Percentage of crossing over is more when

The correct option is linked genes are located far apart from each other.

To understand why this is correct, let us break down the concepts of linkage and crossing over step-by-step:

1. What is Linkage?
Linkage refers to the physical association of genes on the same chromosome. Genes that are located on the same chromosome are called linked genes, and they tend to be inherited together during meiosis.

2. What is Crossing Over?
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis. This process breaks the linkage between genes and creates new combinations of alleles (recombinant gametes).

3. The Relationship Between Distance and Crossing Over:
The probability of a crossover event occurring between two linked genes depends directly on the physical distance between them along the chromosome.
If two genes are located very close to each other, the chance of a chiasma (the point of contact where crossing over occurs) forming precisely in the tiny segment of DNA between them is extremely low. Thus, they tend to remain linked and are rarely separated.
Conversely, if two linked genes are located far apart from each other, there is a much larger physical span of chromosome between them. This increases the likelihood that one or more crossover events will occur in the region separating them.

4. Mathematical Proportionality:
The frequency or percentage of crossing over is directly proportional to the physical distance between the linked genes:
$$\text{Percentage of crossing over}\propto \text{Distance between genes}$$
Therefore, the percentage of crossing over is highest when the linked genes are located far apart from each other.