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35569_Ward's World+MGH Human Genetics

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Human Genetics (continued) somes and their constituent genes, the mechanisms of deoxy- ribonucleic acid (DNA) synthesis and protein synthesis, and the ways in which change in the molecular structure of a gene can lead to a disease. In addition, research in human genetics has been stimulated by concerns about possible genetic damage through environmental agents, particularly ionizing radiation, and the possible harmful effects of hazardous substances in the environment on prenatal development. Chromosome and gene structure In normal humans, the nucleus of each normal cell contains 46 chromosomes, which comprise 23 different pairs (Fig. 2). Of each chromosome pair, one is paternal in origin and the other is maternal in origin. In turn, only one member of each pair is handed on through the reproductive cell (egg or sperm) to each child. Thus, each egg or sperm has only 23 chromosomes, the haploid number; the fusion of egg and sperm at fertilization will restore the double, or diploid, chromosome number of 46. The segregation of chromosome pairs during meiosis allows for a large amount of "shuffling" of genetic material as it is passed down the generation. With 23 pairs of chromosomes, the total possible number of different chromosome combinations in the gametes is 223, or about 8 million. Two parents can provide 223 × 223 different chromosome combinations. This enormous source of variation is multiplied still further by the mechanism of crossing over, in which homologous chromosomes exchange segments during meiosis. Twenty-two of the 23 chromosome pairs, the autosomes, are alike in both sexes; the other pair comprises the sex chromo- somes (Fig. 2). A female has a pair of X chromosomes; a male has a single X, which is paired with a Y chromosome that he has inherited from his father and will transmit to each of his sons. Sex is determined at fertilization and depends on whether the egg (which has a single X chromosome) is fertilized by an X- bearing or a Y-bearing sperm. All chromosomes are composed of DNA (Fig. 3), and genes are segments of DNA. There are about 19,000 – 20,000 genes on the 46 human chromosomes, and many have been mapped to spe- cific chromosomal locations (Fig. 1). The genetic information is coded in DNA in the form of triplets of four bases: two purines, adenine (A) and guanine (G), and two pyrimidines, thymine (T) and cytosine (C). Each triplet combination (codon) codes for a specific amino acid. The sequence of bases in a specific gene dictates the sequence of amino acids in the specific protein coded by that gene. + ward ' s science Fig. 2 Digital illustration of the human karyotype. The karyotype is the complete set of chromosomes in an organism. (Credit: Science Photo Library/Alamy Stock Photo) Fig. 1 A genetic map is a type of chromosome map that shows the relative locations of genes and other important features. The map is based on the idea of linkage (that is, the closer that two genes are to each other on the chromosome, the greater is the probability that they will be inherited together). By following inheritance patterns, the relative locations of genes along the chromosome are established. (Credit: National Human Genome Research Institute)

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