[SagarikaGhosh]

A genome refers to a single set of chromosomes of an organism. Therefore, in diploid organisms, genome represents the total genetic information present in them. A genome map may be defined as a detailed schematic description of the structural and functional organization of the complete genome of an organism.

Three different strategies have been used to construct genetic maps, which are usually referred to as linkage maps, cytogenetic or cytological maps and physical maps.

1. Linkage Maps:

A linkage map depicts the order of genetic markers and the relative distances between them as measured in terms of recombination frequencies between the markers. A map unit or centimorgan (cM) is that distance, which allows 1% recombination between the concerned genes.

2. Cytogenetic Maps:

A cytogenetic map depicts the locations of various genes in a chromosome relative to specific microscopically visible landmarks in the chromosome. In most cases, each chromosome has a characteristic banding pattern, which may be either naturally present, e.g., in polytene chromosomes of Drosophila, or more commonly generated by specific staining protocols, e.g., in case of human chromosomes; the genes are mapped cytologically relative to these band locations. Cytogenetic mapping is far more refined in organisms like Drosophila because either polytene chromosomes have a very detailed banding pattern. It is commonly used ion eukaryotes since they have relatively large microscopically observable chromosomes. Cytogenetic mapping is often used as a first step in the localization of genes in plants and animals.
Cytogenetic mapping can be done by any of the following approaches:
- Fluorescent in situ hybridization (FISH)
- Somatic Cell Hybridization
- Analysis of small changes in polytene chromosomes structure

3. Physical Maps:

In a physical map, genes are depicted in the same order as they occur in chromosome, and the distances between them are shown as a number of base pairs separating the genes.

A genetic marker is a specific location on a chromosome that is defined by a naked eye polymorphism (NEP), protein or DNA sequence; the inheritance of a genetic marker can be followed in a mapping population. Naked eye polymorphism consists of those genetic markers whose inheritance can be followed with the naked eye, and without any specialized biochemical or molecular technique. Mapping population is any population that is studied for the preparation of genetic maps.