Ch_9_PartA

Ch_9_PartA - Chapter 9: Chromosomal Variation Chromosomal...

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Unformatted text preview: Chapter 9: Chromosomal Variation Chromosomal Morphology ¡ Metacentric ¡ Submetacentric ¡ Acrocentric ¡ Telocentric Recombination ¡ Homologous chromosomes pair in prophase I ¡ If crossing over occurs, ½ the gametes will be recombinant Chromosomal Rearrangements ¡ Alter the structure of the chromosome and include duplications, deletions, inversions, and translocations Duplication ¡ Part of the chromosome has been doubled ¡ AB • CDEFG → AB • CDEFEF G ¡ tandem duplication – duplicated region is adjacent to original segment ¡ displaced duplication – duplicated region is at a distance from original segment ¡ reverse duplication – duplicated region is inverted ¡ heterozygotes for a duplication require that the duplicated chromosome loops out during pairing in prophase I ¡ duplications can affect the phenotype Duplication – Bar mutation ¡ Bar mutation results in change in size and shape of eye in D. melanogaster ¢ the greater the number of copies the smaller the eyes ¡ Bar mutation results from a duplication on the X chromosome and is inherited as incompletely dominant ¢ abnormal gene dosage Duplication – Bar mutation ¡ Chromosomes do not align properly resulting in unequal crossover ¡ Double Bar female is created through unequal crossing over between chromosomes containing two copies of Bar Duplications ¡ Duplications can negatively affect the phenotype because of imbalances in the amount of gene products (abnormal gene dosage) ¢ e.g. Bar mutations in Drosophila ¡ Some duplications are beneficial and different gene copies are maintained over millions of years ¢ e.g. globins Gene duplication & evolution ¡ One of the gene copies remains the same and one gene copy mutates and obtains a new function (two copies diverge) ¡ The duplication of whole genes and their subsequent divergence giving rise to multigene families ¢ Can lead to genes with novel functions or pseudogenes ¡ Common in eukaryotic genomes with ~5% of the human genome consisting of duplicated segments ¡ Examples: ¢ Hox genes involved in development comprise of 9-11 genes ¢ Globin gene family consist of 16 similar sequences that all have three exons separated by two introns Globin Gene Tree Globin Gene Tree ¡ Ancestral globin gene diverged into the myoglobin copy and the α / β globin copy ¡ α and β-globin genes diverged ~450-500 mya ¡ β-globin gene family began to diverge ~200 mya ¡ These gene copies are found in many species Gene Trees Species A Species B para para ortho ortho p a r a gene duplication event speciation event turn boxes on side for simplicity orthologs vs. paralogs Gene Trees within Species Trees Deletions ¡ Loss of chromosome segment ¡ AB • CDEFG → AB • CDG ¡ Loop structure similar to that seen in heterozygous duplications Deletions ¡ Phenotypic consequences depend on location of deleted region ¢ If includes centromere, chromosome will not segregate and may be lost ¢ Homozygotes for deletions often dead because lost genes are essential...
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This note was uploaded on 01/30/2010 for the course ECOL 320 taught by Professor Weinert during the Fall '07 term at Arizona.

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Ch_9_PartA - Chapter 9: Chromosomal Variation Chromosomal...

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