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

Options

A. (a) The genes show independent assortment
B. (b) If the genes are present on the same chromosome, they undergo more than one crossovers in every meiosis
C. (c) The genes may be on different chromosomes
D. (d) The genes are tightly linked

Explanation

To solve the question regarding which statement is not true of two genes that show a 50% recombination frequency, we can follow these steps:

### Step-by-Step Solution:

1. **Understand 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 are assorting independently.

**Hint**: Remember that recombination frequency is linked to how genes are inherited together or separately.

2. **Linkage and Independent Assortment**: Genes that are located on the same chromosome may show linkage, which means they do not assort independently. However, if the recombination frequency is 50%, it suggests that the genes are either on different chromosomes or are far apart on the same chromosome, allowing for independent assortment.

**Hint**: Think about how the physical location of genes on chromosomes affects their inheritance patterns.

3. **Evaluate Each Statement**:
- **Statement A**: "The genes show independent assortment." This is true for a 50% recombination frequency.
- **Statement B**: "If the genes are present on the same chromosome, they undergo more than one crossover in every meiosis." This is also true, as multiple crossovers can occur.
- **Statement C**: "The genes may be on different chromosomes." This is true, as a 50% recombination frequency can indicate genes on different chromosomes.
- **Statement D**: "Genes are tightly linked." This is false because tightly linked genes would have a recombination frequency of 0%.

**Hint**: Analyze each statement carefully to determine if it aligns with the principles of genetics regarding linkage and recombination.

4. **Identify the Incorrect Statement**: Based on the evaluation, Statement D is the only one that is not true. Tightly linked genes cannot have a recombination frequency of 50%.

**Hint**: Look for contradictions in the definitions of linkage and recombination frequency.

### Conclusion:
The statement that is not true of two genes that show 50% recombination frequency is **D: "Genes are tightly linked."**