How are genetic linkage maps constructed in genetics?

• A. By sequencing entire genomes
• B. By measuring recombination frequencies between markers in offspring and converting to map distances
• C. By comparing protein structures
• D. By measuring gene expression levels

Answer: (B)

Genetic linkage maps come from tracking how often two genetic markers are inherited together in offspring. The essential idea is that markers that sit close to each other on a chromosome tend to travel as a unit through meiosis, because a crossover between them is unlikely. By crossing individuals with known marker differences and counting how often recombination occurs between pairs of markers in their offspring, we estimate recombination frequencies. Those frequencies are then converted into map distances (centimorgans), with 1% recombination roughly equal to 1 cM, giving a rough scale of how far apart markers are and the order on the chromosome. More distant markers require corrections for multiple crossovers using mapping functions, but the core principle remains measuring how often markers segregate together.

Sequencing entire genomes, while informative for the DNA sequence itself, doesn’t directly provide the recombination-based distances used to build linkage maps. Comparing protein structures or measuring gene expression levels also don’t reveal the chromosomal proximity of markers, which is what a linkage map encodes.