Lecture23S10 - BIS101/Engebrecht Lecture23 5/25/10...

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BIS101/Engebrecht Lecture23 5/25/10 Chromosomal mutations (cont): Deletions (deficiencies) = parts of chromsome missing. There are lots of deletions in Drosophila, these have been mapped cytologically and molecularly (now that the Drosophila genome has been sequenced). These deletions are important tools for mapping genes. By mating deletion heterozygotes with your homozygous mutation, you can determine if the mutation phenotype is uncovered or not. We went over Handout25 on deficiency mapping. In reality, one would first map the mutation of interest by classical 3-point crosses and then when a region has been defined, map the mutation relative to smaller and smaller deletions. CQ1:This is an example of how you would determine the location of a mutation relative to a deletion series. The answer is A . You should understand how to map a gene relative to different deletions. Duplications Chromosome duplications duplicate part of a chromosome as represented below AB . CDEFGHIJ AB . CDEFGHFGHIJ In this example, the region FGH is duplicated and thus will affect linkage of E and I. What is the consequence of duplications? In this case you are not missing DNA however, if the endpoints of the duplication event lie within a gene, this will result in a mutation. A duplication may or may not result in a phenotype depending on 1) the breakpoint and 2) whether a change in gene dosage will result in a phenotype (in heterozygote 3 copies, in homozygote 4 copies). Pairing between a duplicated chromosome and a non-duplicated chromosome results in a loop. Furthermore, homozygotes can have asymmetrical pairing, which would result in formation of a normal (unduplicated) chromosome and a chromosome that has 3 copies. Duplications are also important in evolution, as we will see below. The human globin genes, duplications and the evolution of gene families . Why do we care about chromosome duplications? Duplications provide the basis of evolution. Once a gene has duplicated, it is free to accumulate mutations that can alter its function. In mammals, the globin gene family serves a case in point. An ancestral globin gene that encoded a protein that could carry oxygen underwent an early duplication that led to the present day hemoglobin genes and myoglobin. The ancestral globin gene underwent another gene duplication that led to alpha-type and beta-type globins. The primordial beta and alphas genes underwent additional duplication events giving rise to 6 beta type genes and 5 alpha type genes. The different alpha and beta type genes are differentially expressed in human development. Hemoglobin in general has 2 alpha and 2 beta type chains. Unequal crossing over between different alpha type genes and between different beta type genes can occur, because the alpha genes are very similar to each other just as the beta type genes are very similar to each other.
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This note was uploaded on 07/02/2010 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture23S10 - BIS101/Engebrecht Lecture23 5/25/10...

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