Questions: Genetic Mapping and Recombination Frequency

Question 1

Two genes on the same chromosome show a recombination frequency of 48%. What is the most accurate interpretation?

• A. The genes must be on different chromosomes because 48% is nearly the 50% threshold for independent assortment
• B. The genes are linked but far apart — the observed 48% likely underestimates true genetic distance because double crossovers restore parental configurations
• C. The genes are 48 physical base pairs apart on the chromosome
• D. The genes are in strong negative interference, suppressing crossovers between them

Answer: (B)

A recombination frequency of 48% does not mean the genes are on different chromosomes — genes that are far apart on the same chromosome can recombine so frequently that they approach 50%, appearing to assort independently. The observed frequency likely underestimates the true genetic distance because double crossovers (two crossover events between the genes) restore the parental configuration and are counted as non-recombinant offspring. Mapping functions correct for this underestimation. Option C confuses genetic distance (in centimorgans) with physical distance (in base pairs) — these do not scale linearly.

Question 2

A testcross of an Ab/aB individual crossed with ab/ab produces 218 Ab, 220 aB, 31 AB, and 31 ab offspring (500 total). What is the recombination frequency, and what does it tell you?

• A. 50% — the near-equal parental and recombinant classes indicate independent assortment
• B. 31% — calculate by dividing recombinant count by the larger parental count
• C. 12.4% — the 62 recombinant offspring (AB + ab) divided by 500 total, so the loci are 12.4 cM apart and linked
• D. 62% — the recombinant gametes are more common than expected for linked genes

Answer: (C)

Recombination frequency = recombinants / total = (31 + 31) / 500 = 62/500 = 12.4%. The parental classes (Ab and aB, ~438 total) vastly outnumber the recombinant classes (AB and ab, 62 total), confirming the genes are linked. 12.4% recombination = 12.4 cM. The testcross design is essential: crossing with the homozygous recessive (ab/ab) makes each offspring's phenotype directly reveal which gamete type the heterozygous parent produced, since the recessive parent contributes only recessive alleles.

Question 3

A recombination frequency of 50% between two loci generally means those loci are located on different chromosomes.

• T. True
• F. False

Answer: False

This is a critical misconception. Genes that are on the same chromosome but very far apart can recombine so frequently — through multiple crossovers — that they behave as if they assort independently, producing ~50% recombinant offspring. The 50% recombination frequency is the ceiling for any pair of loci, regardless of whether they are on the same or different chromosomes. 50% indicates independent assortment behavior, but it does not prove the genes are on different chromosomes. Only values significantly below 50% reliably indicate physical linkage.

Question 4

Double crossovers between two loci can cause the observed recombination frequency to underestimate the true genetic distance between them.

• T. True
• F. False

Answer: True

When two crossovers occur between the same pair of genes, the two exchange events cancel out — the result is a chromosome that looks parental (carrying the original allele combinations), not recombinant. These double-crossover offspring are therefore counted in the parental class, even though two crossovers actually occurred. The observed recombination frequency is thus lower than the true frequency of crossover events. This underestimation worsens with increasing distance, which is why observed frequencies plateau near 50% for distant genes and why mapping functions (Haldane, Kosambi) are needed to correct for multiple crossovers.

Question 5

Why doesn't recombination frequency scale linearly with physical distance in base pairs, and what are two distinct reasons that genetic and physical maps diverge?

Model answer: Recombination frequency measures crossover probability, which is influenced by local chromatin structure, DNA sequence context, and regulatory factors — not just raw base-pair distance. Two reasons genetic and physical maps diverge: (1) Double crossovers — two crossover events between distant loci cancel out and are counted as parental, causing observed recombination frequency to plateau below 50% regardless of how far apart the genes physically are. (2) Recombination hotspots — specific genomic regions have crossover rates 10–100× the average, so a small physical stretch may correspond to a large genetic distance, while adjacent gene-dense regions may be recombination deserts with nearly zero map distance despite covering many base pairs.

These two effects operate at different scales: double crossovers distort measurements at larger genetic distances (>20–30 cM), while hotspots create local heterogeneity across the entire genome. Both mean that a genetic map (in cM) and a sequence map (in Mb) of the same chromosome will look different in scale and shape. Regions near centromeres tend to have suppressed recombination (expanding physical distance per cM), while telomeric regions often have elevated recombination (compressing physical distance per cM).