The correct matching sequence is therefore (A) Monosomic $\rightarrow$ (iii) $2\text{n}-1$, (B) Nullisomic $\rightarrow$ (iv) $2\text{n}-2$, (C) Trisomic $\rightarrow$ (ii) $2\text{n}+1$, and (D) Tetrasomic $\rightarrow$ (i) $2\text{n}+2$.

The incorrect option is 1: A(ii) B(i) C(iii) D(iv)

This option incorrectly matches the aneuploidy terms with their corresponding chromosome formulas. Specifically, it states that Monosomic is $2\text{n}+1$ and Trisomic is $2\text{n}-1$. Monosomy always refers to the loss of a single chromosome ($2\text{n}-1$, not $2\text{n}+1$), and Trisomy is the gain of a single chromosome ($2\text{n}+1$, not $2\text{n}-1$). Furthermore, it incorrectly assigns Nullisomic to $2\text{n}+2$ and Tetrasomic to $2\text{n}-2$. Nullisomy is characterized by the *loss* of a pair of chromosomes ($2\text{n}-2$), and Tetrasomy is the *gain* of a pair ($2\text{n}+2$). The pairings in this option are fundamentally reversed for both the single-chromosome (Mono/Tri) and paired-chromosome (Nulli/Tetra) variations.

The incorrect option is 2: A(iv) B(ii) C(i) D(iii)

Option 2 attempts a different set of incorrect pairings, showing a lack of understanding of the numerical prefixes in genetics. It links (A) Monosomic with (iv) $2	ext{n}-2$, which is actually the formula for Nullisomy. It pairs (C) Trisomic with (i) $2	ext{n}+2$, which defines Tetrasomy. Similarly, it incorrectly states (B) Nullisomic as (ii) $2	ext{n}+1$ (Trisomy) and (D) Tetrasomic as (iii) $2	ext{n}-1$ (Monosomy). This configuration systematically misapplies the definitions, confusing the loss of one chromosome with the loss of two, and the gain of one chromosome with the gain of two.

The incorrect option is 3: A(i) B(iii) C(iv) D(ii)

This option represents yet another scramble of the correct biological definitions. It starts by incorrectly matching (A) Monosomic, which means 'one less', with (i) $2	ext{n}+2$, which means 'two more'. It also incorrectly matches (B) Nullisomic, which is 'two less' ($2	ext{n}-2$), with (iii) $2	ext{n}-1$. The pairing of (C) Trisomic (three chromosomes present) with (iv) $2	ext{n}-2$ is wrong, as Trisomy is $2	ext{n}+1$. The final pairing of (D) Tetrasomic (four chromosomes present) with (ii) $2	ext{n}+1$ is also incorrect, as Tetrasomy is correctly defined by $2	ext{n}+2$.