Occurs almost exclusively during the fetal and

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occurs almost exclusively during the fetal and postnatal periods of develop- ment. Experiments in mice support this idea. Reference: Shapiro SD, Endicott SK, Province MA, Pierce JA & Campbell EJ (1991) Marked longevity of human lung parenchymal elastic fibers deduced from prevalence of D -aspartate and nuclear weapons-related radiocarbon. J. Clin. Invest. 87, 1828–1834. 19–82 Marfan’s syndrome is caused by defects in the glycoprotein fibrillin, the major component of microfibrils, which binds to elastin and is essential for the integrity of elastic fibers. Patients with mutations in the fibrillin gene are often tall and lanky in appearance. CALCULATIONS 19–83 A. Since an individual who is heterozygous for a deletion of the a 1(I) gene has one normal gene, and is missing the other gene entirely, all type I collagen molecules will be normal. However, this individual will make only about half the usual number of collagen molecules. If the a 1(I) collagen chain with the point mutation can be incorporated into a collagen molecule, then only about 25% of the collagen molecules will be normal. The probability of incorporating a normal chain at each of two positions in a type I collagen molecule is (1/2)(1/2) = 1/4. Similarly, there is a probability of 1/4 of incorporating a mutated chain at each of two posi- tions in a type I collagen molecule. Finally, there is a 1/2 chance of incorpo- rating one normal and one mutated chain. If the mutant a 1(I) chain cannot be incorporated into a collagen mole- cule, then the result will be the same as that described for the gene deletion. B. A heterozygous individual with one deleted a 1(III) gene will make 100% nor- mal type III collagen, but in half the usual amount. If the a 1(III) gene with the point mutation can be incorporated into a col- lagen molecule, then only (1/2)(1/2)(1/2) or 1/8 of the type III collagen molecules will be normal. Similarly, 1/8 of the type III collagen molecules will consist of three chains with point mutations, 3/8 will have two normal chains and one chain with a point mutations, and 3/8 will have two mutant chains and one normal chain. C. The calculations in parts A and B indicate that point mutations are poten- tially much more severe than gene deletions. If point mutations yield THE EXTRACELLULAR MATRIX OF ANIMALS A451
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products that are incorporated into collagen molecules, and if the collagen molecules with mutant chains can assemble into collagen fibrils, a het- erozygous individual will have vanishingly few completely normal collagen fibrils. By contrast, an individual with one deleted gene will have 50% the usual number of normal collagen molecules and therefore 50% the usual number of normal collagen fibrils. Thus, point mutations are more likely than deletions to be dominant and to display the mutant phenotype.
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