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Chapter 8: Linkage and Gene Mapping *Independent assortment- during gamete formation segregating pairs of unit factors assort independently of one another.

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Presentation on theme: "Chapter 8: Linkage and Gene Mapping *Independent assortment- during gamete formation segregating pairs of unit factors assort independently of one another."— Presentation transcript:

1 Chapter 8: Linkage and Gene Mapping *Independent assortment- during gamete formation segregating pairs of unit factors assort independently of one another *Linkage – condition in which two or more nonallelic genes tend to be inherited together

2 An example of linkage: Purple, long42 Purple, round6 Red, long4 Red, round48 Progeny: 100 TOTAL NP R L r l X RrLlrrll Crossover results in gametes containing non-parental (NP) genotypes: Complete linkage: A condition in which two genes are located so close to each other that no recombination occurs between them Linkage ratio: unique phenotypic ratio (F 2 ) due to linkage of two genes

3 Chromosome Mapping Morgan’s proposal – two genes located close to each other along a chromosome are less likely to have a chiasma form than if two genes are father apart Morgan and Sturtevant and Mapping Determination of genetic distance between linked loci: Genetic distance = # recombinant progeny total # of progeny X 100% = map unit (m.u.) or cM (centiMorgans) Recombinant frequency (RF) Example of linkage between B and L: RL/rl x rl/rl RL/rlPurple, long42 Rl/rlPurple, round6 rL/rlRed, long4 rl/rlRed, round48 Progeny: 100 TOTAL NP # of recombinants10 Total progeny100 = 10 map units between B and L

4 Chromosome Mapping Frequency of recombinants (RF) measures the genetic distance between loci LB 10m.u. Theoretical limit to distance between genes is 50m.u. Even if CO occurs 100% of the time, recombination is only observed in ½ of the gametes (or 50%)

5 G-typesPhenotypesExp.Exp.#Observed# R_L_Purple flowers, long pollen R_llPurple flowers, round pollen37121 rrL_Red flowers, long pollen37121 rrllRed flowers, round pollen12455 Linkage Bateson & Punnett and sweet peas: P 1 : RRLL x rrll Purple flowersRed flowers Long pollenRound pollen F 1 : RrLl Purple flowers Long pollen F 2 : for a total of 381 offspring:

6 The Molecular Basis of Genetics Chapter 10: DNA Structure and Analysis The flow of information: 1. Storage 2. Replication 3. Expression

7 DNA as the Genetic Material Historically: *1868 Friedrick Miescher *1910 Phoebus Levene tetranucleotide hypothesis *1927 Frederick Griffith transforming principle *1944 Avery, MacLeod and McCarty *1952 Alfred Hershey and Martha Chase

8 Evidence for DNA in Eukaryotes: Indirect evidence: *Distribution of DNA *Mutagenesis Organismn2n Human Chicken Trout *RNA – genetic material in some viruses

9 Structural Analysis of DNA: *nucleoside *nucleotide

10 Base Composition: * Chargaff’s Rules SourceATGCA/TG/CA+G/C+TA+T/C+G Human Sea Urchin E. coli T Organism%G+C Phage T236.0 Drosophila45.0 Maize49.1 Euglena53.5 Neurospora53.7


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