Which of the following statements is not true of two genes thath show 50% recombination frequency?

Options

A. The genes may be on different chromomsomes
B. The genes are tightly linked
C. If the genes are present on the same chromosomes, they undergo more than one crossovers in every meiosis.
D. The frequency of crossing over is inversely proportional to the distance between two genes

Answer

To determine which of the statements is not true regarding two genes that show a 50% recombination frequency, we can analyze each option based on our understanding of genetic linkage and recombination.

### Step-by-Step Solution:

1. **Understanding Recombination Frequency**:
- Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. A 50% recombination frequency indicates that the genes assort independently, similar to genes on different chromosomes.

**Hint**: Remember that a 50% recombination frequency suggests independent assortment.

2. **Analyzing Option A**:
- **Option A** states that "the genes may be on different chromosomes." This statement is true because genes that are on different chromosomes will assort independently, leading to a 50% recombination frequency.

**Hint**: Consider the implications of genes being on different chromosomes.

3. **Analyzing Option B**:
- **Option B** states that "the genes are tightly linked." This statement is false because tightly linked genes would have a low recombination frequency, not 50%. A 50% recombination frequency indicates that the genes are not tightly linked.

**Hint**: Think about the relationship between linkage and recombination frequency.

4. **Analyzing Option C**:
- **Option C** states that "if the genes are present on the same chromosome, they undergo more than one crossover in every meiosis." This statement can be true in certain contexts, as multiple crossovers can occur, especially if the genes are far apart.

**Hint**: Consider how crossovers can happen in meiosis.

5. **Analyzing Option D**:
- **Option D** states that "the frequency of genes crossing over is inversely proportional to the distance between two genes." This is a true statement. As the distance between genes increases, the likelihood of crossing over increases, leading to a higher recombination frequency.

**Hint**: Recall the relationship between gene distance and crossing over.

### Conclusion:
The statement that is not true regarding two genes that show a 50% recombination frequency is **Option B**: "the genes are tightly linked."

### Final Answer:
**Option B** is the correct answer as it is not true.