What is the maximum observable recombination frequency for two genes that are linked on the same chromosome?

What is the maximum observable recombination frequency for two genes that are linked on the same chromosome?

• A. 100 percent
• B. 50 percent is the maximum observable recombination frequency for genes on the same chromosome
• C. 0 percent
• D. 25 percent

Answer: (B)

Recombination frequency tells us how often crossing over between two loci creates new allele combinations. For two genes that are linked on the same chromosome, the event that creates recombinants is a crossover between them. In meiosis there are four chromatids, and a single crossover between the two loci alters only two of them, producing recombinant gametes in just half of the resulting gametes. Double crossovers can restore the original parental combinations, so they don’t increase the recombinant fraction beyond this limit. Therefore, the maximum observable recombination frequency for two linked genes is 50 percent. This ceiling also explains why genes far apart on the chromosome or on different chromosomes can appear to assort independently, yielding about 50% recombinants, but never more.

Recombination frequency tells us how often crossing over between two loci creates new allele combinations. For two genes that are linked on the same chromosome, the event that creates recombinants is a crossover between them. In meiosis there are four chromatids, and a single crossover between the two loci alters only two of them, producing recombinant gametes in just half of the resulting gametes. Double crossovers can restore the original parental combinations, so they don’t increase the recombinant fraction beyond this limit. Therefore, the maximum observable recombination frequency for two linked genes is 50 percent. This ceiling also explains why genes far apart on the chromosome or on different chromosomes can appear to assort independently, yielding about 50% recombinants, but never more.