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Chap 13

Chap 13. Meiosis and Sexual Cycles. Sexual Life Cycles. The alternation between meiosis and fertilization is common to all sexually reproducing organisms. In animals and humans, gametes are the haploid cells the diploid zygote is multicellular. Origins of Genetic Variation.

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Chap 13

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  1. Chap 13 Meiosis and Sexual Cycles

  2. Sexual Life Cycles • The alternation between meiosis and fertilization is common to all sexually reproducing organisms. • In animals and humans, gametes are the haploid cells the diploid zygote is multicellular.

  3. Origins of Genetic Variation • The behavior of chromosomes during meiosis and fertilization is responsible for the majority of variation in a sexually reproducing species. • Independent assortment, crossing over and random fertilization are the key mechanisms.

  4. Independent Assortment • There is a 50% chance that a daughter cell will receive a maternal or paternal chromosome. • Each homologous pair is positioned independently at metaphase 1. • The first meiotic division results in each pair sorting maternal and paternal.

  5. Crossing Over • During the pairing of homologs at prophase 1, each gene is aligned precisely with its corresponding gene on the other homolog. • The DNA molecules of 2 nonsister chromatids (one maternal, one paternal), are broken at the same place and rejoined to each other creating a recombined DNA.

  6. Random Fertilization • Each of us produces gametes that contain 1 of 8 million different chromosome combinations. • 8 million x 8 million = 64 trillion diploid combinations. • Adding in variations caused by crossing over and the possibilities are endless!!!!

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