20. The maximum frequency of recombination that can occur between two loci is
(1) 25%         (2) 50%
(3) 75%         (4) 100%

The maximum frequency of recombination that can occur between two loci is 50%. This is because recombination frequency reflects the proportion of recombinant offspring resulting from crossover events between two genes during meiosis. When two genes are very far apart on the same chromosome or on different chromosomes, they assort independently, producing up to 50% recombinant gametes, which is the highest observable recombination frequency.

Explanation of Options

• (1) 25%: This value is a possible recombination frequency when two loci are moderately close on a chromosome, but it is not the maximum limit.

• (2) 50%: This is the maximum recombination frequency between two loci. At this frequency, genes assort independently, meaning they behave as if they are on separate chromosomes or very far apart on the same chromosome with multiple crossovers.

• (3) 75%: This is not a possible recombination frequency because the recombination frequency cannot exceed 50%. Values higher than 50% are not biologically meaningful in this context.

• (4) 100%: This would indicate complete recombination all the time, but due to the mechanism of meiosis and chromatids pairing, it is impossible for recombination frequency to be 100%.

Introduction:
In genetics, recombination frequency between two loci is a crucial concept used to measure the genetic distance and linkage between genes on a chromosome. The maximum frequency of recombination observed is 50%, indicating independent assortment. This article explains this concept clearly and discusses the values that recombination frequency can take based on locus proximity.

Detailed Explanation:
Recombination occurs during meiosis when homologous chromosomes exchange portions through crossover events. The frequency of these crossovers between two loci is called the recombination frequency (RF). RF can range from 0% (complete linkage, no recombination) to 50%. A recombination frequency of 50% means that the loci assort independently, usually because they are on different chromosomes or very far apart on the same chromosome.

The reason RF cannot exceed 50% is that each chromosome consists of two chromatids, and crossover events create recombinant and parental chromatids. Due to the nature of meiosis, at most half of the gametes carry recombinant chromosomes. Therefore, anything higher than 50% would contradict the mechanism of chromatid segregation.

This concept is fundamental in genetic mapping, allowing scientists to estimate the genetic distance between genes by observing their recombination frequencies.

Thus, the maximum frequency of recombination that can occur between two loci is 50%.

The correct answer is option (2) 50%.