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Meiosis and the Chromosomal Theory of Inheritance.docx

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Bhagwati Gupta

October 8 , 2013 Biology 2C03: Genetics Meiosis and the Chromosomal Theory of Inheritance Mendel’s Principle and Meiosis - This similarity in the pattern of chromosome inheritance and genetics inheritance is really a correlation - We need proof that a gene resides on specific chromosome - Metaphase I: principle of independent assortment - Anaphase I: principle of equal segragation Evidence for the Chromosomal Theory of Inheritance - Fruit flies: Drosophila melanogaster - Sex linkage: genes for specific traits are carried on chromosomes - T.H. Morgan and fruit flies  First mutation identified was for white (w) eyes +  Wild type: flies with red eyes, carrying wildtype allele (w )  Mutant; any variation in eye colour, carrying mutant alleles  E.g. white eye colour, white allele (w or w ) -  Note: the gene for eye colour was called white after the mutant phenotype T.H. Morgan’s Experiments - Experiment 1: Crossed white-eyed male to a red-eyed female - Result: all offspring had red eyes.  Is the white-eyed trait dominant or recessive?  Interpretation? - Experiment 2: F1 cross  Result: 3,470 red;782 white and all white-eyed flies were male  Interpretation: Morgan proposed that the eye colour gene is on the X chromosome  Females: two X chromosome = two alleles of the gene  Males: one X chromosome = one allele (hemizygous) - Red-eyed females: w /w ; two wild type alleles of the white gene - White-eyed male: w/Y; one mutant allele of the white gene - All f1 have red eyes - Males:females = 1:1 - If different ratios are seen for the two sexes, the gene is likely on a sex chromosome - White and red-eyed f2 offspring - All females are red-eyed - Males are red and white eyed (what ratio would you expect?) 1:1 ratio Reciprocal Cross - If reciproval crosses give different results, the gene is likely on a sex chromosome - F1, red-eyed females, white-eyed males - Good indication that the gene is on a sex chromosome - F1 cross: female is heterozygous - F2: half offspring have white eyes, half have red eyes. - Females, 1:1 red:white - Males, 1:1 red:white Calvin Bridges’ Thesis: Non-Disjunction as Proof of the Chromosome Theory of Heredity - The exception proves the rule:  Morgan’s u+dergraduate student, Calvin Bridges  w/w x w /Y offspring: females are red and males are white - Bridges’ results:  1/2000 females are white-eyes  1/2000 males are red-eyed - How? - Bridges’ hypothesis: X chromosomes were not segregating properly in meiosis (either anaphase I or anaphase II) - Nondisjunction of the X chromosome: any aberration from the norm; homologues fail to segregate in meiosis I (no sex chromosomes in two gametes) and/or sister chromatids fail to segregate in meiosis II (1 contains no sex information; 1 contains a double of sex information) Bridges’ Cross - Males receive one X chromosome from father = red eyes - Females receive two X-chromosomes from mother = white eyes - Cytological studies confirmed an abnormal normal of chromosomes = aneuploidy - Three copies of the X chromosome or no X chromosome the progeny will die - XXY female: two copies of white allele - X male: red eyes; sterile - Y males: dies Evidence for the Chromosomal Theory of Inheritance - Bridges then crossed XXY females (white eyes) to the normal XY males (red eyes) in order to confirm his theory – write the cross out +  XY/X or XX/Y x X Y X + Y XY X XY XYY X X X XY + XX X XX XXY Y X Y YY - Conclusion: unusual inheritance patterns correlate with aneuploidy (XO and XXY), this is strong evidence that phenotypes are associated with chromosomes and thus that genes are carried on chromosomes Homologous Recombination - Homologous recombination = crossing over; exchange of genetic material from one homologous chromosome to another - For progeny: information does not always segregate together - Mixing and matching on homologous chromosomes - The special events of meiosis allow for the movements that are the basis for the two Mendelian principles - There is another special event in meiosis that is the basis for another essential genetic concept: homologous recombination - Genes on a chromosome are linked, but there is exchange between homologous chromsoomes as well - Homologous recombination occurs during prophase I of meiosis: - A tetrad or bivalent is the associated homologous chromosomes - A chiasma (plural chiasmata) is a site of recombination or crossover: - Crossovers occur during pachytene of prophase I but are visible in diplotene Stages of Prophase I - Leptotene - Zygotene - Pachytne: synaptonemal complex forms; bivaent or tetrad - Diplotene: bivalent or tetrad – chiasmata are visible - Diakinesis: chromsomes are fully condensed Stages of Prophase I: Leptotene - First question, what is holding two replicated sister chromatids together: What is Holding Two Replicated Sister Chromatids Together - In mitosis, the association of the two replicated sister chromatids is established back in S phase via cohesion Cohesion - Similarly, in meiosis, cohesion is established in the pre-meiotic S-phase - It is removed in a two-step process: from the arms at anaphase I and from the centromere at anaphase II Stages of Prophase I: Leptoten - What is holding two replicated sister chromatids together = cohesion - Cohesion protein mediate the association Stages of Prophase I: Process
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