Lecture 2 BIO 311D 2nd Edition Outline of Last Lecture I Review of levels of organization in biology II Chromosomes III Cell division and life cycles IV Significance of meiosis Outline of Current Lecture I Meiosis ploidy and life cycles II Sources of genetic variability in meiosis III Applying meiosis to Mendelian inheritance Current Lecture I Meiosis A 2n Mitosis 2n Meiosis n Mitosis n B If human life cycles were like plant life cycles a multicellular sperm made by mitosis would be equivalent to the independent multicellular haploid structures seen in plants C Meiosis I Chromosomes condense tetrads form homologs separate from each other Meiosis II Twin chromatids separate from each other II Sources of genetic variability in meiosis A Non disjunction is failure of chromosomes to separate during meiosis If a cell with 2n 40 undergoes meiosis but one non disjunction event occurs in meiosis II how many chromosomes will be in each of the four daughter cells What if the event occurred in meiosis I Meiosis II 21 19 20 20 n 1 n 1 n n Meiosis I 21 21 19 19 n 1 n 1 n 1 n 1 B Segregation When a gamete is formed the members of a homologous pair are separated from one another and segregate separate into different gametes this is called the law of segregation this results in gametes with only 23 chromosomes Each has 1 member of each homologous pair C Independent assortment during meiosis produces 2n combinations of chromosomes The way the members of one homologous pair separate into gametes is totally independent from the way other homologous pairs segregate Since each gamete is produced by independent assortment the combination of genes is unique D Another source of genetic variation is recombination through crossing over maternal and paternal homologs break and exchange segments at random sites along the chromosome III Meiosis to Mendelian Inheritance A We care for what happens in meiosis because that s how we get our genes Meiosis is the basis of transmission genetics and of Mendelian Inheritance patterns
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