If linked genes always stayed together, a test cross would give exactly two kinds of offspring. It never does — a few offspring turn up with combinations neither parent had. Those are recombinants, and counting them tells you how far apart the two genes sit.
📚 What you need to know
Parental type offspring have the same allele combination as the parent. Recombinant offspring have a new combination.
Recombinants are produced by crossing over in prophase I, when non-sister chromatids exchange sections of DNA at a chiasma.
Crossing over happens at random positions, and chiasmata form in different places in each meiotic division.
Recombinants are always less frequent than parental types for linked genes.
Genes further apart on a chromosome give a higher recombination frequency, because there is more DNA between them for a chiasma to form in.
Recombination frequency = recombinant offspring ÷ total offspring × 100%. A test cross is the standard way to measure it.
Crossing over makes new combinations
In prophase I of meiosis, homologous chromosomes pair up. Non-sister chromatids touch at points called chiasmata, break, and rejoin to the other chromatid. Whole blocks of alleles are swapped.
The result is chromatids carrying combinations that did not exist before. A chromatid that arrived carrying A and B might leave carrying A and b.
The two untouched chromatids keep the parental combinations. Only the two that took part in the chiasma carry something new.
A crossover between two linked genes only counts if the chiasma forms between the two loci. A crossover somewhere else on the chromosome swaps DNA, but leaves those two alleles sitting together as before.
Spotting recombinants with a test cross
You cannot see genotypes, only phenotypes. A test cross fixes that: cross the individual with one that is homozygous recessive for both genes.
That partner can only donate recessive alleles, so it contributes nothing visible. Every offspring phenotype is a direct read-out of the gamete it received from the parent being tested.
🧩 Finding the recombinants in a data table
Identify the parent’s allele combinations — for (AB)(ab), those are AB and ab.
Match each offspring phenotype to the gamete it must have received.
Any offspring showing AB or ab is parental. Anything else — Ab or aB — is recombinant.
Confirm with the numbers. The two largest classes are parental; the two smallest are recombinant.
Add the two recombinant classes together before calculating the frequency.
Recombination frequency
RF = (number of recombinant offspring ÷ total offspring) × 100%
Distance and recombination frequency
Chiasmata form at random points along the chromosome. So the chance of one falling between two genes depends on how much DNA lies between them.
Genes far apart → many possible crossover points between them → more recombinants → higher RF.
Genes close together → few possible crossover points between them → fewer recombinants → lower RF.
For linked genes RF is always below 50%, because the two parental classes stay in the majority. An RF of about 50% means the genes are behaving as if unlinked — either they are on different chromosomes, or they are so far apart on the same one that crossovers between them are almost certain.
This is how genetic maps were built. Morgan and his students crossed thousands of fruit flies, measured RF for pair after pair of genes, and used those percentages as distances. A 1% recombination frequency became one centimorgan — a map drawn entirely from counting offspring.
Worked examples
WORKED EXAMPLE 1
A plant with genotype (PL)(pl) is test crossed. Of 1000 offspring: 412 purple/long, 388 red/round, 105 purple/round, 95 red/long. Identify the recombinants and calculate the recombination frequency.
Step 1: what were the parental combinations?
The tested parent is (PL)(pl), so PL and pl are parental
Step 2: match phenotypes to gametes
Purple/long = PL, red/round = pl → parental. Purple/round = Pl, red/long = pL → recombinant
Step 3: count the recombinants105 + 95 = 200Step 4: apply the formulaRF = (200 ÷ 1000) × 100 = 20%RF = 20%, so the loci are about 20 centimorgans apartThe two big classes are parental and the two small ones are recombinant — the standard pattern for linked genes.
Offspring phenotype
Gamete received
Type
Number
Purple flowers, long pollen
PL
Parental
412
Red flowers, round pollen
pl
Parental
388
Purple flowers, round pollen
Pl
Recombinant
105
Red flowers, long pollen
pL
Recombinant
95
WORKED EXAMPLE 2
Genes X and Y give a recombination frequency of 4%. Genes X and Z give 31%. Deduce which pair of loci lies closer together, and explain why recombinants appear at all.
Step 1: compare the frequencies4% is much lower than 31%Step 2: link frequency to distance
A low frequency means a chiasma rarely forms between the two loci, so there is little DNA between them
X and Y are closer togetherStep 3: why any recombinants at all?
Crossing over in prophase I exchanges sections between non-sister chromatids, breaking the linkage in some cells
Both values are below 50%, so both pairs are linked — X and Z are just far enough apart to be separated often.
💡 Exam tip
Work out the parental combinations first from the tested parent’s genotype. Everything else is recombinant.
Add both recombinant classes before dividing — forgetting one halves your answer.
Give RF as a percentage unless the question asks for centimorgans, and remember 1% = 1 cM.
Name prophase I and the chiasma when explaining where recombinants come from.
Say the crossover must occur between the two loci — that detail separates good answers from average ones.
An RF near 50% means the genes are effectively unlinked. Say so if the data points that way.
⚠ Common mix-up
Calling the two biggest classes recombinant. Recombinants are always the minority for linked genes.
Dividing by the recombinant total instead of the grand total. The denominator is all the offspring.
Saying crossing over happens in meiosis II. It is prophase I, between homologous chromosomes.
Thinking a crossover changes all four chromatids. One chiasma changes two of them.
Giving an RF above 50% for linked genes. That is not possible — check your arithmetic.
Confusing crossing over with independent assortment. Crossing over exchanges parts of chromosomes; independent assortment shuffles whole ones.
That completes Genetics & Inheritance. Up next: Gene Pools & Speciation — what happens to all these alleles at the scale of a whole population.
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