If the idiogram of Allium cepa shows 4 metacentric, 4 submetacentric, 4 acrocentric and 4 telocentric chromosomes, what is the ratio of centromeres to arms?

Options

A. ` 1:2`
B. ` 3:1`
C. ` 1:1`
D. ` 4:7`

Answer

To find the ratio of centromeres to arms in Allium cepa (onion) based on the given idiogram, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Types of Chromosomes**:
- We have 4 metacentric, 4 submetacentric, 4 acrocentric, and 4 telocentric chromosomes.

2. **Determine the Centromeres**:
- Each chromosome has 1 centromere.
- Therefore, for each type:
- Metacentric: 4 chromosomes × 1 centromere = 4 centromeres
- Submetacentric: 4 chromosomes × 1 centromere = 4 centromeres
- Acrocentric: 4 chromosomes × 1 centromere = 4 centromeres
- Telocentric: 4 chromosomes × 1 centromere = 4 centromeres
- Total centromeres = 4 + 4 + 4 + 4 = 16 centromeres.

3. **Determine the Arms**:
- Metacentric chromosomes have 2 arms each.
- Submetacentric chromosomes have 2 arms each.
- Acrocentric chromosomes have 2 arms each.
- Telocentric chromosomes have 1 arm each (since the short arm is absent).
- Therefore, for each type:
- Metacentric: 4 chromosomes × 2 arms = 8 arms
- Submetacentric: 4 chromosomes × 2 arms = 8 arms
- Acrocentric: 4 chromosomes × 2 arms = 8 arms
- Telocentric: 4 chromosomes × 1 arm = 4 arms
- Total arms = 8 + 8 + 8 + 4 = 28 arms.

4. **Calculate the Ratio of Centromeres to Arms**:
- The ratio of centromeres to arms = Total Centromeres : Total Arms
- Ratio = 16 : 28.

5. **Simplify the Ratio**:
- To simplify the ratio, divide both numbers by their greatest common divisor (GCD), which is 4.
- 16 ÷ 4 = 4
- 28 ÷ 4 = 7
- Therefore, the simplified ratio is 4 : 7.

### Final Answer:
The ratio of centromeres to arms in Allium cepa is **4 : 7**.

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