Mapping with markers is a technique used to link chromosomal regions without relying on the direct observation of alleles, which is particularly useful in organisms like humans where traditional linkage mapping methods are not feasible. In linkage mapping, researchers typically cross organisms with different phenotypes to analyze the percentage of recombinant phenotypes, which helps determine the distance between alleles on chromosomes. However, ethical concerns prevent similar approaches in human studies.
A molecular marker is a specific DNA segment that exhibits polymorphism, meaning it varies among individuals within a population. These markers are invaluable for identifying locations of unknown genes or alleles, allowing researchers to map chromosomes without prior knowledge of the alleles involved. One common type of molecular marker is the restriction fragment length polymorphism (RFLP). RFLPs utilize restriction enzymes, which are proteins that cut DNA at specific sequences. The cutting pattern varies among individuals due to differences in their DNA sequences, resulting in fragments of different lengths. These variations can arise from mutations, single nucleotide polymorphisms, deletions, or duplications.
To analyze RFLPs, DNA is treated with restriction enzymes and then subjected to gel electrophoresis, where the lengths of the resulting fragments are compared. This process generates an RFLP map, which serves as a linkage map for the markers on an organism's genome. For example, in a pedigree analysis, different genotypes can be identified based on the presence of specific bands on the gel, allowing researchers to infer the genotypes of parents and offspring without prior knowledge of their genetic makeup.
Another type of molecular marker is the microsatellite, which consists of repetitive DNA sequences, such as CA repeats, that can vary in length and frequency among individuals. These microsatellites can be identified using techniques like polymerase chain reaction (PCR) and Southern blotting. Each individual in a species will have a unique number of these repeats, which can be used to differentiate between organisms and assess recombination rates when they are mated.
Both RFLPs and microsatellites function similarly to traditional linkage mapping, but instead of focusing on phenotypes, they analyze the positions and lengths of DNA sequences. By utilizing recombination frequencies from matings, researchers can effectively map the genome using these molecular markers. This approach highlights the versatility of molecular markers in genetic mapping, providing a powerful tool for understanding genetic relationships and inheritance patterns.
