Gene linkage and gene maps
If two genes are on the same chromosome they are inherited together. New combinations only arise through crossing over, and the further apart the genes are, the more often that happens.
Rules
- Test cross AaBb × aabb: four classes of offspring
- The two largest classes are parental, the two small ones recombinant
- Recombination frequency = recombinants / all offspring
- 1% recombination = 1 centimorgan (cM) map distance
- Read coupling (cis) AB/ab or repulsion (trans) Ab/aB off the parental classes
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
A double heterozygote (AaBb) is test-crossed with aabb. The table shows the offspring by the alleles they received from the heterozygous parent. Work out the recombination frequency and the map distance and say how the alleles were arranged in the parent.
| Alleles from the AaBb parent | Count |
|---|---|
| AB | 87 |
| ab | 88 |
| Ab | 168 |
| aB | 157 |
Recombination frequency in %
Map distance in cM
Arrangement in the parent
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
A double heterozygote (AaBb) is test-crossed with aabb. The table shows the offspring by the alleles they received from the heterozygous parent. Work out the recombination frequency and the map distance and say how the alleles were arranged in the parent.
Worked solution
- 1.The two large classes are the parental combinations: Ab and aB → repulsion Ab / aB
- 2.Recombinants: AB + ab = 134 + 138 = 272
- 3.Recombination frequency = 272 / 1,700 = 16%; 1% equals 1 cM → 16 cM
Typical mistake: Always taking AB and ab as the parental classes. The counts tell you which combinations are parental: the two largest ones.
Common questions
Why is the recombination frequency at most 50%?
For genes very far apart every combination is equally likely, as if they were on different chromosomes: that gives 50%.