tailieunhanh - Chapter 062. Principles of Human Genetics (Part 17)

Genotypes describe the specific alleles at a particular locus. For example, there are three common alleles (E2, E3, E4) of the apolipoprotein E (APOE) gene. The genotype of an individual can therefore be described as APOE3/4 or APOE4/4 or any other variant. These designations indicate which alleles are present on the two chromosomes in the APOE gene at locus . In other cases, the genotype might be assigned arbitrary numbers (., 1/2) or letters (., B/b) to distinguish different alleles. A haplotype refers to a group of alleles that are closely linked together at a genomic locus (Fig. 62-8). Haplotypes. | Chapter 062. Principles of Human Genetics Part 17 Genotypes describe the specific alleles at a particular locus. For example there are three common alleles E2 E3 E4 of the apolipoprotein E APOE gene. The genotype of an individual can therefore be described as APOE3 4 or APOE4 4 or any other variant. These designations indicate which alleles are present on the two chromosomes in the APOE gene at locus . In other cases the genotype might be assigned arbitrary numbers . 1 2 or letters . B b to distinguish different alleles. A haplotype refers to a group of alleles that are closely linked together at a genomic locus Fig. 62-8 . Haplotypes are useful for tracking the transmission of genomic segments within families and for detecting evidence of genetic recombination if the crossover event occurs between the alleles Fig. 62-3 . As an example various alleles at the histocompatibility locus antigen HLA on chromosome 6p are used to establish haplotypes associated with certain disease states. For example 21-hydroxylase deficiency complement deficiency and hemochromatosis are each associated with specific HLA haplotypes. It is now recognized that these genes lie in close vicinity to the HLA locus which explains why HLA associations were identified even before the disease genes were cloned and localized. In other cases specific HLA associations with diseases such as ankylosing spondylitis HLA-B27 or type 1 diabetes mellitus HLA-DR4 reflect the role of specific HLA allelic variants in susceptibility to these autoimmune diseases. The recent characterization of common SNP haplotypes in four populations from different parts of the world through the HapMap project is providing a novel tool for association studies designed to detect genes involved in the pathogenesis of complex disorders Table 62-1 . The presence or absence of certain haplotypes may also become relevant for the customized choice of medical therapies pharmacogenomics or for preventative strategies. .

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• Mendels principles of heredity