Genetic Analysis of Complex Disease. Группа авторов

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a single base‐pair (bp) change, such as the adenine‐to‐thymine substitution in the HBB gene that alters the B chain of hemoglobin A from its wild type to its hemoglobin sickle cell state. Alternatively, allelic differences can be as extensive as large, multicodon deletions, such as those observed in Duchenne muscular dystrophy. Some bp changes have no deleterious effect on the function of the gene; nevertheless, these functionally neutral changes in the DNA still represent different forms of a gene. Rare changes in the genetic code that cause functional change of the gene or protein are often termed “mutations,” while functionally neutral changes that are more common in the population (>1%) are given the name “polymorphism.” The term “pathogenic variant” is also commonly used to refer to rare genetic changes that cause functional change in the gene or protein. “Mutation” and “pathogenic variant” may be used interchangeably throughout the chapter.

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      (Source: Reprinted by permission from Jorde et al. (1995).)

      Differences in alleles can be detected via laboratory testing. The ability to detect allele differences accurately within families, between families, and between laboratories is critically important for tracking the alleles that may be involved in Mendelian and genetically complex common disorders through linkage or association analysis. Allele detection strategies may be as simple as the presence (+) or absence (−) of a deletion or point mutation or as complicated as assessing the allele size in bp of DNA. The latter application is common when highly polymorphic microsatellite repeat markers are used in linkage analysis.

      Genes and Chromosomes

      Genes are organized as linear structures called chromosomes, with many thousands of genes on each chromosome. Each chromosome has distinguishable sites, known as centromeres that aid in cell division and in the maintenance of chromosome integrity. The centromere is visualized as the central constriction on a chromosome, and it separates the p (short) and the q (long) arms from one another. The centromere enables correct segregation of the duplicated chromosomal material during meiosis and mitosis. Telomeres are present at both ends of the chromosome and are required for stability of the chromosomal unit.

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      (Source: Courtesy of Mazin Qumsiyeh, Duke University Medical Center, Durham, NC.)

      Because two copies of each chromosome are present in a normal somatic (body) cell, the human organism is diploid. In contrast, egg and sperm cells have haploid chromosomal complements, consisting of a single member of each chromosome pair. The correct number of chromosomes in the normal human cell was finally established in 1956, three years after the double‐helical structure of DNA was described, when Tjio and Levan (1956) demonstrated unequivocally that the chromosomal complement is 46.

      Regions of chromosomes are defined by patterns of alternating light and dark regions called bands, which become apparent after a chemical treatment has been applied. One of the most common types of banding process, called Giemsa or G banding, involves digesting the chromosomes with trypsin and then staining with a Giemsa dye. G banding identifies late‐replicating, heterochromatic regions of DNA; these are the dark bands. Other chemical processes will produce different banding patterns and identify unique types of DNA.

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      The X and Y chromosomes vary in their genetic composition except for an area at the distal end of the p arm of each, termed the pseudoautosomal region. The pseudoautosomal region behaves similarly to the autosomes during meiosis by allowing for recombination of the sex chromosomes. Just proximal to the pseudoautosomal region on the Y chromosome are the sex‐determining region on the Y and testes‐determining factor genes, which are critical for the normal development of male reproductive organs. When a recombination event extends past the boundary of the pseudoautosomal region and includes one or both of these genes, sexual development will most likely be adversely affected. For instance, the rare occurrences of chromosomally XX males and XY females are due to such aberrant recombination.

      A cell’s ability to reproduce itself is critical to the survival of an organism. This cell duplication process, utilized by somatic cells, is called mitosis. Similarly, an organism’s ability to reproduce itself is critical to the survival of the species. In sexual organisms, the reproductive process involves the union of gametes (sperm and egg cells), which are haploid. Meiosis is the process by which these haploid gametes are formed from a diploid cell and is the biological basis of linkage analysis.


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