Difference between gene and marker

Young graduates often confused with the exact difference between gene and marker. The definition for gene and markers could be found in 100's of text books and websites. But nowhere the difference between them is mentioned. I will put a simple explanation for differentiating gene from markers.

Genes and markers are segments of DNA reside on chromosomes. Gene is a coding part of the DNA , encodes protein or RNA molecules which has specific function in the cells. The phenotypic traits of an organism are corresponding to particular genes.  Molecular tools has identified thousands of genes, and each gene was assigned to a particular function. But the sequence information and locations of the genes on chromosomes are very less studied and for most of the genes  the information is not available. Genetic mapping and physical mapping studies has been carried out to locate the genes on chromosomes. This mapping studies are limited to only those genes which has visible phenotypic trait. Irrelevant of the location, the genes encodes a function.

Markers which have no specific functional expression in cells as like genes. They are DNA segments which sequence is fully or partly known and  the relative location of the markers on chromosome is known. Genetic mapping studies  located markers on a specific chromosomes and estimated the relative distance between the markers and to the markers and genes. A gene with known function and unknown location can be tagged with a marker with known location if the gene  resides close to the marker on the chromosomes. If the marker position on the chromosome is unknown, those markers cannot be used for genetic mapping or breeding programs. They can be used only for diversity studies.

To generalize the above discussion, a gene has function but the sequence and location on the chromosome is mostly unknown. The marker has known sequence information and mapped on a chromosome, but do not have any functional role. This definition is only a generalization and not applicable to all genes and markers.

When a gene qualifies as a marker?

If the sequence of the gene and its location on the chromosome is known, that gene can be used as the marker for genetic s studies as like markers.

Do not confuse the genic markers with marker genes. Marker genes encodes a functional protein which can be detected or assayed by experiments and being used in the cytological studies and transgenic developments.