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40 Cards in this Set
- Front
- Back
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Two types of events that reshape genomes
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- rearrangements
- changes in chromosome number |
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Types of Chromosomal Rearrangements
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- Deletion
- Duplication - Inversion - Translocation (Reciprocal and Nonreciprocal) - Transposition |
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Deletion
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- removal of a segment of DNA
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Duplication
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- Increase in the number of copies of a chromosomal region
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Inversion
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- Half circle rotation of a chromosomal region
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Nonreciprocal Translocation
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- Unequal exchanges b/tw nonhomologous chromosomes
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Reciprocal Translocation
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- Parts of two nonhomologous chromosomes trade places
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Transposition
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- Movement of short DNA segments from one position in the genome to another
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Two Categories of Chromosomal Rearrangements
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- Unbalanced rearrangements
- Balanced rearrangements |
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Unbalanced rearrangements
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- gene dosage is changed (DNA is lost or gained)
- deletion - duplication |
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Balanced rearrangements
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- gene dosage is not changed (no gain or loss of DNA)
- Inversion - Translocation |
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Deletion for Homozygosity
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- Del/Del
- Del/ Y (hemizygotes) - often but not always lethal |
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Deletion for heterozygosity
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- Del/ +
- can survive deletion b/c carries a nondeleted wt homolog of deleted chromo - cannot be too large or will be lethal - may cause a phenotype |
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Deletion Loop
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- during pairing of homologs in prophase of meiosis I, region of nondeleted chromosome forms loop
- unpaired bulge of normal chromo that corresponds to the area deleted from the other homolog |
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Deletion Loop (2)
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- progeny of Del/+ will always inherit markers in del loop
- recombination cannot occur with genes within deletion loop; genes can not be separated |
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Pseudominance
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- normal gene carries a mutant recessive allele of one of the genes, individual will exhibit mutant phenotype
- used to determine whether a deletion has removed a particular gene |
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Deletion can be used to locate genes
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- double mutant will have abnormal phenotype if recessive mutations are in region that is deleted
- if del result in loss of bands from chromo and also reveals recessive mutation of a gene -> locate gene - < # of bands < accuracy of gene localization |
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Deletion Mapping
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- using strain with known deletions to map another recessive mutation
- if double heterozygote has mutant phenotype, then genes b must be in the same region that is removed in the deleted seq of strain W - if double heterozygote has normal phenotype, then gene b must be in a different region than is removed in the deleted seq of strain Y |
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Duplications add seq to genome
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- Two types
- Tandem duplication: duplicate regions are closely-linked; same order or reverse order (repeats) - nontandem (dispersed) duplication: duplicate regions are not closely linked or are unlinked - duplications can be produced by chromosome breakage |
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Duplications can affect phenotype
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- dupl are less likely then deletions to cause mutant phenotypes
- dupl hetero (dp/+) can cause phenotypes through increased gene dosage of through altered gene expression |
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Unequal Crossing over
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- can increase or decrease gene copy #
- in drosophila, tandem duplications of an x-linked gene causes Barr eye - most progeny of Bar-eye homozygotes are Bar-eye - some progeny of Bar-eye homozygotes have double Bar-eye |
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Summary of phenotypic effects of dupl and del
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- dup/+ or Df/+, an imbalance in gene product may cause an altered phenotype
- dup/+ or Df/+, a dupl or del can cause a gene(s) to have altered expression levels b/c of being in new chromo contact - del/dupl drive evolution - dupl generate multi-gene families |
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Inversions
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- can occur through chromosome breakage or intrachromosomal recombination
- double strand break followed by 180 rotation of chromosomal region - crossover b/tw inverted repeats on same chromo |
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Two Kinds of Inversions
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- Paracentric and Pericentric
- in hetero, both kinds of inversions cause problems for chromo segregation during meiosis |
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Paracentric Inversion
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- inverted portion involves only one arm
- exclude centromere - single crossover occurs w/in inversion loop; imbalance in centromere # (one product is accentric other is dicentric) - accentric cannot attach to spindle fiber, chromo is lost - dicentric chromo breaks |
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Pericentric Inversion
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- inverted portion involves both arms
- includes centromere - single crossover occurs w/in inversion loop - each recomb chromatid will have single centromere will carry dupl and del |
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An inversion can affect a phenotype if it disrupts a gene
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- inversion breakpoints can also be in noncoding seq and affect regulatory seq or affect chromatin structure over large physical distance "position effects"
- if one inversion lies w/in DNA of a gene, a phenotype can occur - intragenic break (separate 2 parts of gene; relocating one part to a distant region of chromo; other part is at original site); disrupts gene's function |
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Inversion loops
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- are produced in inversion hetero during meiosis
- only way crossovers can occur w/in inverted segment is trough realignment of one sister chromatid |
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In/+ individuals produce aberrant gametes
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-In/+ inversion hetero
- inversion loop: one chromo region rotates to conform to similar region in other homolog - crossing over w/in inversion loop produces aberrant recomb chromatids |
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Translocation attach of a chromo to another
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- two breaks on non-homologous chromo
- part of a chromo moved to another chromo |
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Reciprocal Translocation
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- results when two breaks (in each chromo) yield DNA frag that do not religate to chromo of origin but switch places and attach to other chromo
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Heterozygosity for Translocation
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- causes reduced fertility and pseudolinkage
- chromo with translocation will pair and segregate normally during meiosis - chromosome with translocation form a crosslike config during meiosis that can produce genetically unbalanced gametes |
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Tansposable elements (TEs)
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- general term for genetic DNA unit that can insert into a chromosome and reinsert elsewhere in genome
- mobile genetic elements |
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Transposition
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- process which TEs move from one genomic location to another
- intact TEs encode transposase for insertion and excision |
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TE insertion
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- cause complete or partial loss-
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