Crossover Probabilities
Crossover Interference
Lower values = stronger interference (double crossovers less likely)
Perform three-point crosses to map gene order—account for crossover interference in recombination
Lower values = stronger interference (double crossovers less likely)
When genes are located on the same chromosome, they tend to be inherited together—a phenomenon called genetic linkage. However, during meiosis, crossing over between homologous chromosomes can separate linked genes, producing recombinant offspring.
A three-point cross involves three genetic markers (A, B, and C) on the same chromosome. By analyzing the frequencies of different offspring classes, we can:
In a three-point cross, there are eight possible gamete classes:
The occurrence of one crossover often reduces the probability of a second nearby crossover—called interference. This is measured by the coefficient of coincidence (COC):
The genetic map distance between two loci is measured in centiMorgans (cM), where 1 cM = 1% recombination frequency. For closely linked genes, map distance ≈ recombination frequency × 100. However, for distant loci, multiple crossovers can lead to underestimation of the true genetic distance.
This mapping technique was pioneered by Thomas Hunt Morgan and his students (including Alfred Sturtevant) working with Drosophila melanogaster in the 1910s. Their work provided the first physical evidence that genes are linearly arranged on chromosomes and laid the foundation for modern genetic mapping and genomics.