93. A scientist performs a series of experiments to determine the recombination frequencies between the
following genes. He acquires the following data: P – Q: 3%; Q – R: 2%; R – S: 13%; P – S: 8% Which one of the
following represents the correct order of genes?
(1) PQRS
(2) QPSR
(3) SPQR
(4) PRSQ

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Introduction:

In genetics, determining the order of genes on a chromosome is crucial for understanding inheritance patterns and genetic linkage. Recombination frequencies, calculated from experimental data, play a key role in determining this order. In this article, we will explore how recombination frequencies help scientists map genes and deduce the correct order of genes on a chromosome using an example with the genes P, Q, R, and S.

Understanding Recombination Frequencies:

Recombination frequencies are a measure of how often a crossover event occurs between two genes during meiosis. This frequency is directly proportional to the physical distance between the genes on the chromosome: the greater the distance, the higher the recombination frequency.

The recombination frequency between two genes can be calculated by dividing the number of recombinant offspring by the total number of offspring. This is often expressed as a percentage, which gives us an idea of the relative distances between genes.

The Experiment and Data:

In this experiment, the scientist has determined the recombination frequencies between four genes: P, Q, R, and S. The data provided are as follows:

• P – Q: 3%

• Q – R: 2%

• R – S: 13%

• P – S: 8%

Now, the goal is to determine the correct order of the genes along the chromosome based on these recombination frequencies.

Analyzing the Data:

To deduce the correct gene order, we need to look for clues in the recombination frequencies:

1. P – Q: 3% and Q – R: 2%: These values suggest that genes P, Q, and R are closely linked together.

2. P – S: 8%: This suggests that gene P is somewhat further from gene S than from Q or R.

3. R – S: 13%: The larger recombination frequency indicates that R and S are further apart compared to the others.

Based on this analysis, we can start to order the genes:

• Since P is close to both Q and R, it is likely placed at one end of the sequence.

• Q and R are also closely linked, suggesting that R and S are farther apart, which places S towards the end of the sequence.

Determining the Correct Order:

From the recombination frequencies, we can deduce the following order of genes:

• P – Q – R – S

This order places P and Q near each other, followed by R, and finally S at the other end. The data supports this order because the recombination frequencies make sense based on the distances between the genes.

Conclusion:

The correct order of genes based on the provided recombination frequencies is P – Q – R – S. This order aligns with the data from the experiment, where the smaller recombination frequencies suggest closer proximity of genes, and the larger values indicate genes that are farther apart on the chromosome.