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Perspectives on gene genetics: mutation types
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Perspectives on gene genetics: mutation types
Human genetic diseases show different patterns of inheritance depending on the type of mutation that causes them. Duchenne muscular dystrophy, a degenerative muscle disease that specifically affects males, is caused by a recessive mutation in the X chromosome and shows a typical sex-linked segregation pattern.
Cystic fibrosis depends on a recessive mutation in one chromosome. This type of mutation has a very different segregation pattern. Males and females can be equally affected by the disease. Both parents must be heterozygous carriers of the mutated allele for their children to be at risk of the disease. Each child of heterozygous parents has a 25 percent chance of taking both mutated genes from the parents and thus being affected by the disease, a 50 percent chance of receiving one normal and one mutated allele and being a carrier, and a 25 percent chance of being normal.
Huntington’s chorea, a degenerative disease of the nervous system that strikes in adulthood, on the other hand, is caused by a third segregation pattern: autosomal dominant mutations. If both parents are carriers, each child (despite gender) has a 50 percent chance of inheriting the mutated gene and being ill.
Mapping an autosomal gene (on a specific chromosome) is certainly more complicated than mapping sex-linked mutations. Once the specific chromosome has been identified, one proceeds with linkage analysis (proximity of other genes or markers) to identify other known markers, on the same chromosome to draw a map.
Through analysis of the recombination frequencies between two specific genes or markers, the distance of the markers themselves can be determined, and in fact the further apart two genes are on the same chromosome, the more likely it is that recombination will occur between them.
Based on this principle of classical genetics, distance units can be established, and a genetic map unit corresponds to the distance between two positions that have a recombination frequency of 1% and is referred to as a centimorgan in honor of the great Drosophila geneticist, T. H. Morgan. In this organism 1 centimorgan corresponds to about 400 kilobases of DNA. These distances vary from organism to organism because they also depend on the composition of the genome itself.
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