Q.Distance between the genes and percentage of recombination shows:

The distance between two genes on a chromosome is directly proportional to the frequency of recombination between them. This is because crossing over is more likely to occur between genes that are farther apart, leading to a higher percentage of recombinant offspring. Conversely, genes that are very close together recombine less frequently. This principle is the foundation of genetic mapping, where the recombination frequency (expressed as a percentage) is used as a measure of the physical distance between genes.

• A direct relationship means that as one value increases, the other also increases.
• An inverse relationship means that as one increases, the other decreases.
• A parallel relationship is not a standard term for this concept.
• No relationship would mean the two are independent, which is false.

Therefore, the correct option is the one that states the two quantities increase together.

This relationship is one of the most elegant and practically useful ideas in genetics. It comes directly from the behaviour of chromosomes during meiosis, specifically during crossing over in prophase I.

When two genes are located on the same chromosome, they tend to be inherited together — we call them linked genes. But linkage is rarely absolute. During meiosis, homologous chromosomes exchange segments in a process called crossing over. If a crossover occurs between the two genes, the alleles get reshuffled, producing recombinant gametes. The key point is this: the chance that a crossover will happen between two given genes depends on how far apart they are on the chromosome.

Think of it like a long rope. If you randomly cut the rope at one point, the chance that the cut falls between two specific marks is higher if those marks are far apart. Similarly, the greater the physical distance separating two genes, the more likely a crossover event will occur between them. And more crossovers mean more recombinant offspring.

This direct relationship is the foundation of genetic mapping. By calculating recombination frequencies from test cross data, geneticists can determine the relative positions of genes on a chromosome. A recombination frequency of 1% is defined as one map unit (or centimorgan). So if genes A and B show 12% recombination, they are 12 map units apart. If genes B and C show 5% recombination, they are 5 map units apart. This allows us to build a linear map of gene order and distance.

Now let's look at the options given in the question:

• Direct relationship — This is correct. As distance increases, recombination percentage increases.
• Inverse relationship — This would mean that greater distance gives less recombination, which is the opposite of what happens.
• Parallel relationship — This is not a standard term in genetics for this context. Distance and recombination do not change in parallel in the sense of being equal; rather, one causes the other.
• No relationship — This is false. The relationship is well-established and forms the basis of chromosome mapping.

The NCERT textbook explicitly states that the frequency of recombination is directly proportional to the distance between genes. This is a core principle in the chapter on Principles of Inheritance and Variation.

Picture two pins stuck into a long piece of string representing a chromosome, and a pair of scissors making one random cut somewhere along its length: the farther apart the two pins are, the more of the string lies between them, and so the higher the chance the random cut (crossing over) falls between them rather than outside them. This 'more space between the pins = more chances of a cut landing there' picture is a faster way to recall why greater gene distance means higher recombination frequency than re-deriving it from meiosis diagrams each time.