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MEIOSIS

MEIOSIS. Mendel’s principles. Each organism carries two copies of each gene—one from each parent Sex cells carry only one copy of each gene. Process that separates an organism’s two sets of genes—segregation Each pair of genes separates independently of the others—independent assortment.

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MEIOSIS

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  1. MEIOSIS

  2. Mendel’s principles • Each organism carries two copies of each gene—one from each parent • Sex cells carry only one copy of each gene. • Process that separates an organism’s two sets of genes—segregation • Each pair of genes separates independently of the others—independent assortment

  3. MEIOSIS • Discovery of chromosomes proved Mendel correct; except chromosomes separate, not genes • Process of reduction division; # of chromosomes per cell is cut in half through separation of homologous chromosomes in a diploid cell. • Creates gametes (sex cells) which have 1 chromosome from each pair (1/2 DNA)

  4. Chromosome # • Homologous chromosomes = pair of chromosomes alike in appearance and genetic information 1 comes from mother, 1 from father • Diploid cell: a cell which has both sets of homologous chromosomes • somatic cells; 2N (46 in humans) • Haploid cell: a cell which has only 1 set of chromosomes (not in pairs !) • gametes; 1N (23 in humans)

  5. Why is Meiosis Necessary? • Zygote = fertilized egg (diploid), fused gametes • Which of the following is correct fertilization • 2N + 2N = 4N • 1N + 1N = 2N

  6. Stages of Meiosis • Meiosis I: DNA is replicated and chromosomes form • Tetrads (4 chromatids) form • Crossing over takes place • Cell divides; sister chromatids stay together • Meiosis II: DNA is not replicated • Sister chromatids separate • 4 haploid cells are formed • 1 egg and 3 polar bodies in women • 4 sperm cells in men

  7. Dihybrid CrossIndependent Assortment BbSs BbSs

  8. GS Gs gS gs GS GGSS GGSs GgSS GgSs Gs GGSs GGss GgSs Ggss gS GgSS GgSs ggSS ggSs gs GgSs Ggss ggSs ggss • G = green g = striped • S = short s = long Parents = GgSs x GgSs

  9. Principle of Independent Assortment • Genes for different traits are inherited independently during the formation of gametes.

  10. DIHYBRID CROSS: involves 2 pairs of alleles, 2 traits. • Same principles for monohybrid apply for dihybrid

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