Monday, February 12, 2007

Section 11-B(2): Positional cloning approach to glaucoma (linkage analysis)

Linkage analysis is a method for identifying glaucoma-causing genes that is dependent solely on the availability of large families with several members that have glaucoma. DNA is collected from each member of these families and is tested to see which segments of the DNA are always passed down through the family along with glaucoma. Genes that cause glaucoma are located within these linked regions of DNA (Figure 11-1).

Figure 11-1

Figure 11-1. Linkage analysis of glaucoma. These diagrams represent the family tree of a family affected with glaucoma. The males in the family are represented by squares and the females are represented by circles. Deceased family members have a diagnonal line across the symbol. Family members affected with glaucoma are indicated by shading the pedigree symbols black. Most spouses were omitted from the diagram. In linkage analysis the inheritance of genetic markers (represented in the diagrams of a family tree with colored squares) is compared to inheritance of glaucoma (represented by darkly shaded pedigree symbols). The inheritance of a genetic marker on chromosome 1 is represented by the green boxes. Notice that the green boxes are always passed down through the family together with glaucoma. The gene causing glaucoma in this family is located near this genetic marker on chromosome 1.

Linkage analysis identifies where glaucoma genes are located within the genome. The next step is testing members of the family for disease-causing defects in the genes that are located with the linked regions.

Linkage analysis has some advantages that make it particularly well-suited for searching for glaucoma genes. This approach to finding disease-causing genes is possible even when little is known about the basic biological mechanisms of the disease being studied (such as glaucoma). This research is dependent on the identification and enrollment of large families with many members that are affected with glaucoma. Most of the known glaucoma genes were discovered with linkage analysis of large glaucoma families.

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