A linkage map is a chromosome map of a species that shows the position of its known genes or markers relative to each other, rather than as specific physical points on each chromosome.A Linkage map is different from gene map. Thomas Hunt Morgan observed that the amount of crossing over between linked genes is different. It gives the idea that crossover frequency indicates the distance separating genes on the chromosome. Morgan’s student Alfred Sturtevant developed the first genetic –map, also called a linkage map.
Sturtevant proposed that the greater the distance between linked genes, the greater the chance of crossing over between non-sister chromatids. If number of recombinant is measured, then the distance between the genes can be measured. This distance is called a genetic map unit (m.u.), or a centimorgan. It is defined as the distance between genes for which one product of meiosis in 100 is recombinant. A recombinant frequency (RF) of I % is equivalent to I m.u. A linkage map is created by finding the map distances between a numbers of traits that are present on the same chromosome. Significant gaps between traits are avoided. It can cause inaccuracies due to multiple recombination events.
Linkage mapping is critical for identifying the location of genes that cause genetic diseases. In a normal population, genetic traits and markers will occur in all possible combinations. The frequencies of combinations are determined by the frequencies of the individual genes.
A linkage map is based on the frequencies of recombination between markers during crossover of homologous chromosomes. The greater the frequency of recombination (segregation) between two genetic markers, the farther apart the) are will be. Conversely, the higher the frequency of association between the markers, the smaller the physical distance between :hem. Historically, the markers originally used were detectable phenotypes (enzyme production, eye color) derived from coding DNA sequences.
Genetic maps help researchers to locate other markers, such as other genes by testing for genetic linkage of the already known markers.
- Frequency of recombination: Firstly, recombination frequency of different genes is measured from the given data. The proportion of recombinant types between two gene pairs as compared to the sum of all combinations is called cross over or recombination frequency.Recombination Frequency — Sum of all combinationsTwo linked genes are backcrossed for calculating the recombination frequencies. The cross between the heterozygote to a homozygous double recessive is called back cross.
- The recombination frequency is directly proportional to the distance between the linked gene loci. Genes can be mapped on a chromosome on the basis of their recombination frequencies. 1% of recombination frequency is equal to 1 unit map distance.
- Recombination frequencies of the entire linked gene are measured. These genes are arranged on chromosomes according to these recombination frequencies.
- Example:In Drosophila, ebony body colour (b) and vestigial wings (vg) are linked. The F1, progeny back crossed or test crossed with recessive parent. It gives following results:
|Total number of progeny
||100 = 17 %
Thus the map unit between ebony colour and vestigial wing genes is 17 m.u. Similarly, gene frequency of different genes is calculated. These genes are arranged on chromosomes according to these frequencies.