Genetic Linkage
Genetic linkage refers to the tendency of genes located close together on the same chromosome to be inherited together during meiosis, rather than independently assort as predicted by Mendel's Law of Independent Assortment. The degree of linkage between two genes is inversely proportional to the physical distance between them; closely linked genes rarely recombine, while distantly located genes recombine more frequently and approach independent assortment. Linkage mapping uses recombination frequencies to construct genetic maps that estimate distances between genes in centimorgans (cM).
Key Formula
Recombination Frequency (%) = (Recombinant offspring / Total offspring) × 100
LaTeX: \text{Recombination Frequency (cM)} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100
| Symbol | Meaning | Unit |
|---|---|---|
| RF | Recombination frequency (map distance) | centimorgans (cM) or % |
| Recombinant offspring | Offspring showing new combinations of parental alleles | count |
| Total offspring | All offspring in the cross | count |
Worked Example
Problem
In a testcross of Drosophila, 800 offspring are produced: 340 parental type A, 340 parental type B, 60 recombinant type C, and 60 recombinant type D. What is the recombination frequency and map distance between the two genes?
Solution
Step 1: Count recombinant offspring. Recombinants = 60 + 60 = 120. Step 2: Count total offspring. Total = 340 + 340 + 60 + 60 = 800. Step 3: Apply formula. RF = (120 / 800) × 100 = 15%. Step 4: Convert to map units. 15% recombination = 15 centimorgans (cM).
Answer
Recombination frequency = 15%; the two genes are 15 cM (centimorgans) apart on the same chromosome.
Relationship Between Map Distance and Recombination Frequency
| Map Distance (cM) | Recombination Frequency (%) | Degree of Linkage | Example |
|---|---|---|---|
| 0–5 cM | 0–5% | Very tightly linked | Genes very close on chromosome |
| 5–20 cM | 5–20% | Linked | Detectable coupling in crosses |
| 20–40 cM | 20–40% | Loosely linked | Some independent assortment behaviour |
| 50+ cM | 50% | Unlinked (independent) | Different chromosomes or far apart |
Interactive Tools
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Related Terms
Chromosomal Crossover
Chromosomal crossover (also called crossing over or recombination) is the exchange of genetic material between homologous chromosomes at sites called chiasmata during prophase I of meiosis, resulting in recombinant chromosomes with new combinations of alleles. This process is a major source of genetic variation in sexually reproducing organisms and is essential for proper chromosome segregation in most eukaryotes. Crossover frequency between two loci is used to calculate genetic map distances and construct linkage maps.
Hardy-Weinberg Equilibrium
The Hardy-Weinberg Equilibrium (HWE) is a principle stating that allele and genotype frequencies in an ideal, infinitely large, randomly mating population will remain constant from generation to generation in the absence of evolutionary influences such as mutation, selection, gene flow, and genetic drift. It provides a mathematical null hypothesis against which real populations can be compared to detect evolutionary change. The principle was independently formulated by Godfrey Hardy and Wilhelm Weinberg in 1908.
Genetic Drift
Genetic drift is a mechanism of evolution referring to random changes in allele frequencies in a population due to chance sampling events rather than natural selection, most pronounced in small populations. Unlike natural selection, genetic drift is non-directional and can lead to the fixation (frequency = 1) or loss (frequency = 0) of alleles regardless of their adaptive value. Two important forms are the bottleneck effect (sudden population reduction) and the founder effect (small group establishes a new population), both of which reduce genetic diversity.
From Latin ligare (to bind). The concept of genetic linkage was discovered by Thomas Hunt Morgan and Alfred Sturtevant at Columbia University around 1910–1913 through experiments on Drosophila melanogaster. Sturtevant created the first genetic linkage map in 1913. The unit "centimorgan" is named in honour of Morgan.