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Medical Genetics and Cancer

Medical Genetics and Cancer - Medical Genetics and Cancer...

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1 Medical Genetics and Cancer Genetic Analysis of Human Diseases A genetic basis for a human disease may be suggested from a variety of observations. Human traits are determined both by genetics and the environment. The following suggest a genetic basis for a disease: When an individual exhibits a disease, this disorder is more likely to occur in genetic relatives than in the general population. Identical twins (monozygotic twins) share the disease more often than non-identical (dizygotic) twins. Concordance is the degree to which a disease is inherited. It is figured by calculating the percentage of twin pairs in which both twins exhibit the disorder relative to pairs where only one twin has the disorder. Diseases caused by a single gene should have 100% concordance. For dizygotic twins, concordance of dominant disorders is expected to be 50%. For recessive diseases, the concordance among dizygotic twins would be 25%. Frequently these are lower due to the penetrance of the disease or if one twin acquires the disease due to a mutation after fertilization. The disease does not spread to individuals sharing similar environmental situations. Different populations tend to have different frequencies of the disease. The disease tends to develop at a characteristic age (the age of onset). The human disorder may resemble a disorder that is already known to have a genetic basis in an animal (e.g., the albino phenotype – see Figure 22.1). A correlation is observed between a disease and a mutant human gene or chromosomal aberration. Inheritance patterns of human diseases may be determined via pedigree analysis. Human diseases that are caused by a mutation in a single gene may be studied using pedigree analysis.
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2 Autosomal recessive inheritance (e.g., Tay-Sachs Disease – Figure 22.2) has the following characteristics: The affected offspring usually has two unaffected parents. When two unaffected heterozygotes have children, the average percentage of affected children is 25%. Two affected individuals will have 100% affected children. The trait occurs with the same frequency in both sexes. Table 22.1 lists some human recessive disorders. Autosomal dominant inheritance (e.g., Huntington Disease – Figure 22.3) has the following characteristics: The affected offspring usually has one or both affected parents. An affected individual, with only one affected parent, is expected on average to produce 50% affected children. Two affected, heterozygous individuals will have on average 25% unaffected offspring. The trait occurs in the same frequency in both sexes. For most dominant, disease-causing alleles, the homozygote is more severely affected than the heterozygote. In some cases, a dominant allele may be lethal in the homozygous condition. A list of autosomal dominant human disorders is provided in Table 22.2.
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