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Chapter 4

Chapter 4 notes (Exam II).docx

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University of Rochester
BIO 190
Elaine Sia

4.1 Why is gene mapping so important?  Crucial information is needed to build complex genotypes required for experimental purposes or for commercial applications.  It is a useful way of zeroing on structure and function.  Geneticists can deduce the evolutionary lineages and genetic mechanisms. What are the differences between recombination based maps and physical maps? Recombination based maps ma0p the genes that have been identified in mutant phenotypes showing single gene-inheritance. Physical maps show tge genes as sengments arranged along the long DNA molecule tht constitutes a chromosome. How are recombination maps assembled? Two or three genes at a time. TO stay linkage, why is a tester used to cross with the F1 generation? The analyst will only have to worry about the genotype of one of the parents rather than both. What is the general pattern that is observed with dihybrid crosses in consideration of recombination? Two equally frequent nonrecombinant classes totaling in excess of 50%. Two equally frequent recombinatnt classes totaling less than 50%. How do crossovers produce recombinants for linked genes? When homologous chromosomes pair during meiosis, the chromosomes occasionally break and exchange parts in a process called crossing over. This results in two new combinations called crossover products. The process involves the formation of a crossshaped structure called chiasma, when two dyads unite as a bivalent during meiosis. What are the two conformations of linked genes in a dihybrid? In onr arrangement both the wt alleles are present t in one homolog- cis. In the other they are on diferent homologs (trans). Cis (AB/ab); Trans (Ab/aB) What is the evidence that cross-over must occur in the 4 chromatid stage? Tetrad analyses of genes that are linked show that many tetrads contain four different allele combinations which is only possible if crossovers take place in the four-chromatid stage, otherwise if crossover took place in the two-chromatid stage then there would only be a max of two different genotypes in an individual tetrad. How can multiple cross-overs occur? Between more than two chromatids. However, single crossovers only occur between two chromatids. Cross0overs between sister chromatids are very rare however, do not produce new allele combinations so are not usually considered. 4.2 What is the relationship with RF and mu? The greater the RF the further apart the genes are What is a linkage map? IT is a map that uses percentage of recombinant as a quantitative index pf the linear distance between two genes on a genetic map. What is a genetic mu? It is the distance between genes for which 1 product of meiosis in 100 is recombinant. What is a three-point test cross? It is used to deduce whether the three genes are linked and if they to deduce their order and the map distance between them. How is a three-pouit tst cross?(page 132) A parental cross is performed. Then the F1 trihybrid progeny is crossed with a triple recessive individual. From any trihybrid 8 gamete genotypes are possible. These are the genotypes that are seen in the testross prgeny. Looking back at the gametes acquired from the P generation, it is possible to recognize which genotypes are the result of recombination. We can then analyze the loci two at a time and find out which ones are recombinants. In th
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