Preset buttons
These buttons load common classroom scenarios. They include tight linkage, a 50 cM unlinked control, coupling phase, repulsion phase, and linked intercross examples.
Genetic Linkage Calculator
Calculate linked-gene gamete probabilities, recombinant classes, and expected offspring ratios from recombination frequency. Use this tool for AB/ab coupling phase, Ab/aB repulsion phase, test crosses, and linked dihybrid intercrosses.
Live genetic linkage and recombination calculator
Adjust recombination frequency, phase, cross type, and offspring number. The linked gamete and offspring probabilities update instantly.
Start with a linked-gene scenario
Load a common linkage example, then adjust phase, recombination frequency, cross type, and sample size.
AB/ab × aabb with 12% recombination. Parental classes dominate the offspring.
Parent 1: linked heterozygote
Choose whether the dominant alleles sit on the same homolog or opposite homologs.
Parent 2 and offspring scale
Choose a double-recessive tester or a second linked heterozygote.
Parent 2 gametes
ab100.00%
tester
Live linkage result
Strong linkage pattern
The genes sit close enough that parental gametes strongly outnumber recombinant gametes.
Distance
12.0 cM
Recombinants
12.0%
Top phenotype
44.0%
Parent 1 gamete probabilities
Parental gametes preserve the original chromosome arrangement. Recombinant gametes require a crossover between A and B.
AB (parental)44.00% · 220 expected
Ab (recombinant)6.00% · 30 expected
aB (recombinant)6.00% · 30 expected
ab (parental)44.00% · 220 expected
Offspring phenotype probabilities
These probabilities combine the gametes from both parents and convert genotypes into phenotype classes.
A_B_ · Both dominant traits44.00% · 220 expected
A_bb · Trait A dominant only6.00% · 30 expected
aaB_ · Trait B dominant only6.00% · 30 expected
aabb · Both recessive traits44.00% · 220 expected
Offspring genotype table
Linked intercrosses can produce several genotype classes. The table ranks them from most likely to least likely.
| Genotype | Probability | Expected count | Relative bar |
|---|---|---|---|
| aabb | 44.00% | 220 | |
| AaBb | 44.00% | 220 | |
| aaBb | 6.00% | 30 | |
| Aabb | 6.00% | 30 |
What is genetic linkage?
Genetic linkage describes genes that sit on the same chromosome and travel together through meiosis more often than independent assortment predicts. Thomas Hunt Morgan linked chromosome inheritance with visible fruit fly traits in the early twentieth century. Alfred Sturtevant then used recombination frequencies to build the first genetic map in 1913.
Crossing over breaks linkage when homologous chromatids exchange DNA between two loci. A short interval produces fewer crossovers, so parental allele combinations remain common. A longer interval produces more recombinant chromatids, but the observable recombination frequency reaches a 50% ceiling.
OpenStax explains that Sturtevant used map units, now called centimorgans, where 0.01 recombination frequency corresponds to 1 cM. Read the OpenStax linkage chapter.
How to use the genetic linkage calculator
- 1
Select the parental phase
Choose AB/ab for coupling phase or Ab/aB for repulsion phase in the linked heterozygous parent.
- 2
Enter recombination frequency
Type the map distance in centimorgans from 0 to 50. The calculator treats 1 cM as about 1% recombination for short intervals.
- 3
Choose the second parent
Use aabb for a mapping test cross, or use another linked heterozygote for an intercross-style probability model.
- 4
Read gamete and offspring probabilities
Compare parental gametes, recombinant gametes, phenotype probabilities, and expected offspring counts.
What each part of the tool does
Parent 1 phase card
This card sets the chromosome arrangement in the heterozygous parent. AB/ab makes AB and ab parental gametes, while Ab/aB makes Ab and aB parental gametes.
Recombination frequency control
The slider represents map distance from 0 to 50 cM. Lower values increase parental classes, while 50 cM produces the independent assortment limit.
Parent 2 selector
The aabb tester makes offspring phenotypes reveal parent 1 gametes directly. The linked heterozygote option models a linked dihybrid-style cross.
Result banner
The banner summarises linkage strength, map distance, recombinant percentage, and the most common phenotype class for the current settings.
Probability tables
The bars and genotype table show exact probabilities and expected offspring counts. They help you compare parental classes with recombinant classes without manual Punnett square arithmetic.
Coupling phase, repulsion phase, and recombinant classes
Phase decides which allele combinations count as parental. In coupling phase, a heterozygote carries AB on one homolog and ab on the other. In repulsion phase, it carries Ab on one homolog and aB on the other.
A 12 cM coupling-phase test cross gives AB and ab gametes at 44% each. Ab and aB recombinants appear at 6% each. The total recombinant fraction equals 12%, which matches the map distance for a short interval.
