Final answer:
To detect cross-overs between closely linked genes, a large population is needed to ensure sufficient sample size for calculating recombination frequencies and accurately mapping the genes on chromosomes.
Step-by-step explanation:
For obtaining cross-overs between closely linked genes, a very large population is required. This is due to the fact that when two genes are located in close proximity on the same chromosome, they are considered linked, and their alleles are more likely to be transmitted through meiosis together. However, as the distance between two genes increases, the probability of crossovers between them also increases. Geneticists use the proportion of recombinant gametes to measure the genetic distance between linked genes. To detect crossovers and hence calculate the recombination frequency for closely linked genes, a large population is necessary to ensure a statistically significant sample size. This will then allow for accurate mapping of the genes on chromosomes.