Lesson 5 of 7 / Inheritance and genetic problems
Linkage changes gamete probabilities
What do excess parental classes tell you about two loci?
I can explain linkage, crossing-over and altered dihybrid/test-cross ratios.
I can solve dihybrid problems involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage and epistasis.Syllabus
Syllabus 9477, Core 2(x). Solve dihybrid problems involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage and epistasis.
I can solve test-cross problems.Syllabus
Syllabus 9477, Core 2(y). Solve test-cross problems.
I can explain linkage, crossing-over and altered dihybrid ratios.Syllabus
Syllabus 9477, Core 2(z). Explain linkage, crossing-over and altered dihybrid ratios.
About 8 min
Make a guess. You are not marked.
Two genes are on the same chromosome. Are their alleles always inherited together?
Show the answer
No. Crossing-over can separate them.
Crossing-over between the loci produces recombinant gametes. The further apart the loci, the more often it happens.
The key idea
Linked 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.
Write the cross in chromosome-line notation
Capital letters in a phenotype mean the dominant feature is shown. Alleles written above the double line are on one homologue, and alleles below are on the other.
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
Linked genes are on the same chromosome, so in a genetic diagram you must show which alleles share a chromosome. Use chromosome-line notation: write the alleles on one homologue above a pair of parallel lines and the alleles on the other homologue below them. A and B above the lines with a and b below shows that A and B are on one chromosome. Some school mark schemes deduct a mark for the slash shorthand AB/ab inside a genetic diagram, so keep the slash for sentences only. AB/ab and Ab/aB are different arrangements even though both individuals are AaBb. Without crossing-over between the loci, only the two parental combinations are produced.
Label the gametes and offspring as parental or recombinant. Parental gametes carry the same combination of alleles as a chromosome of the parent: AB and ab from A B over a b. Recombinant gametes carry a new combination, Ab or aB, made by crossing-over. In a test cross the parental offspring classes are the most frequent, and the two recombinant classes are smaller and roughly equal.
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
Parental genotypes in line notation
A B above the double line and a b below, crossed with a b over a b.
- 2
Circle the gametes and label them
AB and ab are parental; Ab and aB are recombinant.
- 3
Offspring genotypes and phenotypes
Draw each offspring in line notation, state its phenotype, and label it parental or recombinant.
- 4
Recombinant frequency
Recombinant offspring / total offspring x 100.
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.
Watch out for this
A student says: "Two linked genes can never be separated." What is wrong with this?
Show the answer
Crossing-over between the loci can generate recombinant combinations.
Check your understanding
Which are recombinant gametes from an AB/ab parent?
Show the answer
Ab and aB
These combine alleles from different original homologues.
Exam-style questions
Check your understanding
A test cross of AaBb x aabb gives AB 410, ab 390, Ab 105, aB 95. Which conclusion is best supported?
Show the answer
A and B are linked, about 20 map units apart.
Parental classes far outnumber recombinants: (105 + 95) / 1000 = 20% recombination, so the genes are linked.
Quick recall
What do excess parental classes tell you about two loci?