Linkage and Recombination
பிணைப்பு மற்றும் மறுசேர்க்கை
Mendel’s Law of Independent Assortment fails when genes are physically close on the same chromosome. This is Linkage. If you ignore the physical distance between loci, you will fail the genetics section of NEET.
The idea
Linkage is the physical association of genes on a chromosome, preventing independent assortment. Recombination is the generation of non-parental gene combinations due to crossing over during meiosis. The frequency of recombination is directly proportional to the physical distance between two genes. Tightly linked genes show low recombination frequency, while loosely linked genes show high frequency.
Why it matters
Linkage exists because genes are physically linked on the same DNA molecule; recombination exists due to crossing over during meiosis to create genetic diversity.
NEET lens
NEET focuses on the inverse relationship between linkage strength and recombination frequency. Memorize the Morgan experiment on Drosophila: white-eyed and miniature-winged flies. Expect calculation-based questions where you must derive map units (centimorgans) from recombination percentages. Do not confuse linkage with crossing over; linkage reduces recombination.
Clinical significance
Linkage maps are essential for identifying disease-causing loci in humans. When linkage is disrupted or genes are incorrectly mapped, diagnostic markers for genetic disorders like Duchenne Muscular Dystrophy fail. Precision in mapping prevents misdiagnosis in clinical genetics.
Introduction
Linkage describes the tendency of genes on the same chromosome to be inherited together (not independently). Recombination (crossing over during meiosis I) can separate linked genes, with the frequency of recombination used to map relative gene positions on chromosomes.
Core concept
Linkage: genes on same chromosome = linked. Tend to be inherited as a unit (linkage group = genes on same chromosome). Each chromosome = 1 linkage group. Humans: 23 pairs → 23 linkage groups. Morgan's Drosophila work (1910): discovered linkage by observing deviations from 9:3:3:1 dihybrid ratios. Recombination: crossing over between homologous chromosomes at prophase I → recombinant gametes (new allele combinations). Recombination frequency (RF): RF = (recombinant offspring / total offspring) × 100%. RF = 0-50%. RF = 50% = independent assortment (very far apart on same chromosome or different chromosomes). RF < 50% = linked. RF proportional to distance → genetic map (1 cM = 1% RF = 1 map unit = 1 Morgan). Parental types: more common (retain original combination). Recombinant types: less common (new combination from crossover).
Go deeper
Complete vs incomplete linkage: Complete linkage: genes so close that no crossing over occurs between them → only parental types in offspring. Incomplete linkage: some crossing over → both parental and recombinant types (parental >> recombinant). Example: Morgan's Drosophila: body colour (b) and wing length (vg) on chromosome 2 → incomplete linkage → some recombinants (but less than 50%).
From Class 10
Mendel's Law of Independent Assortment applies when genes are on different chromosomes. But when two genes are on the same chromosome (linked), they tend to be inherited together — unless recombination separates them.