Lecture 7 - From Tuesday Recessive lethal allele: Coat...

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From Tuesday… Recessive lethal allele: Coat color in mice: the yellow phenotype (Y) is a dominant mutation…the mutant phenotype is seen in heterozygote. BUT: the lethality factor inherent in the Y allele is only observed in the homozygote (Y/Y), or rather it is not observed (!) as carriers of this genotype do not progress beyond the embryonic stage.
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Lecture 7 Recombination and linkage
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Recombination frequency: the proportion or percentage of recombinant cells or individuals Result: 50% Parental types and 50% recombinants Analysis: genes are likely on different chromosomes… independent assortment What the ratio is different from the expected? 1:1:1:1 For unlinked genes/loci
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Linked alleles tend to be inherited together Two autosomal alleles are physically connected on the chromosome and will not assort independently (recombination frequency < 50%) Dihybrid progeny Parental genotypes are far more abundant in progeny than are recombinant genotypes… 89.3 % parental genotypes Purple eye Vestigial wing
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Figure 4-3 Crossing over produces new allelic combinations How are any recombinants produced (10.7% recombination frequency) if genes are linked? Crossing over occurs between non-sister chromatids, following replication, during synapsis through the end of prophase I of meiosis
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Chromosomal crossover (or crossing over) is the exchange of genetic material between chromatids of homologous chromosomes. Meiotic recombination initiates with double- stranded breaks that are introduced into the DNA Crossing over/recombination Crossing over produces new allelic combinations Dyad = sister chromatids
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Crossing over is between chromatids, not chromosomes Only 2 different products 4 different products Established using organisms that produce ascus-bound tetrads: 4 meiotic products from a single meiocyte remain together.
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Notation used to indicate linkage Cis and trans allelic conformation of linked genes in a dihybrid… Cis (adjacent; on the near side of): like alleles, i.e. dominant/wild type, occur on the same homolog VS Trans (opposite): like alleles are on different homologs Cis AB/ab or ++/ab Trans Ab/aB or +b/a+ Alleles on the same homolog have no punctuation between them Slash (/) indicates two homologs
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Multiple crossover events can occur along a chromosome pair Exchanges can occur between more than two chromatids… Simplest case: double crossover involving three linked genes
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This note was uploaded on 12/17/2009 for the course BIO Genetics taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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Lecture 7 - From Tuesday Recessive lethal allele: Coat...

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