You are mapping genes on chromosome 9 of the giant anteater. You are studying three genes that appear to be linked. One gene is sharp claws (SC). One gene is for long tongue (LT). One gene is for grey fur (GF). The recombination frequency between SC and LT is 14%. The recombination frequency between LT and GF is 16%. The recombination frequency between GF and SC is 27%. What is the likely order of these genes with respect to one another on chromosome 9?

Question

You are mapping genes on chromosome 9 of the giant anteater.  You are studying three genes that appear to be linked. One gene is sharp claws (SC).  One gene is for long tongue (LT).  One gene is for grey fur (GF).  The recombination frequency between SC and LT is 14%.  The recombination frequency between LT and GF is 16%.  The recombination frequency between GF and SC is 27%.  What is the likely order of these genes with respect to one another on chromosome 9?

BlackTom AI · Accepted Answer

We’re given three genes SC (sharp claws), LT (long tongue), and GF (grey fur) with pairwise recombination frequencies: SC-LT = 14%, LT-GF = 16%, and GF-SC = 27%. First, note that when three genes are linked on a chromosome, the two adjacent intervals typically show the smallest recombination frequencies, while the recombination frequency between the two outer genes tends to be larger (roughly the sum of the two adjacent intervals, possibly reduced by double crossover events).

Option analysis:
- Option A: These genes are not linked
  This would imply no linkage and recombination frequencies around 50% for all pairs, which contradicts the given low values (14%, 16%, 27%). Thus, not linked is inconsistent with the data.
- Option B: SC-LT-GF
  Here, the adjacent pairs would be SC-LT = 14% and LT-GF = 16%, which matches the provided values. The recombination between the outer genes SC and GF would be the sum of the two adjacent intervals (14% + 16% = 30%) in the simplest additive expectation, though actual outer recombination can be slightly reduced by double crossovers, yielding 27% as given. This order is consistent with all three pairwise frequencies.
- Option C: SC-GF-LT
  If SC-GF were adjacent, the SC-GF recombination would be 27%, and GF-LT would be 16%. Then the SC-LT distance would be the sum 27% + 16% = 43%, which does not match the observed 14%. This contradiction shows this order cannot be correct.
- Option D: LT-SC-GF
  If LT-SC were adjacent, LT-SC would be 14% (okay), but then SC-GF would be the second adjacent interval and should be 27% in this arrangement, while LT-GF would be the sum 14% + 27% = 41%, not 16%. This fails to fit the given data.

In evaluating all options, only the arrangement where SC and LT are adjacent with 14% and LT and GF are adjacent with 16% remains consistent with all three observed recombination frequencies, while the outer SC-GF frequency (27%) can be explained by the two-step distance adjusted for crossovers. Therefore, the candidate ordering that fits best is the SC-LT-GF arrangement.

Taken together, the data align with SC and LT being close together, LT and GF being the next distant pair, and SC and GF being the farthest pair among the three, which matches the SC-LT-GF configuration.

Bio 93 Section CDE 2025 Final- Section E

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