Inheritance, complex patterns

Certain mutant hemoglobins are common in many populations, and a patient may inherit more than one type. Hemoglobin disorders thus present a complex pattern of clinical phenotypes. The use of DNA probes for their diagnosis is considered in Chapter 40. 

Secondary metabolites with similar structural types and pharmacophoric groups can be seen in several bacteria (where they are often termed antibiotics if they have antimicrobial or cytotoxic properties). Since eukaryotic cells had taken up a-proteobacteria (which became mitochondria) and cyanobacteria (which became chloroplasts), they also inherited a number of genes that encode enzymes for pathways leading to secondary metabolites. Therefore, we may speculate that early plants already had the capacity of building defense compounds and that alkaloids were among the first. Since the numbers and types of herbivores and other enemies have increased within the last 100 million years, angiosperms have had to face more enemies and as a consequence have developed a more complex pattern of defense and signal compounds. 

Multifactorial diseases involve complex patterns of inheritance and represent a very large class of genetic... 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat because the specific factors that cause most of these disorders have not yet been identified. By 2010, however, researchers predict they will have found the major contributing genes for many common complex disorders. 

Alzheimer s disease exists in two major forms, the so-called early and late onset types. The former follows typical Mendelian inheritance while the latter shows a more complex, non-Mendelian, pattern of inheritance. The early onset form of the disease has permitted the identification of several genes which are causally related to the condition. 

Behavior-related illnesses and patterns, both the so-called healthy and nonhealthy, are complex and involve many factors including external ones that are both learned and simply exposed to. Two features emerge from this ongoing study. First, is a tendency (not certainty) for this trait to be inherited. This tendency is generally greater than is the tendency with respect to physical disease. Second, environment plays a role. Similar environments produce similar people and different environments for related people produce different people. 

Keratoconus is an ectatic corneal dystrophy that tends to be bilateral but may be asymmetric and generally manifests in the second or third decade of life. There is evidence that keratoconus is a hereditary condition, with a family history reported in 6% to 8% of patients with the disease.The prevalence in first-degree relatives is 15% to 67% higher than in the general population, and the incidence has been reported at approximately 1 in 2,000. The inheritance pattern has been variably reported as sporadic, autosomal recessive, and autosomal dominant. Keratoconus is likely a multigenic disease with a complex mode of inheritance, and its manifestation likely involves environmental factors. 

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