Recombination between three linked genes is 10%, 4% and 14%, how do I get the gene order?
Pooja Iyer12th Sample
7 Sept 2026 · 1 min read
ok so I'm stuck on gene mapping in the linkage part of Principles of Inheritance. NCERT says Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between them and "mapped" their positions. Fine, I get the idea, but then I met a question: three genes A, B, C are on the same chromosome. Recombination between A and B is 10%, between B and C is 4%, between A and C is 14%. Find the order.
What I did: biggest number = farthest apart, so A and C are at the two ends. Then B has to be somewhere... I wrote A–B–C because the letters looked natural lol. But what if it was B–A–C and I just got lucky? I also don't really see why recombination % should mean distance. Morgan's group said tightly linked genes show very low recombination, but why? And I once saw a statement with "50%" and got confused about whether there is an upper limit. Oh and which stage does crossing over happen in, I keep mixing it up with synapsis.
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[+][–]Tanvi Deshmukh12thSample· 1mo ago
You didn't get lucky, you can actually check it! 1% recombination = 1 map unit (cM), and distances add up along a straight line. If the order was B–A–C, then B–C would be B–A + A–C = 10 + 14 = 24%, but the question says 4%. Doesn't fit. With A–B–C: A–B + B–C = 10 + 4 = 14 = A–C. Fits perfectly! So the rule is simple, the two ends are the pair with the biggest value, and the gene in the middle is the one whose two distances add up to that biggest value. Do this check every time, it takes 10 seconds.
[+][–]Pooja Iyer12thSample· 1mo agoAuthor
omg the add-up check is so clean, thank you. I was trying to pick the middle gene by feel
[+][–]Siddharth RaoDropperSample· 1mo ago
On the "why". Crossing over happens at random points along the paired chromosomes. If two genes are far apart there are more places in between where a chiasma can form, so they get separated more often. Genes sitting very close almost never get a break between them, so they travel together (that's your "parental combinations much higher than non-parental" line).
The 50% thing: if two genes show 50% recombinants in a test cross, they are behaving exactly like genes on different chromosomes, i.e. independent assortment. So recombination frequency of linked genes stays below 50%, even if they're really far apart on the same chromosome.
[+][–]Nisha Verma12thSample· 1mo ago
Crossing over is in meiosis II, zygotene stage, when the chromatids pair up and exchange pieces. That's the one I wrote in my notes at least. And yes below 50 always.
[+][–]Kavya NaikDropperSample· 1mo ago
Careful, I think that stage is wrong. Crossing over is in prophase I of meiosis, specifically pachytene, and it's between non-sister chromatids of the homologous pair. Zygotene is when synapsis (pairing of homologues) happens, so the two get mixed up very easily. Then in diplotene the homologues start separating and you can see the crossing points as chiasmata.
My old trick: Z for zygotene = Zipping up the pair, P for pachytene = Pieces swapped. And meiosis II is basically like mitosis, no pairing of homologues there so no crossing over.
[+][–]Nisha Verma12thSample· 1mo ago
ahh yes I definitely mixed it with synapsis. pachytene it is, noting it down
[+][–]Aarav Kulkarni11thSample· 1mo ago
not an expert but I made a tiny list from NCERT while reading this: Morgan's cross with yellow-bodied, white-eyed female and brown-bodied, red-eyed male... the two genes were on X chromosome and stayed together in the F2 far more often than 9:3:3:1 would predict. so parental types >> recombinants. then he named the swapping "recombination". is the 9:3:3:1 breaking down the real proof of linkage?
[+][–]Tanvi Deshmukh12thSample· 1mo ago
yes aarav, that's exactly the point. Independent assortment predicts 9:3:3:1 in a dihybrid F2 (or 1:1:1:1 in a test cross). If you get way more parental types than that, the genes can't be assorting independently, hence linked.
