Use a testcross to estimate recombination and retain limits

Mechanism before a label

A testcross to a double recessive can make the gametes of a heterozygous parent visible in offspring categories, provided phenotype classification and viability are suitable. The most frequent reciprocal classes often suggest parental phase, but phase evidence should be stated rather than assumed from names.

Recombination frequency is recombinant offspring divided by all scored offspring. For small intervals, percent recombination approximates map distance in centimorgans. Multiple crossovers can restore parental combinations, so observed recombination underestimates long genetic distances. Under the ordinary linkage and segregation model the population recombinant fraction approaches, but does not exceed, 0.5; a finite sample can exceed 50% through sampling variation or model violations. Map distance itself can exceed 50 centimorgans.

Linkage maps are relative genetic maps, not base-pair rulers. Recombination varies by genomic region and biological context. A deficit in a class can also reflect viability or scoring bias, requiring reciprocal crosses and independent evidence.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Testcross offspring	Value (offspring)
AB	82
ab	78
Ab	21
aB	19

A stipulated AB/ab heterozygote crossed to ab/ab; all classes detectable.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Use a testcross to estimate recombination and retain limits

Supplied synthetic evidence: A stipulated AB/ab heterozygote crossed to ab/ab; all classes detectable.

Testcross offspring: reference condition

Quantity	Value	Unit
AB	82	offspring
ab	78	offspring
Ab	21	offspring
aB	19	offspring

Task: Why can a low recombinant percentage support linkage, while 50% cannot prove different chromosomes?

The small-interval distance approximation is not an exact physical distance.

Compare with the conditional interpretation

40/200=20% recombinant offspring under the class assumptions.

Reference recombinant count 21+19=40 out of 200 yields 0.20. Doubling every class count would leave the estimate unchanged but usually improve precision under independent sampling.

Case index for this investigation

0. Track homologs and chromatids through meiosis
1. Explain crossing over and assortment without mixing mechanisms
2. Derive cross probabilities and distinguish expectation from outcome
3. Use a testcross to estimate recombination and retain limits
4. Test an inheritance null with chi-square reasoning
5. Distinguish dominance, codominance and multiple alleles
6. Interpret chromosome-linked and cytoplasmic inheritance
7. Separate genotype, environment and their interaction

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Change the counts, not the prespecified model

Original synthetic practice. Start the case, predict a result, then edit one class. These scratch values are not saved or graded. The four categories must remain mutually exclusive; repeated counts are not new replicates.

Start / reset this calculation

Worked reasoning

Reference recombinant count 21+19=40 out of 200 yields 0.20. Doubling every class count would leave the estimate unchanged but usually improve precision under independent sampling.

40/200=20% recombinant offspring under the class assumptions.

Apply to a new case

Why can a low recombinant percentage support linkage, while 50% cannot prove different chromosomes?

Compare your reasoning with the worked response

Close linked loci retain excess parental combinations. At large separation, multiple crossovers can randomize observed combinations to about one half even on one chromosome; physical location needs other evidence.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

The small-interval distance approximation is not an exact physical distance. Do not attribute the difference to environment without matched design and uncertainty.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 5.3–5.4. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.