Molecular markers - Codominant and Dominant markers

Definition


A genetic marker  is a particular gene or DNA sequence with a known location on a chromosome and associated with a particular gene or trait.

• it may be a short DNA sequence, such as a sequence surrounding a single base-pair change SNP 
• or a long one, like minisatellites

Genetic markers fall into 3 classes -  Morphological , Biochemical and Molecular

A molecular marker should not be considered as normal genes as they do not have any biological effect , and instead they can be thought as constant landmarks in the genome.

Molecular markers are independent of G*E interaction.

They are identifiable DNA sequences found at specific locations of the genome and transmitted by the standard laws of inheritance from one generation to the next.

Characteristics of ideal molecular marker::

An ideal molecular marker should  - 

• Have high level of polymorphism
• Should be evenly distributed throughout the entire genome
• Genetic differences should be clearly visible
• Preferably Co-dominant in nature
• Have linkage to distinct phenotypes

Based on whether markers can discriminate between Homozygotes and Heterozygotes Molecular markers  may be categorized as :

1. CODOMINANT MARKERS 

2. DOMINANT MARKERS

in this gel diagram (a) is a codominant marker (b) is dominant marker

CODOMINANT MARKERS 

• Using Codominant markers we can clearly discriminate between Homozygotes and Heterozygotes , they also indicate differences in sizes clearly in gel diagrams

• examples of Codominant markers are -> RFLP , SNPs , CAPS , SSR , Allozymes

DOMINANT MARKERS

• Dominant markers do not tell about the condition whether the allele is heterozgous/homozygous , it simply depicts  the presence or absence of allele

• examples of Dominant markers are -> RAPD , AFLP , SCAR , AP-PCR

Abbrevations used

RFLP - Restriction Fragment Length Polymorphism
RAPD - Randomly Amplified Polymorphic DNA
AFLP -  Amplification Fragment Length Polymorphism
CAPS - Cleaved Amplified Polymorphic Sequences
SSR - Simple Sequence Repeats (microsatellites)
SNP - Single Nucleotide Polymorphism
SCAR - Sequence Characterized Amplified Regions
APPCR - Arbitrarily Primed PCR

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