TAILIEUCHUNG - Chapter 062. Principles of Human Genetics (Part 28)

Allelic Association, Linkage Disequilibrium, and Haplotypes Allelic association refers to a situation in which the frequency of an allele is significantly increased or decreased in individuals affected by a particular disease in comparison to controls. Linkage and association differ in several aspects. Genetic linkage is demonstrable in families or sibships. Association studies, on the other hand, compare a population of affected individuals with a control population. Association studies can be performed as case-control studies that include unrelated affected individuals and matched controls, or as family-based studies that compare the frequencies of alleles transmitted or not transmitted to affected children. . | Chapter 062. Principles of Human Genetics Part 28 Allelic Association Linkage Disequilibrium and Haplotypes Allelic association refers to a situation in which the frequency of an allele is significantly increased or decreased in individuals affected by a particular disease in comparison to controls. Linkage and association differ in several aspects. Genetic linkage is demonstrable in families or sibships. Association studies on the other hand compare a population of affected individuals with a control population. Association studies can be performed as case-control studies that include unrelated affected individuals and matched controls or as family-based studies that compare the frequencies of alleles transmitted or not transmitted to affected children. Allelic association studies are particularly useful for identifying susceptibility genes in complex diseases. When alleles at two loci occur more frequently in combination than would be predicted based on known allele frequencies and recombination fractions they are said to be in linkage disequilibrium . In Fig. 62-13 a mutation Z has occurred at a susceptibility locus where the normal allele is Y. The mutation is in close proximity to a genetic polymorphism with allele A or B. With time the chromosomes carrying the A and Z alleles accumulate and represent 10 of the chromosomes in the population. The fact that the disease susceptibility gene Z is found preferentially or exclusively in association with the A allele illustrates linkage disequilibrium. Though not all chromosomes carrying the A allele carry the disease gene the A allele is associated with an increased risk because of its possible association with the Z allele. This model implies that it may be possible in the future to identify Z directly to provide a more accurate prediction of disease susceptibility. Evidence for linkage disequilibrium can be helpful in mapping disease genes because it suggests that the two loci in this case A and Z are tightly .

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• Principles of Human Genetics
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• Mendels principles of heredity