chromo_struct-1 - Chromosome Structure Variations Causes...

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Chromosome Structure Variations
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Causes and Problems Chromosome structure variations result from chromosome breakage. Broken chromosomes tend to re-join; if there is more than one break, rejoining occurs at random and not necessarily with the correct ends. The result is structural changes in the chromosomes. Chromosome breakage is caused by X-rays, various chemicals, and can also occur spontaneously. --General problems with structural variants: 1. breaking a critical gene. This destroys the gene and thus can result in a mutant phenotype. 2. aneuploidy, usually after meiosis. We will explore #2 with the individual types of chromosome variation.
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Types --Types: Consider a normal chromosome with genes in alphabetical order: abcdefghi -- deletion : part of the chromosome has been removed: abcghi -- duplication : part of the chromosome is duplicated: abcdefdefghi -- inversion : part of the chromosome has been re-inserted in reverse order: abcfedghi -- ring : the ends of the chromosome are joined together to make a ring -- translocation : parts of two non-homologous chromosomes are joined: if one normal chromosome is abcdefghi and the other chromosome is uvwxyz, then a translocation between them would be abcdefxyz and uvwghi.
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Deletions When homozygous, most deletions are lethal, because most genes are necessary for life and a homozygous deletion would have zero copies of some genes. When heterozygous, the genes on the normal homologue are hemizygous : there is only 1 copy of those genes, and thus they are expressed even if recessive (like genes on the X in male mammals). Heterozygous deletions are aneuploid, because the genes in the deleted region are present in only 1 copy instead of the normal two copies. Some genes need to be present in two copies, so heterozygous deletions sometimes give rise to defects in the affected individual, especially if the deletions are large.
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Duplications Genes are duplicated if there is more than one copy present in the haploid genome. Some duplications are “dispersed”, found in very
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chromo_struct-1 - Chromosome Structure Variations Causes...

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