Lesson 5 of 7 / Inheritance and genetic problems
Linkage changes gamete probabilities
What do excess parental classes tell you about two loci?
In this lesson: Explain linkage, crossing-over and altered dihybrid/test-cross ratios.
About 8 min
The key ideaLinked alleles are often transmitted together; crossing-over creates recombinant gametes whose frequency can be estimated from an informative test cross.
Build from gametes
How does recombination change gamete frequencies?
Parental arrangement: AB / ab. The tester produces only ab gametes.
Ab and aB are recombinant classes, each 5%. AB and ab are parental classes, each 45%. At 50%, the frequencies resemble independent assortment; this alone does not prove the genes are on different chromosomes.
Bar lengths represent expected proportions, not an observed sample. The equal reciprocal classes assume a simple cross with equal viability. Recombination frequency between two loci does not exceed 50% in this model; multiple crossovers can conceal some exchange events.
Explanation
Write linked phase explicitly. AB/ab means A and B are on one homologue while a and b are on the other. Ab/aB is a different arrangement even though both individuals are AaBb. Without crossing-over between the loci, only the two parental combinations are produced.
A crossover between non-sister homologous chromatids between the loci can create recombinant Ab and aB chromatids from AB/ab. Only chromatids involved in that crossover are recombinant. More distant loci are more likely to have a crossover between them, but multiple crossovers limit a simple distance interpretation.
In a test cross with ab/ab, offspring classes reveal the heterozygote's gametes. If counts are AB 420, ab 420, Ab 80 and aB 80, recombinant frequency = (80 + 80)/1000 x 100 = 16%. Under a symmetric model each parental gamete is 42% and each recombinant 8%.
A recombination frequency near 50% can indicate unlinked loci or loci far apart on the same chromosome; it does not prove different chromosomes. Genetic map distances are a useful extension, but the core task is explaining departures from independent dihybrid ratios using linkage and crossing-over.
Step by step
- 1
Identify parental phase
Use the most frequent classes and the stated cross.
- 2
Sum recombinant classes
Divide by all informative offspring.
- 3
Predict gamete probabilities
Do not force all four classes to 1/4.
Worked example
Work through the evidence
AB/ab is test-crossed with ab/ab. Recombination frequency is 10%. What are the four expected gamete probabilities?
One way to explain it
AB and ab are 45% each; Ab and aB are 5% each, assuming symmetric recovery and no viability differences.
Why this answer works
- Recombinants sum to 10%.
- The remaining 90% is split between parental classes.
Is this true? "Two linked genes can never be separated."
Crossing-over between the loci can generate recombinant combinations.