MCAT content library / Biology & biochemistry / Genetic linkage and mapping
Turn offspring counts into a chromosome map, then recognize when recombination frequency stops being a direct measure of physical distance.
About 18 minutes, including the three-point cross and checks.
Useful foundations
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Start with parental and recombinant classes
Genes on the same chromosome are physically linked, but crossing over during meiosis can separate their alleles. Loci that are close together are less likely to be separated by a crossover between them than loci that are farther apart. This is why recombinant offspring can be used to infer relative gene positions.2
In a testcross, the genotype of each offspring can reveal which gamete came from the heterozygous parent. The most common classes are usually the noncrossover, or parental, arrangements. Recombinant classes reflect one or more crossovers. That frequency information is useful only if you keep the denominator explicit: recombinant offspring divided by all scored offspring.
Concept: recombination frequency estimates map distance
For two nearby loci, the recombination fraction is approximately the probability that a meiotic product is recombinant for those loci. A 1% recombinant frequency is conventionally expressed as about 1 map unit, or 1 centimorgan, for short intervals. This is a genetic distance, not a fixed number of DNA base pairs.
The approximation becomes less direct across longer intervals because multiple crossovers can occur. An even number of crossovers between the two loci can restore the parental arrangement of the outer markers and go unnoticed in a simple two-locus count. Genetic map distances can therefore add across intervals even when the observed pairwise recombinant fraction is lower.
Worked application: map three linked loci
An original testcross follows three loci, A, B, and C. A heterozygous individual produces the gametes below, and the tester is recessive at all three loci. Assume every offspring is scored correctly, viability is equal among classes, and no gene conversion occurs.
| Gamete from heterozygote | Offspring count |
|---|---|
| ABC | 360 |
| abc | 350 |
| Abc | 18 |
| aBC | 22 |
| ABc | 120 |
| abC | 110 |
| AbC | 10 |
| aBc | 10 |
The two largest classes, ABC and abc, identify the parental chromosome arrangements. The two smallest classes, AbC and aBc, are the best candidates for double crossovers because two crossovers are generally less frequent than one under the supplied model.
Compare each double-crossover class with the matching parental class. ABC becomes AbC: A and C keep their parental states, while B changes. Likewise, abc becomes aBc: again B is the only locus that switches relative to its parental chromosome. Therefore B is the middle locus, and the gene order is A–B–C.
Pattern: double crossovers reveal the middle locus
Once the order is known, classify the single-crossovers by interval. The A–B single-crossover classes are Abc and aBC. Add the double crossovers because a double crossover includes one crossover in the A–B interval:
A–B recombinants = 18 + 22 + 10 + 10 = 60. The estimated A–B distance is therefore 60/1000 = 0.060, or 6 map units.
The B–C single-crossover classes are ABc and abC. Again include both double-crossover classes:
B–C recombinants = 120 + 110 + 10 + 10 = 250. The estimated B–C distance is 250/1000 = 0.250, or 25 map units.
The resulting map is therefore approximately A — 6 — B — 25 — C, for an additive genetic map distance of about 31 map units from A to C.
| Interval | Recombinant classes counted | Frequency | Map estimate |
|---|---|---|---|
| A–B | Abc, aBC, AbC, aBc | 60/1000 = 0.060 | 6 cM |
| B–C | ABc, abC, AbC, aBc | 250/1000 = 0.250 | 25 cM |
If you ignore B and look only at the outer loci A and C, the directly observed recombinant classes total 270, or 27%. That is lower than the 31-map-unit sum because the double crossovers restore the parental arrangement of A and C. The middle marker makes those otherwise hidden events visible.
Rule: recombination frequency has a 50% ceiling
An observed recombination fraction near 50% does not prove that two genes lie on different chromosomes. Unlinked loci can assort to give 50% recombinant classes, but loci far apart on the same chromosome can also approach 50% because multiple crossovers increasingly erase the relationship between simple recombinant counts and crossover number.
That ceiling creates an important distinction: 50% recombination means the loci behave as though unlinked in that cross. It does not, by itself, establish their physical chromosome locations. Additional markers or direct physical mapping would be needed to distinguish those possibilities.
ACTIVE PRACTICE · GENETIC MAPPING
Infer Gene Order From the Rarest Classes
The worked testcross yielded ABC 360, abc 350, Abc 18, aBC 22, ABc 120, abC 110, AbC 10 and aBc 10 offspring. All eight classes total 1,000.
A–B: 6.0 cM; B–C: 25.0 cM.
Recombination fraction approximates map distance for short intervals; multiple crossovers complicate larger distances. The initial values reproduce the original testcross.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
In a two-locus testcross, 84 of 600 offspring are recombinant. What is the observed recombination frequency?
A. 7%
Review this choice. This would result from dividing 42 by 600, but all 84 recombinant offspring belong in the numerator.
B. 14%
Correct. 84/600 = 0.14, or 14%.
C. 42%
Review this choice. The numerator is the recombinant count, not half of the total offspring.
D. 84%
Review this choice. Eighty-four is a count, not a percentage of 600.
Check 2
For three linked loci, the parental classes are ABC and abc. The two least frequent classes are AbC and aBc. Which locus is in the middle?
A. A
Review this choice. A retains its parental state when each double-crossover class is paired with its corresponding parental class.
B. B
Correct. B is the only locus whose allele changes relative to the parental arrangements in the double-crossover classes.
C. C
Review this choice. C retains its parental state in the double-crossover comparison.
D. The order cannot be inferred from these classes.
Review this choice. Given the stated assumptions, the parental and double-crossover classes are enough to identify the middle locus.
Check 3
Two loci show an observed recombinant frequency of 50% in a large, well-controlled testcross. Which conclusion is justified by that result alone?
A. The loci must be on different chromosomes.
Review this choice. Widely separated loci on the same chromosome can also behave as though unlinked.
B. No crossing over occurred.
Review this choice. A 50% recombinant fraction does not imply absence of crossing over.
C. The loci behave as though unlinked in this cross.
Correct. That is the inference supported directly by the observed frequency.
D. Their genetic distance is exactly 50 centimorgans.
Review this choice. At long distances, observed recombination fraction saturates and is not a direct additive map-distance measurement.
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