Discover how crossing over during meiosis creates recombinant chromosomes — and how scientists exploit recombination frequency to map gene positions.
In prophase I, homologous chromosomes pair tightly (synapsis). At points called chiasmata, non-sister chromatids exchange segments. This produces chromosomes with new allele combinations — recombinants.
Slide to see how physical distance between genes affects how often they recombine. Each map unit equals 1% recombination frequency.
Mendel's Law of Independent Assortment states that genes on different chromosomes assort independently, producing equal ratios of all four phenotype classes in a testcross (1:1:1:1). But genes on the same chromosome violate this law.
Thomas Hunt Morgan (1910s) studied Drosophila and found pairs of genes that did not assort independently. He realized they must be physically located on the same chromosome — linked. Crossing over partially breaks the linkage, producing recombinants at a rate that reflects the distance between loci.
| Gene Relationship | Expected Testcross Ratio | Example Counts (n=400) |
|---|---|---|
| Unlinked — different chromosomes | 1 : 1 : 1 : 1 | 100 : 100 : 100 : 100 |
| Loosely linked — far apart, same chromosome | Slightly skewed | 120 : 130 : 80 : 70 |
| Tightly linked — close together | Strongly skewed | 188 : 194 : 9 : 9 |
| Completely linked — no crossover possible | 1 : 1 : 0 : 0 | 200 : 200 : 0 : 0 |
A step-by-step walkthrough of how to calculate recombination frequency and build a chromosome map.
Gene mapping requires crossing a doubly heterozygous individual with a homozygous recessive individual. The recessive parent contributes only recessive alleles, so each offspring's phenotype directly reveals which gamete the heterozygous parent produced.
Examine the offspring counts. The two most abundant classes are Parental types — they match the original chromosome configuration (no crossover between the two loci). The two rarest classes are Recombinant types — they arose from a crossover.
| Phenotype | Count | Class | Why? |
|---|---|---|---|
| Purple long | 450 | Parental | Gamete LP — original chromosome 1 intact |
| White short | 445 | Parental | Gamete lp — original chromosome 2 intact |
| White long | 18 | Recombinant | Gamete Lp — crossover between L and P loci |
| Purple short | 15 | Recombinant | Gamete lP — crossover between L and P loci |
Using the sample data:
So a RF of 3.56% means the L and P genes are 3.56 map units apart on the chromosome.
Important limit: Even genes very far apart on the same chromosome cannot show more than 50% recombination (because multiple crossovers cancel out). Anything approaching 50 map units looks statistically similar to unlinked genes.
Work through each problem step by step. Use hints when stuck.
In a plant species, long stems (L) are dominant over short stems (l), and purple flowers (P) are dominant over white flowers (p). Both genes are located on the same chromosome.
| Phenotype | Count |
|---|---|
| Purple long | 450 |
| White short | 445 |
| White long | 18 |
| Purple short | 15 |
Total offspring: 928
In pea plants, yellow seeds (Y) are dominant over green seeds (y), and smooth seeds (S) are dominant over wrinkled seeds (s). Both genes are on chromosome 1.
| Phenotype | Count |
|---|---|
| Yellow smooth | 452 |
| Green wrinkled | 448 |
| Yellow wrinkled | 51 |
| Green smooth | 49 |
Total offspring: 1000
In a flowering plant, red petals (R) are dominant over white petals (r), and broad leaves (B) are dominant over narrow leaves (b). Both gene loci are on the same chromosome.
| Phenotype | Count |
|---|---|
| Red broad | 330 |
| White narrow | 324 |
| Red narrow | 75 |
| White broad | 71 |
Total offspring: 800