Linkage Recombination Mapping

Mendel's dihybrid 9:3:3:1 ratio depends on one assumption: the two genes sit on different chromosomes. When two genes sit on the same chromosome, they tend to travel together into gametes — that is linkage. The closer two loci are on a chromosome, the more tightly they are linked and the less likely crossing over will separate them.

T.H. Morgan's work on Drosophila (NCERT Class 12 Biology Chapter 4, page 80) established that genes on the same chromosome do not assort independently. Instead, parental (non-recombinant) combinations appear in offspring far more often than recombinant combinations. When crossing over occurs during meiosis I (specifically at the pachytene stage of prophase I), homologous chromatids exchange segments, producing recombinant gametes. The frequency of this recombination is the basis of genetic mapping.

Recombination frequency (RF) measures how often recombinant offspring appear:

RF = (number of recombinants ÷ total progeny) × 100

One percent recombination frequency equals one centiMorgan (1 cM) of map distance. The upper limit of RF is 50% — at that point the two loci behave as if they are on separate chromosomes (effectively unlinked).

The core trap in this topic is confusing linkage with independent assortment. If a test cross yields parental types far exceeding 50%, the genes are linked. If recombinants approach 50%, the genes are either very far apart on the same chromosome or on different chromosomes entirely.

Watch out: RF gives a relative map distance, not a physical distance in base pairs. Two loci 10 cM apart will show roughly 10% recombinant offspring in a test cross — but double crossovers at large distances can underestimate the true map distance.

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