The distance between the genes is measured by

Options

A. angstrom
B. map unit
C. Dobson unit
D. millimetre

Explanation

To solve the question regarding how the distance between genes is measured, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks about the unit used to measure the distance between genes on a chromosome.

2. **Identify the Options**: The options provided are:
- A) Angstrom
- B) Map unit
- C) Toxin unit
- D) Millimeter

3. **Recall Genetic Terminology**: In genetics, the distance between genes is often measured in terms of recombination frequency, which is related to the concept of crossing over during meiosis.

4. **Define Map Unit**: The term "map unit" is synonymous with "centimorgan." A centimorgan (cM) is defined as the distance between genes for which 1% of the time, crossing over occurs.

5. **Differentiate Between Units**:
- **Centimorgan (cM)**: Represents 1% recombination frequency.
- **Decimorgan**: Represents 10% recombination frequency.
- **Morgan**: Represents 100% recombination frequency.

6. **Evaluate the Options**:
- **Option A (Angstrom)**: A unit of length used to measure atomic scales, not applicable here.
- **Option B (Map unit)**: Correct, as it refers to centimorgans used to measure genetic distance.
- **Option C (Toxin unit)**: Not relevant to genetics.
- **Option D (Millimeter)**: A unit of length, not used for measuring genetic distance.

7. **Conclusion**: The correct answer is **Option B: Map unit**, as it is the recognized unit for measuring the distance between genes based on recombination frequency.

### Final Answer:
The distance between the genes is measured by **Map unit** (Option B).