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Genetics380_Oct20_09-1

Genetics380_Oct20_09-1 - Eukaryotic Gene Mapping Map of...

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Genetics 380 Eukaryotic Gene Mapping Reading: Chapter 7 of textbook. Map of Rutgers Busch Campus, Piscataway
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Inheritance of genes on the same chromosome Genes located close together on the same chromosome are called linked genes and belong to the same linkage group . Linked genes travel together during meiosis, and are not expected to assort independently. What is independent assortment?
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In meiosis, each pair of homologous chromosomes assorts independently of other homologous pairs.
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Sturtevant’s map of the Drosophila X chromosome.
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Independent Assortment vs. Complete Linkage Unlinked genes on different chromosomes Linked genes on the same chromosome A B b a 2 n combinations,where n = # of chromosomes A B a B A b a b A B a b A B A B a b a b 100% nonrecombinant gametes 50% recombinant gametes 50% nonrecombinant gametes
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If F1 is selfed, Mendel predicts 9:3:3:1 ratio If F1 is backcrossed (to aabb), Mendel predicts 1:1:1:1
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Nonindependent assortment
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Why is it important to obtain a map of gene positions on chromosomes? Gene position is crucial information needed to build complex genotypes required for experimental purposes of commercial applications. Knowing the position occupied by a gene provides a way of zeroing in on its structure and function . The genes present and their arrangement on chromosomes are often slightly different in related species. For example, long human chromosome 2 is split into two shorter chromosomes in the great apes.
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Chromosome banding is revealed by special staining techniques. Shown here is the staining of chromosomes with a special dye called Giesma, which reveals G bands that distinguish areas of DNA that are rich in adenine-thymine (A-T base pairs).
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Large metacentric human chromosome 2 compared to two acrocentric chromosomes found in three other primates.
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The arrangement of genes on chromosomes is represented diagrammatically as a uni-dimensional chromosome map , showing gene positions, known as loci (sing., locus) and the distances between the loci on some kind of scale. Two basic types of chromosomal maps: Recombination-based maps: map the loci of genes that have been identified by mutant phenotypes showing single-gene inheritance. We will talk about these today. Physical maps: show genes as segments arranged along the long DNA molecule that constitutes a chromosome.
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Notation for crosses with linkage (genes on the same chromosome) X A B A B a b a b A B a b = A B a b A a B b A B b a NOT NOT
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Additional linkage symbolism and terminology Linked genes in a dihybrid are present in one of two combinations: Cis (adjacent) conformation: two dominant, or wild-type, alleles are present on the same homolog Trans (opposite) conformation: on different homologs cis AB/ab or ++/ab trans Ab/aB or +b/a+ The following conventions pertain to linkage symbolism: 1. A slash symbolically separates the two homologs.
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