bio214d - Biology 214/414 2009 Packet 3 Omit section 5.12...

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Biology 214/414 2009 P a c k e t # 3 Omit section 5.12 Suggested problems: Chapter 5: 1, 3, 4, 8, 10, 12, 14, 20, 22, 24, 26, 34 Linkage, genetic recombination, and chromosome mapping in higher organisms I. Independent assortment of alleles (Mendel’s second law) occurs (mainly) because of independent assortment of non-homologous chromosomes in the first meiotic division; but each chromosome contains many alleles (ie, genes) and these “linked ” alleles/genes would not be expected to be able to assort independently from each other. If we consider examples in which we are following two genes at a time, each heterozygous with simple dominance- recessiveness applying, we can see how this works ( Figures 5-1 to 5-3 ). a. If the two genes are on different chromosomes, four gamete types are produced in equal amounts. b. If the two genes are very closely linked, only two types of gametes are formed, and the configuration of alleles occurring originally in the parent remains in the gametes (and ultimately the offspring). c. In “partial linkage ”, the configuration of alleles occurring originally in the parent tends to remain but not 100 % of the time; this is intermediate between a. and b. above. It is called “genetic recombination ” because the arrangement of the alleles on the parental chromosomes is “recombined ”. II. Genetic recombination is a result of a process/phenomenon called “crossing over ” ( Figures 5-5,6 ) a. A gene is sometimes called a “genetic marker ” if a certain allele is readily identifiable (eg, by the phenotype it produces). b. On each homologue of a pair analogous genes occur in the same position or “locus ”; loci are arranged in linear order on the chromosomes. c. Recombination of alleles on homologous chromosomes is accomplished by crossing over, during which homologous parts
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of homologous chromosomes are interchanged by breakage and rejoining . i. This involves the breakage and rejoining of 2 chromatids at a time. ii. It appears to occur during synapsis in first meiotic prophase, ie after DNA replication. iii. Four chromatids of a pair of homologues pair locus to locus along their lengths; because there are four chromatids, this is called a “tetrad ”. Although each individual crossover event occurs between two chromatids at a time, more than one individual crossover event can occur in any given tetrad. ( Figure 5-12 ) iv. Recombination of linked genes occurs by crossing over between the two loci. The probability of crossing over increases as the distance between the two loci increases . v.
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This note was uploaded on 03/03/2010 for the course BIOL 214 taught by Professor Stark during the Fall '06 term at Illinois Tech.

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bio214d - Biology 214/414 2009 Packet 3 Omit section 5.12...

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