5. Lab 3 Genetic Mapping (aka 'The Tomato Lab')

5. Lab 3 Genetic Mapping (aka 'The Tomato Lab') - D RY LAB...

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44 DRY LAB – GENETIC MAPPING THE TOMATO Expectations Upon completion of this lab, you should be able to: Describe how recombination frequencies can be used to infer relative gene locations on a chromosome. Explain why non-recombinants & double recombinants are the most & least common gametes when looking at linked genes. Explain why corrections for distances calculated from double recombinants need to be made. Define and describe a RFLP. Reading Hartwell et al., (2008), p124-135 (5.1) – gene linkage and recombination Hartwell et al., (2008), p135-141 (5.2) – gene mapping Hartwell et al., (2008), p303-309 (9.1) – restriction length polymorphisms Quiz At end of lab; on concepts presented within this lab (readings above are helpful for understanding). Due Hand-in assignment sheet due at end of lab. INTRODUCTION Genes close together on the same chromosome ( i.e., linked genes) (Fig. 4.1) tend to segregate together (co-segregate) in genetic during meiosis ( i.e., go into the same gamete). This means that allelic combinations for specific pairs of genes may be transmitted together to gametes. However, the presence of recombination does allow some novel allelic combinations to occur (in recombinant gametes), just at a lower rate than expected. Linked genes thus show a departure from the typical Mendelian phenotypic ratio for dihybrid crosses (9:3:3:1) expected when genes assort independently. Because genes that are farther apart experience more recombination, the proportion of recombinant gametes provides information about the distance between the genes of interest. A genetic map can therefore be produced based on the proportions of recombinants ( i.e., new allelic combinations for genes on the same chromosome) observed for the different genes, or even between sites within a gene. Remember: recombination occurs between non-sister chromatids of homologous chromosomes. Because more recombinant gametes will be produced if genes are farther apart, the proportion of recombinant gametes in a pool of gametes can be used as a measure of genetic distance. If three linked genes are mapped, the proportions of recombinant gametes for each pair of genes can be used to construct a genetic map. In the example below (Box 4.1), a cross is performed between two different inbred lines and the F 1 generation is then back-crossed to one parental line. The results of the backcross are shown in Table 4.1. Figure 4.1: Linked genes . The genes vg , pr , and b are all located on the same chromosome and thus are linked genes. Alleles vg + , pr + , and b + are the wild-type alleles for these genes. Taken from Hartwell et al., (2008).
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45 Box 4.1: Cross between two inbred lines, with 3 genes (D, E, and F), all located on the same chromosome, with backcrossing of the F 1 generation to the P generation.
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