This logic also explains why linked genes can disrupt a dihybrid 9:3:3:1 ratio. Independent assortment requires four equal gamete classes from an AaBb parent. Linkage makes the two parental gametes more common than the two recombinant gametes.
Worked examples
Example 1: AB/ab × aabb at 10 cM
The linked heterozygote makes AB and ab parental gametes at 45% each. It makes Ab and aB recombinant gametes at 5% each. A double-recessive tester contributes only ab gametes.
Among 1,000 offspring, the expected counts equal 450 A_B_, 50 A_bb, 50 aaB_, and 450 aabb. The two rare classes identify the crossover products.
Example 2: Ab/aB × aabb at 20 cM
Repulsion phase changes the parental classes. Ab and aB gametes each appear at 40%, while AB and ab recombinants each appear at 10%. The map distance still equals 20 cM.
In 500 offspring, you expect 50 A_B_, 200 A_bb, 200 aaB_, and 50 aabb. The dominant-dominant and double-recessive classes now represent recombination.
Practical value in genetics and breeding
Linkage mapping helps geneticists locate genes by tracking how often markers separate during meiosis. Plant breeders use the same concept when they follow favourable allele combinations across generations. Medical genetics uses linkage analysis in families when researchers need to track a disease locus with nearby markers.
Genome mapping extends this idea across many markers. OpenStax describes linkage analysis as studying recombination frequency between genes, with higher recombination indicating greater distance. Review the OpenStax genome mapping section.
Limitations and caveats
This calculator models two loci with one recombination frequency. It does not estimate gene order, interference, coefficient of coincidence, or double-crossover correction. Three-point test crosses handle those questions better.
A recombination frequency near 50% does not prove that genes sit on different chromosomes. Distant genes on the same chromosome can also produce 50% observable recombination. Long intervals require extra markers and mapping functions.
This tool supports genetics education and research planning. It does not provide clinical genetic counselling, diagnostic interpretation, or professional breeding certification.
Frequently asked questions
What does a genetic linkage calculator do?
A genetic linkage calculator estimates gamete and offspring probabilities when two genes sit on the same chromosome. It uses recombination frequency to split gametes into parental and recombinant classes. A 10 cM distance gives 10% total recombinant gametes and 90% parental gametes. This pattern changes a dihybrid test cross away from the independent 1:1:1:1 expectation.
How do centimorgans relate to recombination frequency?
One centimorgan corresponds to about 1% recombination for short genetic intervals. A 12 cM distance therefore predicts 12% recombinant gametes in a mapping cross. This relationship weakens across long intervals because double crossovers can restore the parental allele combination. A recombination frequency cannot exceed 50%, because 50% behaves like independent assortment.
What is the difference between coupling and repulsion phase?
Coupling phase means the two dominant alleles sit on the same homolog, usually written AB/ab. Repulsion phase means each homolog carries one dominant and one recessive allele, written Ab/aB. The recombination frequency can stay the same while the parental offspring classes change. In a test cross, AB/ab produces AB and ab as parental classes, while Ab/aB produces Ab and aB as parental classes.
Why do linked genes deviate from a 9:3:3:1 ratio?
The 9:3:3:1 ratio assumes independent assortment of two genes. Linked genes violate that assumption because allele combinations travel together on the same chromosome more often than meiosis separates them. Crossing over creates recombinant gametes, but the recombinant total stays below 50% when genes show linkage. Linked intercrosses therefore produce phenotype probabilities that differ from the standard dihybrid ratio.
Which cross works best for mapping linked genes?
A test cross works best for basic linkage mapping. The heterozygote supplies AB, Ab, aB, and ab gametes, while the aabb tester supplies only ab gametes. That design lets each offspring phenotype reveal one gamete from the heterozygous parent. Recombinant offspring divided by total offspring gives recombination frequency.
Can a 50 cM distance mean genes are on the same chromosome?
Yes. A 50% recombination frequency can describe genes on different chromosomes or genes very far apart on the same chromosome. Multiple crossovers between distant loci can hide linkage in a two-point cross. Geneticists use additional markers and mapping functions when they need long-distance chromosome maps. This calculator treats 50 cM as the independent assortment limit.
How do I calculate recombinant offspring from a linkage test cross?
Identify the two lowest-frequency classes only after you know the parental phase. In AB/ab coupling phase, Ab and aB offspring count as recombinants. In Ab/aB repulsion phase, AB and ab offspring count as recombinants. Add recombinant offspring, divide by total offspring, and multiply by 100 to get recombination frequency in percent. For short intervals, that percent approximately equals map distance in cM.
Does this calculator include interference or double crossovers?
No. This calculator models a two-point cross with one recombination frequency between two loci. It does not estimate coefficient of coincidence, interference, or three-point map order. Double crossovers can make long intervals look shorter in simple two-point data. Use a three-point test cross when you need marker order and interference estimates.
Related genetics calculators
Use these tools to compare independent assortment, test crosses, and linked-gene inheritance models.