Principles of Genetics and Breeding

7.1.4.1 Genetic distance

• It is expressed in units of crossing over or centimorgan (cM).
• One CM equals 1% crossing over and contains approximately 1000 kb. This means, two loci which show 1% recombination are 1 CM apart on a genetic map.
• In a linkage a new locus is assigned based on an already known locus. It shows that a reference locus is a prerequisite for the purpose.
• Different types of DNA markers act as reference points or land marks in mapping genes.

The genetic linkage map of various fishes is presented in Table.3.

Sl. No.	Fish	No. of markers
1	Tilapia	174
2	Salmon	More than 250
3	Zebra fish	Latest map contain 2000 markers

New methodologies,especially somatic–cell hybridization and chromosome banding techniques, have facilitated the construction of linkage maps for each of the 23 pairs of human chromosomes. All genes in a given linkage group can be shown to map in a linear way. Most importantly, it has no branches. The linkage maps of viruses and prokaryotes and eukaryotic organelles (mitochondria and chloroplasts) are usually circular.

The rationale behind genetic mapping is that the probability of a crossover occurring between two loci is a function of the length of the interval separating the loci. If two loci are located wide apart there is more chance for recombination. The frequency of recombinant gametes produced be used as an index of the distance between two loci on a chromosome. The relative locations of genes on chromosomes can be mapped.

• One map unit is defined as the linkage distance that yields 1% recombination. When a large number of genes are mapped in any given species, the genes are observed to occur in linkage groups, with one linkage group corresponding to each pair of chromosomes.
• Gene map provides an ordered array or sequence tagged sites along the chromosome. A physical map is an ordered, overlapping array of DNA clones covering an entire chromosome or chromosome region. Gene map provides a framework upon which the physical map can be built by linking adjacent STSs on the genetic map into a set of overlapping DNA clones.